|
Application of Deep Learning Workflow for Autonomous Grain Size Analysis
A Bodras, J Zhang, JC Nino
Application of Computer Simulation in Materials Science of Molecules
Traditional grain size determination in materials characterization involves microscopy images and a laborious process requiring significant manual input and human expertise. In recent years, the development of computer vision (CV) has provided an alternative approach to microstructural characterization with preliminary implementations greatly simplifying the grain size determination process. Here, an end-to-end workflow to measure grain size in microscopy images without any manual input is presented. Following the ASTM standards for grain size determination, results from the line intercept (Heyn’s method) and planimetric (Saltykov’s method) approaches are used as the baseline. A pre-trained holistically nested edge detection (HED) model is used for CV-based edge detection, and the results are further compared to the classic Canny edge detection method. Post-processing was performed using open-source image processing packages to extract the grain size. In optical microscope images, the pre-trained HED model achieves much higher accuracy than the Canny edge detection method while reducing the image processing time by one to two orders of magnitude compared to traditional methods. The effects of morphological operations on the predicted grain size accuracy are also explored. Overall, the proposed end-to-end convolutional neural network (CNN)-based workflow can significantly reduce the processing time while maintaining the same accuracy as the traditional manual method.
|
|
|
Sb-doped BiI3–Developing the Next Room Temperature Spectrometer
PM Johns, M Bliss, JC Nino
Nuclear Science and Engineering
|
2022 |
|
Irradiation Behavior of Oxide Ceramics for Inert Matrices
DT Moore, CA Papesch, BD Miller, PG Medvedev, JC Nino
Nuclear Science and Engineering
|
2022 |
|
Connectomic analysis of Alzheimer’s disease using percolation theory
P Kotlarz, JC Nino, M Febo
Network Neuroscience 6 (1), 213-233
Alzheimer’s disease (AD) is a severe neurodegenerative disorder that affects a growing worldwide elderly population. Identification of brain functional biomarkers is expected to help determine preclinical stages for targeted mechanistic studies and development of therapeutic interventions to deter disease progression. Connectomic analysis, a graph theory–based methodology used in the analysis of brain-derived connectivity matrices was used in conjunction with percolation theory targeted attack model to investigate the network effects of AD-related amyloid deposition. We used matrices derived from resting-state functional magnetic resonance imaging collected on mice with extracellular amyloidosis (TgCRND8 mice, n = 17) and control littermates (n = 17). Global, nodal, spatial, and percolation-based analysis was performed comparing AD and control mice…
|
2022 |
|
Complementary resistive switching in core–shell nanowires
SE Vasisth, JC Nino
Journal of Applied Physics 130 (15), 155,104
Highly dense, energy-efficient, and fast neuromorphic architectures emulating the computational abilities of the brain use memristors to emulate synapses in the analog or digital systems. Core–shell nanowires provide us with new opportunities for neuromorphic hardware integration. In this work, we have fabricated core–shell nanowires using a combination of bottom-up and top-down techniques. Additionally, we have demonstrated eightwise and counter-eightwise bipolar resistive switching (BRS). Remarkably, for the first time along with BRS, we have also demonstrated complementary resistive switching (CRS) in core–shell nanowires. Here, Pt was used as the conductive core and HfO2 as the memristive shell with Ti as the top electrode. The resistive switching properties were characterized by I–V curves and pulse operation modes. The cycling endurance in the BRS mode was 1000 cycles with an off–on ratio of ∼13 and resistance was retained for 104 s. Additionally, the compliance current used to form the nanowire in the BRS mode influenced the CRS operation by lowering the peak operating current. Additionally, current density–electric field analysis performed to determine charge conduction mechanisms revealed that the wires exhibit a thermionic emission mechanism in the high resistance state and Ohmic conduction mechanism in the low resistance state during the BRS mode of operation and hopping conduction mechanism in state 0 and space-charge-limited conduction mechanism in state 1 during the CRS mode of operation. This observed versatility in the mode of operation makes core–shell nanowires of significant interest for use as synaptic elements in neuromorphic network architectures.
|
2021 |
|
Microstructure evolution of gadolinium doped cerium oxide under large thermal gradients
H Maruyama, JC Nino
Ceramics International 47 (19), 27718-27729
The effects of large thermal gradient annealing on the microstructure of 10 mol% gadolinium doped ceria (GDC) were investigated. GDC powder was prepared by solvent deficient method and sintered at 1650 °C for 10 h to achieve dense ceramics with ~8 μm grain size. The densified GDC samples were subsequently annealed using a 60 W infrared laser at over 2100 °C for 1 h under a thermal gradient equivalent to ~0.3–0.5 °C/μm. The post-annealed samples at 2150 °C for 1 h exhibit grains with average length and width of 37 and 28 μm, respectively. Electron backscattered diffraction (EBSD) analysis revealed that the post-annealed sample at 2150 °C consists of grains oriented close to five principal directions (<4 3 10>, <0 0 1> and <13 1 14> on [0 0 1], and <7 6 20> and <7 2 7> on [0 1 0]) within a tolerance angle of ±10°, whereas the grains of the pre-annealed sample are randomly oriented.
|
2021 |
|
Effect of Reduced Atmosphere Sintering on Blocking Grain Boundaries in Rare-Earth Doped Ceria
S Sulekar, M Mehr, JH Kim, JC Nino
Inorganics 9 (8), 63
Rare-earth doped ceria materials are amongst the top choices for use in electrolytes and composite electrodes in intermediate temperature solid oxide fuel cells. Trivalent acceptor dopants such as gadolinium, which mediate the ionic conductivity in ceria by creating oxygen vacancies, have a tendency to segregate at grain boundaries and triple points. This leads to formation of ionically resistive blocking grain boundaries and necessitates high operating temperatures to overcome this barrier. In an effort to improve the grain boundary conductivity, we studied the effect of a modified sintering cycle, where 10 mol% gadolinia doped ceria was sintered under a reducing atmosphere and subsequently reoxidized. A detailed analysis of the complex impedance, conductivity, and activation energy values was performed. The analysis shows that for samples processed thus, the ionic conductivity improves when compared with conventionally processed samples sintered in air. Equivalent circuit fitting shows that this improvement in conductivity is mainly due to a drop in the grain boundary resistance. Based on comparison of activation energy values for the conventionally processed vs. reduced-reoxidized samples, this drop can be attributed to a diminished blocking effect of defect-associates at the grain boundaries.
|
2021 |
|
Trivalent Dopant Size Influences Electrostrictive Strain in Ceria Solid Solutions
M Varenik, JC Nino, E Wachtel, S Kim, SR Cohen, I Lubomirsky
ACS Applied Materials & Interfaces 13 (17), 20269-20276
The technologically important frequency range for the application of electrostrictors and piezoelectrics is tens of Hz to tens of kHz. Sm3+– and Gd3+-doped ceria ceramics, excellent intermediate-temperature ion conductors, have been shown to exhibit very large electrostriction below 1 Hz. Why this is so is still not understood. While optimal design of ceria-based devices requires an in-depth understanding of their mechanical and electromechanical properties, systematic investigation of the influence of dopant size on frequency response is lacking. In this report, the mechanical and electromechanical properties of dense ceria ceramics doped with trivalent lanthanides (RE0.1Ce0.9O1.95, RE = Lu, Yb, Er, Gd, Sm, and Nd) were investigated. Young’s, shear, and bulk moduli were obtained from ultrasound pulse echo measurements. Nanoindentation measurements revealed room-temperature creep in all samples as well as the dependence of Young’s modulus on the unloading rate. Both are evidence for viscoelastic behavior, in this case anelasticity. For all samples, within the frequency range f = 0.15–150 Hz and electric field E ≤ 0.7 MV/m, the longitudinal electrostriction strain coefficient (|M33|) was 102 to 104-fold larger than expected for classical (Newnham) electrostrictors. However, electrostrictive strain in Er-, Gd-, Sm-, and Nd-doped ceramics exhibited marked frequency relaxation, with the Debye-type characteristic relaxation time τ ≤ 1 s, while for the smallest dopants—Lu and Yb—little change in electrostrictive strain was detected over the complete frequency range studied. We find that only the small, less-studied dopants continue to produce useable electrostrictive strain at the higher frequencies. We suggest that this striking difference in frequency response may be explained by postulating that introduction of a dopant induces two types of polarizable elastic dipoles and that the dopant size determines which of the two will be dominant.
|
2021 |
|
EFFECT OF DEGREE OF SULFONATION IN NANOCELLULOSE/CHITOSAN COMPOSITE ON ADSORPTION OF CATIONIC DYE AS OPIOID SIMULANT
M Frye, SE Vasisth, A Atassi, D Mazyck, JC Nino
CELLULOSE CHEMISTRY AND TECHNOLOGY 55 (1-2), 87-99
In this study, the effect of nanocellulose sulfonate group content on adsorption of an opioid simulant was tested. The opioid simulant used was Victoria blue R, an amine dye. Nanocellulose filters were fabricated by crosslinking cellulose nanocrystals (CNCs) with chitosan to improve the mechanical stability of freeze-dried CNCs. Thermogravimetric analysis confirmed the filter’s thermal stability and operating temperatures. Conductometric titration, Fourier transform infrared spectroscopy, and scanning electron microscopy techniques were used to characterize the degree of nanocellulose functionalization. Lastly, the adsorption performance of the sulfonated nanocellulose filter was tested and fitted to kinetic models and adsorption isotherms. The adsorption of the dye by the sulfonated nanocellulose followed pseudo-second order kinetics and the Langmuir isotherm. The maximum adsorption of Victoria blue R dye by sulfonated nanocellulose (68.56 mg/g) is significantly higher than those of other adsorbents, like activated carbon (0.59-2.97 mg/g) and magnetic microparticles (40.98 mg/g). Thus, sulfonated cellulose nanocrystals are a promising material for the sequestration of opioids from water.
|
2021 |
|
22 K superconductivity in exposed to
GN Tam, H Maruyama, JC Nino, GR Stewart
Physical Review B 102 (13), 134507
Several previous reports on undoped AEFe2As2 (AE= alkaline earth metal) point to drops in resistivity indicative of possible superconductivity and some degree of Meissner effect in the magnetic susceptibility. Also, based on both resistivity and magnetic susceptibility measurements, controlled exposure to water vapor has been shown to induce superconductivity in AEFe2As2. In our study, BaFe2As2 single crystals grown using the self-flux method showed a full resistive drop around 22 K and magnetic shielding, when exposed to fluorine gas postgrowth. Our measurements indicate electron (donor) doping via atomic substitution of F for As is concurrent with the observed superconductivity, which sheds light on the likely effect of exposure to water vapor in previous work. Like doping experiments (such as substitution of Co for Fe, P for As, or K for Ba) in AEFe2As2 to date, the present work is consistent with suppression of the spin density wave transition coincident with the appearance of Tc. In addition to answering the puzzle of superconductivity in undoped or water vapor exposed AEFe2As2, our results also represent a fast (20 min exposure to 5% F in He) and reliable method suitable for inducing superconductivity in thin films of AEFe2As2. Some supportive work on BaFe2As2 exposed to Cl2 is also presented.
|
2020 |
|
Dopant Concentration Controls Quasi-Static Electrostrictive Strain Response of Ceria Ceramics
M Varenik, JC Nino, E Wachtel, S Kim, O Yeheskel, N Yavo, I Lubomirsky
ACS applied materials & interfaces 12 (35), 39381-39387
Electromechanically active ceramic materials, piezoelectrics and electrostrictors, provide the backbone of a variety of consumer technologies. Gd- and Sm-doped ceria are ion conducting ceramics, finding application in fuel cells, oxygen sensors, and, potentially, as memristor materials. While optimal design of ceria-based devices requires a thorough understanding of their mechanical and electromechanical properties, reports of systematic study of the effect of dopant concentration on the electromechanical behavior of ceria-based ceramics are lacking. Here we report the longitudinal electrostriction strain coefficient (M33) of dense RExCe1–xO2–x/2 (x ≤ 0.25) ceramic pellets, where RE = Gd or Sm, measured under ambient conditions as a function of dopant concentration within the frequency range f = 0.15–350 Hz and electric field amplitude E ≤ 0.5 MV/m. For >100 Hz, all ceramic pellets tested, independent of dopant concentration, exhibit longitudinal electrostriction strain coefficient with magnitude on the order of 10–18 m2/V2. The quasi-static (f < 1 Hz) electrostriction strain coefficient for undoped ceria is comparable in magnitude, while introducing 5 mol % Gd or 5 mol % Sm produces an increase in M33 by up to 2 orders of magnitude. For x ≤ 0.1 (Gd)–0.15 (Sm), the Debye-type relaxation time constant (τ) is in the range 60–300 ms. The inverse relationship between dopant concentration and quasi-static electrostrictive strain parallels the anelasticity and ionic conductivity of Gd- and Sm-doped ceria ceramics, indicating that electrostriction is partially governed by ordering of vacancies and changes in local symmetry. |
2020 |
|
Machine learning of octahedral tilting in oxide perovskites by symbolic classification with compressed sensing
SR Xie, P Kotlarz, RG Hennig, JC Nino
Computational Materials Science 180, 109690
The steady growth of online materials databases, coupled with efforts in materials informatics, has invited the reexamination of existing empirical models through the lens of modern machine learning techniques. Inspired by recent efforts to improve on the Goldschmidt tolerance factor for perovskite formation, we apply the symbolic regression to the problem of predicting octahedral tilting. In addition to its impact on the crystal structure, octahedral tilting is related to functional properties, including dielectric permittivity, ferroelectricity, magnetic properties, and metal–insulator transitions. By relating a selection of physical parameters (e.g., atomic radii, electronegativity) with mathematical operations (e.g., addition, exponentiation), we identify an analytical equation that correctly predicts the octahedral tilting classification for 49 perovskite oxides in a dataset of 60 materials. Using the same training dataset, we additionally fit and compare seven models generated by other common machine learning methods. Despite the increased complexity afforded by support vector machines, decision trees/random forests, and artificial neural networks, we find that our equation outperforms the other models as well as the original tolerance factor in predicting octahedral tilting.
|
2020 |
|
Resistive switching in atomic layer deposited HfO2/ZrO2 nanolayer stacks
L Tang, H Maruyama, T Han, JC Nino, Y Chen, D Zhang
Applied Surface Science 515, 146015
The resistive switching properties of HfO2/ZrO2 nanolayers with the total thickness of 16 nm prepared using atomic layer deposition (ALD) were investigated. Current-voltage behavior, pulse time mode measurement, retention and endurance tests were carried out to characterize the memristive (memory-resistive) properties. Resistive switching was observed in all nanolayer stacks, and the set voltage (Vset) decreased with increasing the number of layers (i.e., increasing number of hafnia-zirconia interfaces). Grazing incidence x-ray diffraction (GI-XRD) results demonstrate that the hafnia transforms from monoclinic to orthorhombic crystal structure during the post metallization annealing. Shifts in the binding energy of the x-ray photoelectron spectra (XPS) implies the existence of hafnia and zirconia suboxide (HfO2-δ and ZrO2-δ). Moreover, the blocking nature of the inserted oxide/oxide interfaces serves as a barrier to oxygen ion/vacancy migration. It is shown that memristive/insulating nanostructures like HfO2/ZrO2 can help modulate the resistive switching of memristor-based devices.
|
2020 |
|
Deep Learning in Memristive Nanowire Networks
JD Kendall, RD Pantone, JC Nino
arXiv preprint arXiv:2003.02642
Analog crossbar architectures for accelerating neural network training and inference have made tremendous progress over the past several years. These architectures are ideal for dense layers with fewer than roughly a thousand neurons. However, for large sparse layers, crossbar architectures are highly inefficient. A new hardware architecture, dubbed the MN3 (Memristive Nanowire Neural Network), was recently described as an efficient architecture for simulating very wide, sparse neural network layers, on the order of millions of neurons per layer. The MN3 utilizes a high-density memristive nanowire mesh to efficiently connect large numbers of silicon neurons with modifiable weights. Here, in order to explore the MN3’s ability to function as a deep neural network, we describe one algorithm for training deep MN3 models and benchmark simulations of the architecture on two deep learning tasks. We utilize a simple piecewise linear memristor model, since we seek to demonstrate that training is, in principle, possible for randomized nanowire architectures. In future work, we intend on utilizing more realistic memristor models, and we will adapt the presented algorithm appropriately. We show that the MN3 is capable of performing composition, gradient propagation, and weight updates, which together allow it to function as a deep neural network. We show that a simulated multilayer perceptron (MLP), built from MN3 networks, can obtain a 1.61% error rate on the popular MNIST dataset, comparable to equivalently sized software-based network. This work represents, to the authors’ knowledge, the first randomized nanowire architecture capable of reproducing the backpropagation algorithm. |
2020 |
|
Palm readings: Manicaria saccifera palm fibers are biocompatible textiles with low immunogenicity
BD James, WN Ruddick, SE Vasisth, K Dulany, S Sulekar, A Porras, …
Materials Science and Engineering: C 108, 110484
Plant-based fibers are a potential alternative to synthetic polymer fibers that can yield enhanced biocompatibility and mechanical properties matching those properties of tissue. Given the unique morphology of the bract of the Manicaria saccifera palm , being an interwoven meshwork of fibers, we believe that these fibers with this built-in structure could prove useful as a tissue engineering scaffold material. Thus, we first investigated the fiber’s in vitro biocompatibility and immunogenicity. We cultured NIH/3T3 mouse fibroblasts, human aortic smooth muscle cells, and human adipose-derived mesenchymal stem cells on the fiber mats, which all readily attached and over 21 days grew to engulf the fibers. Importantly, this was achieved without treating the plant tissue with extracellular matrix proteins or any adhesion ligands. In addition, we measured the gene expression and protein secretion of three target inflammatory cytokines (IL-1β, IL-8, and TNFα) from THP-1 human leukemia monocytes cultured in the presence of the biotextile as an in vitro immunological model. After 24 h of culture, gene expression and protein secretion were largely the same as the control, demonstrating the low immunogenicity of Manicaria saccifera fibers. We also measured the tensile mechanical properties of the fibers. Individual fibers after processing had a Young’s modulus of 9.51 ± 4.38 GPa and a tensile strength of 68.62 ± 27.93 MPa. We investigated the tensile mechanical properties of the fiber mats perpendicular to the fiber axis (transverse loading), which displayed upwards of 100% strain, but with a concession in strength compared to longitudinal loading. Collectively, our in vitro assessments point toward Manicaria saccifera as a highly biocompatible biotextile, with a range of potential clinical and engineering applications.
|
2020 |
|
Solvent‐deficient method lowers grain‐boundary resistivity of doped ceria
H Maruyama, S Zeljković, JC Nino
Journal of the American Ceramic Society 103 (2), 819-830
Nanoparticles of gadolinium-doped cerium oxide (GDC) were synthesized using solvent-deficient method and their sinterability and electrical properties were investigated using the powder and cold sintering process. The GDC powder was uniaxially pressed into cylindrically-shaped pellets with a mixture of nitric acid and hydrogen peroxide at 200°C to encourage particle arrangement during forming process. These bulk samples were annealed using two different temperature profiles: at 800°C for 5 hours and at 1300°C for 1 minute—800°C for 5 hours. The samples produced using HNO3/H2O2 mixture showed higher relative density than ones without it. Ionic conductivity of the sample sintered through the two-step profile was obtained from electrochemical impedance spectroscopy. Although the grain conductivity for the samples (8.0 × 10−3 S cm−1 at 500°C, and 3.3 × 10−2 S cm−1 at 700°C) is on par with a conventionally sintered sample, the measured total conductivity (3.9 × 10−3 S cm−1 at 500°C, and 2.5 × 10−2 S cm−1 at 700°C) is about 10 times higher than the conventionally sintered one and is comparable to the values seen in the previous studies for GDC which employed higher sintering temperature, pointing to the effectively lower grain-boundary impedance. This result could be attributed to no significant phase segregation along grain boundaries due to the low-temperature processing.
|
2020 |
|
Simplified sol-gel processing method for amorphous TiO x Memristors
EN Moreira, J Kendall, H Maruyama, JC Nino
Journal of Electroceramics, 1-7
The memristor, a two-terminal memory device with units of resistance, has continued to gain momentum as simpler and more versatile memristive devices are discovered. Amorphous metal-oxide devices have emerged as potential replacements for organic and silicon materials in thin-film electronics. This work presents memristive devices based on amorphous TiOx which were synthesized using a simplified sol-gel process that does not require a dry nitrogen flow step to fabricate amorphous films of titanium oxide (TiOx) for memristive devices. This simplified process significantly decreases the cost and complexity of the fabrication of memristive devices. The memristive behavior was characterized by I-V curves and read-write sequential pulses. We report on the effects of different TiOx layers on I-V curve behavior, stability, aging of the devices as well as the influence of interfaces and electrode materials in the memristive properties. Devices made as a stack of copper electrode, different TiOx layers and aluminum electrode showed best results for on/off ratio than other devices in this work, as well better stability of resistive switching properties. |
2020 |
|
Structural, magnetic and optical properties of BiFeO3 synthesized by the solvent-deficient method
S Zeljković, T Ivas, H Maruyama, JC Nino
Ceramics International 45 (16), 19793-19798
Bismuth ferrite (BiFeO 3, BFO) powders were synthesized by a simple and cost-effective solvent-deficient method using bismuth nitrate, iron nitrate, and ammonium bicarbonate as the only precursors. Single phase BiFeO 3 powder was fabricated after the Bi:Fe ratio was adjusted and after the precursor mixture was calcined for one hour at 600 °C. We investigated the formation reactions, crystal structure, particle size distribution, magnetic and optical properties of synthesized BiFeO 3. X-ray diffraction revealed the formation of well-crystallized BFO nanocrystallites starting at a temperature of 450 °C. BiFeO 3 powder calcined at 600 °C showed very weak ferromagnetism at room temperature which is different from the linear M– H relationship in bulk BiFeO 3 ceramics. The remnant magnetization value (Mr) was found to be 5 × 10 −3 emu g −1 and a coercive field value (Hc) nearly 500 Oe. The UV–visible spectra showed maximum adsorption at ∼464 nm with a derived bandgap value of 1.85 (1.8449 ± 0.0013) eV for BFO nanocrystallites supporting their potential application as visible light-response photocatalysts. Direct bandgap value obtained from reflectance measurement is determined to be 2.25 (2.2464 ± 0.0065) eV. |
2019 |
|
Effect of Pt3Pb on the permittivity and conductivity of lead zirconate titanate thin films
H Maruyama, G Baure, T Jones, J Nikkel, MM Moharam, V Craciun, …
Thin Solid Films 685, 420-427
The evolution and interaction of buried interfaces during the synthesis of lead zirconate titanate (PZT) ferroelectric thin film devices have been previously predicted by density functional theory modeling and observed experimentally via in–situ structural characterization. Moreover, the formation and disappearance of an intermetallic phase (Pt 3Pb) during the crystallization of the film has been identified as a key process expected to affect the resulting electrical properties of the devices. To elucidate this effect, a combination of direct current (leakage current measurements) and alternating current (impedance spectroscopy) characterization techniques are used to examine the electrical properties of sol–gel–derived PZT thin films. Films with the intermetallic phase exhibit a high potential barrier at the metal–insulator interface (0.83 eV) similar to the fully crystallized films (0.81 eV) and a lower current density than the films without the intermetallic phase, as shown in the Richardson plot. Impedance measurements also revealed that the conductivity of the films with the intermetallic phase, σ = 1.9 × 10 −11 S cm −1, and the real part of the relative permittivity at 10 kHz, ε r′=42, are lower than the other films (fully crystallized film: σ = 3.5 × 10 −10 S cm −1 ε r′=566, amorphous crystallized film: σ = 4.6 × 10 −11 S cm −1 ε r′=121). It is shown that these electrical characterization methods can serve as non–destructive examination tools to track the evolution of secondary phases during device synthesis and fabrication. |
2019 |
|
Room temperature semiconductor detectors for nuclear security
PM Johns, JC Nino
Journal of Applied Physics 126 (4), 040902
Preventing radioactive sources from being used for harmful purposes is a global challenge. A requirement for solving the challenge is developing radiation detectors that are efficient, sensitive, and practical. Room temperature semiconductor detectors (RTSDs) are an important class of gamma-ray sensors because they can generate high-resolution gamma-ray spectra at ambient operating temperatures. A number of diverse and stringent requirements must be met for semiconducting materials to serve as sensors in RTSD spectrometers, which limits the number of candidates of interest that receive attention and undergo focused research and development efforts. Despite this, the development of new compounds for sensors in RTSDs is a thriving research field, and a number of materials with stunning potential as RTSD materials have emerged within the last decade. In this perspective, the state of the art in RTSD materials is examined, and emerging semiconducting compounds are reviewed. The highly developed CdTe, CdZnTe, HgI2, and TlBr are first discussed to highlight the potential that can emerge from RTSD compounds in advanced stages of technological development. Thereafter, emerging compounds are reviewed by class from chalcogenides, iodides and chalcohalides, and organic-inorganic hybrid compounds. This work provides both a compilation of the physical and electronic properties of the emerging RTSD candidates and a perspective on the importance of material properties for the future of compounds that can transform the field of radiation detection science.
|
2019 |
|
Solvent-deficient synthesis of cerium oxide: Characterization and kinetics
S Zeljković, D Jelić, H Maruyama, JC Nino
Ceramics International 45 (8), 10063-10071
The reaction mechanism and kinetics of CeO2 synthesis using a solvent-deficient method are investigated by simultaneous thermogravimetric analysis (TGA)/differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The decomposition process of the cerium(III) nitrate hexahydrate and ammonium bicarbonate precursor mixture with four observed stages is monitored using TGA/DSC measurements in a nonisothermal regime with heating rates of 5, 10, 15 and 20 °C min−1. The proposed mechanism indicates a complex synthesis with several parallel reactions, some of which occur at room temperature. A detailed kinetic analysis is performed using isoconversional (expanded Friedman, modified Coats-Redfern and Kissinger) and model fitting (Nth order and nucleation and growth models) methods. The first three stages are best described by the Nth order model with activation energy values of 21, 53 and 90 kJ mol−1. The last stage, during which ammonium nitrate decomposition occurs, is best fit by the nucleation and growth model and has an activation energy of 129 kJ mol−1. The proposed mechanism, supported by the kinetic analysis in our study, indicates that CeO2 has already formed before the reaction reaches 200 °C. The average crystallite size of CeO2 synthesized at 300 °C, which was calculated from the XRD measurements and observed in the SEM and TEM data, is between 10 and 20 nm.
|
2019 |
|
Effect of a DC bias on the conductivity of gadolinia doped ceria thin films
SS Sulekar, JE Ordonez, IC Arango, ME Gomez, JC Nino
Electrochimica Acta 303, 275-283
This paper studies the phenomenon of mixed ionic and electronic conductivity in magnetron sputtered gadolinia doped ceria thin films under the effect of an applied DC bias. Electrochemical impedance spectroscopy was used to measure the change in impedance under an alternating voltage of 300 mV, at temperatures between 25 °C and 150 °C and applied biases of ∼4–20 kV/mm, which are much higher than any prior study. The application of a DC bias produces a reversible decrease in both the grain and grain boundary resistances for GDC, and the films exhibit bias-induced mixed ionic and electronic conductivity. Additional features become visible in the Nyquist plots, indicating possible novel mechanisms in response to bias. Particularly interesting is the appearance of inductive loops in the low frequency regime, at very high bias values, rarely seen for such materials. Here this novel behavior was analyzed by fitting the data using equivalent circuits to understand the underlying mechanisms at play. Through this work, it is established that the change in behavior is attributed to electronic conduction through grain boundaries along the direction of the applied field.
|
2019 |
|
Oxygen vacancy ordering and viscoelastic mechanical properties of doped ceria ceramics
M Varenik, S Cohen, E Wachtel, AI Frenkel, JC Nino, I Lubomirsky
Scripta Materialia 163, 19-23
Young’s, shear and bulk moduli of Ce1-xSmxO2-x/2 (x ≤ 0.55) were studied using ultrasonic time of flight and nanoindentation techniques. Sound velocity measurements, corrected for sample porosity, demonstrate decrease in the unrelaxed ceramic moduli with increasing Sm-content. Room temperature creep under indenter load-hold, as well as time-dependent material stiffness, reveal a transition from prominent anelasticity in the fluorite phase to prominent elasticity in the double fluorite phase. This supports rearrangement of elastic dipoles under anisotropic stress, which occurs more readily when oxygen vacancies are not ordered on the crystal lattice,
as the source of ceria anelastic behavior.
|
2019 |
|
Resistive switching in multiferroic BiFeO3 films: Ferroelectricity versus vacancy migration
AC Rodríguez, IC Arango, MF Gomez, C Dominguez, J Trastoy, C Urban, …
Solid State Communications 288, 38-42
We studied the voltage-induced resistive switching (RS) in ferroelectric/metal (BiFeO3/Nb:SrTiO3) vertical devices. We found switching with RON and ROFF ratios of ΔR = 1-RON/ROFF = 0.82 at voltages starting at VSET, RESET = ±2 V. Upon increasing voltage, ΔR also increases until dielectric breakdown is reached. Interestingly, the VSET, RESET values at which the RS becomes significant, coincides with the coercive voltage of the ferroelectric polarization, as measured by piezoelectric force microscopy in similar BiFeO3 films. This suggests that the driving mechanism of the RS effect in our films is connected to the BiFeO3 ferroelectricity. However, the increase of the RS effect after complete ferroelectric saturation points to an additional mechanism that may be related to vacancy displacements. This is further supported by forming process necessary to induce resistance bi-stability, typical of an RS effect. |
2019 |
|
Evaluation of the computational capabilities of a memristive random network (MN3) under the context of reservoir computing
LE Suarez, JD Kendall, JC Nino
Neural Networks 106, 223-236
This work presents the simulation results of a novel recurrent, memristive neuromorphic architecture, the MN3 and explores its computational capabilities in the performance of a temporal pattern recognition task by considering the principles of the reservoir computing approach. A simple methodology based on the definitions of ordered and chaotic dynamical systems was used to determine the separation and fading memory properties of the architecture. The results show the potential use of this architecture as a reservoir for the on-line processing of time-varying inputs.
|
2018 |
|
Memristive nanowires exhibit small-world connectivity
RD Pantone, JD Kendall, JC Nino
Neural Networks 106, 144-151
Small-world networks provide an excellent balance of efficiency and robustness that is not available with other network topologies. These characteristics are exhibited in the Memristive Nanowire Neural Network (MN 3), a novel neuromorphic hardware architecture. This architecture is composed of an electrode array connected by stochastically deposited core–shell nanowires. We simulate the stochastic behavior of the nanowires by making various assumptions on their paths. First, we assume that the nanowires follow straight paths. Next, we assume that they follow arc paths with varying radii. Last, we assume that they follow paths generated by pink noise. For each of the three methods, we present a method to find whether a nanowire passes over an electrode, allowing us to represent the architecture as a bipartite graph. We find that the small-worldness coefficient increases logarithmically and is consistently greater than one, which is indicative of a small-world network. |
2018 |
|
Comparison of the in-and across-plane ionic conductivity of highly oriented neodymium doped ceria thin films
G Baure, H Zhou, CC Chung, MN Buck, MA Stozhkova, JL Jones, JC Nino
Acta Materialia 147, 10-15
To determine the effect of grain boundaries and grain orientation on the electrical properties of solid oxide fuel cell electrolytes, a comparison of the in-plane and across-plane ionic conductivity of both strongly and poorly textured, columnar-grained doped ceria thin films was performed within equivalent temperature ranges (150–300 °C). Additionally, the in-plane conductivity of partially amorphous films, polycrystalline films with randomly oriented grains, and single crystal, epitaxial films with no grain boundaries was determined. Pulsed laser deposition permitted the growth of all these types of films and the ability to grow columnar-grained doped ceria on both conducting and insulating surfaces enabled testing of the films both in-plane and across-plane. Compared to the columnar-grained samples, partially amorphous films exhibited a lower conductivity, while epitaxial doped ceria exhibited an enhancement in conductivity of 2 orders of magnitude. Between 300 and 400 °C, the in-plane conductivity of the strongly textured film was higher than the poorly textured one. The conductivity and activation energy in-plane and across-plane for the strongly textured film was similar (2.75 × 10 −5 S/cm, 0.70 eV vs. 5.50 × 10 −5 S/cm, 0.68 eV at 250 °C). In contrast, for the poorly textured films, the in-plane and across-plane conductivity values differed by almost an order of magnitude (2.86 × 10 −5 S/cm, 0.55 eV vs. 1.99 × 10 −4 S/cm, 0.78 eV at 250 °C) suggesting that the boundaries between oriented grains were less resistive. These results further strengthen the argument that grain orientation affects ionic transport through grain boundaries.
|
2018 |
|
Unexpectedly high piezoelectricity of Sm-doped lead zirconate titanate in the Curie point region
SB Seshadri, MM Nolan, G Tutuncu, JS Forrester, E Sapper, G Esteves, …
Scientific reports 8 (1), 1-13
Large piezoelectric coefficients in polycrystalline lead zirconate titanate (PZT) are traditionally achieved through compositional design using a combination of chemical substitution with a donor dopant and adjustment of the zirconium to titanium compositional ratio to meet the morphotropic phase boundary (MPB). In this work, a different route to large piezoelectricity is demonstrated. Results reveal unexpectedly high piezoelectric coefficients at elevated temperatures and compositions far from the MPB. At temperatures near the Curie point, doping with 2 at% Sm results in exceptionally large piezoelectric coefficients of up to 915 pm/V. This value is approximately twice those of other donor dopants (e.g., 477 pm/V for Nb and 435 pm/V for La). Structural changes during the phase transitions of Sm-doped PZT show a pseudo-cubic phase forming ≈50 °C below the Curie temperature. Possible origins of these effects are discussed and the high piezoelectricity is posited to be due to extrinsic effects. The enhancement of the mechanism at elevated temperatures is attributed to the coexistence of tetragonal and pseudo-cubic phases, which enables strain accommodation during electromechanical deformation and interphase boundary motion. This work provides insight into possible routes for designing high performance piezoelectrics which are alternatives to traditional methods relying on MPB compositions.
|
2018 |
|
Thin film organic photodetectors for indirect X-ray detection demonstrating low dose rate sensitivity at low voltage operation
DJ Starkenburg, PM Johns, JE Baciak, JC Nino, J Xue
Journal of Applied Physics 122 (22), 225502
Developments in the field of organic semiconductors have generated organic photodetectors with high quantum efficiency, wide spectral sensitivity, low power consumption, and unique form factors that are flexible and conformable to their substrate shape. In this work, organic photodetectors coupled with inorganic CsI(Tl) scintillators are used to showcase the low dose rate sensitivity that is enabled when high performance organic photodetectors and scintillator crystals are integrated. The detection capability of these organic-inorganic coupled systems to high energy radiation highlights their potential as an alternative to traditional photomultiplier tubes for nuclear spectroscopy applications. When exposed to Bremsstrahlung radiation produced from an X-ray generator, SubPc:C60, AlPcCl:C70, and P3HT:PC61BM thin film photodetectors with active layer thicknesses less than 100 nm show detection of incident radiation at low and no applied bias. Remarkably low dose rates, down to at least 0.18 μGy/s, were detectable with a characteristic linear relationship between exposure rate and photodetector current output. These devices also demonstrate sensitivities as high as 5.37 mC Gy−1 cm−2 when coupled to CsI(Tl). Additionally, as the tube voltage across the X-ray generator was varied, these organic-inorganic systems showed their ability to detect a range of continuous radiation spectra spanning several hundred keV.
|
2017 |
|
Grain orientation effects on the ionic conductivity of neodymia doped ceria thin films
G Baure, H Zhou, CC Chung, MA Stozhkova, JL Jones, JC Nino
Acta Materialia 133, 81-89
It is generally accepted that grain boundaries in the path of transport are detrimental to ionic conductivity. To delve deeper into the connection between grain boundaries and ionic transport, the relative orientations of the grains were determined using the transmission Kikuchi diffraction technique. Nanocrystalline (grain size ∼ 40 nm) neodymia doped ceria thin films grown via pulsed laser deposition amplify the effect of these intrinsic interfaces. In addition, this deposition technique allowed the growth of partially amorphous and columnar grained films. Further, the strength of the texture in the columnar grained films was modified by changing substrates. The in-plane impedance measurements were able to isolate the response of the film from the response of the electrode interface and confirmed the majority carriers were oxygen vacancies at low temperatures. The anionic conductivity improved as the strength of the texture in the films increased. The conductivity of the strongly textured films was 2 orders of magnitude higher than the conductivity of the randomly oriented ones between 300 and 400 °C. Also, the in-plane conductivity per grain was more than 3 orders of magnitude higher in the strongly textured film than in the poorly textured one indicating conductivity is not dependent on grain boundary density. I  V measurements revealed that grain boundaries posed a potential barrier to anions in the poorly textured and randomly oriented films, but not in the strongly textured samples. The type of grain boundary was deemed a contributing factor. Boundaries between more misaligned grains were more resistive decreasing the total conductivity.
|
2017 |
|
Suppressed grain growth in highly porous barium titanate foams by two‐step sintering
M Mehr, J Pineiro, BD Santos, JC Nino
Journal of the American Ceramic Society 100 (2), 539-545
Porous barium titanate has gained significant attention in recent years for their potential use in applications such as scaffolds for bone tissue engineering, stress sensors, gas sensors, and many others. However, there is very little control over the grain size of the material during the sintering processes specially to achieve little or no growth of the starting powders. Here, using the two-step sintering method barium titanate foams were shown to be synthesized with controlled grain size of the struts without significant differences in the pore structure of the materials. In order to evaluate the applicability of two-step sintering for a variety of processing methods, highly porous (>80% porosity) foams synthesized through the direct polyurethane foaming method were used to create conditions furthest from bulk where two-step sintering has shown success. Two-step sintering parameters were identified and the processing conditions were confirmed to not alter the mechanical properties of the samples due to expected residual stresses or thermal shock resulting from the rapid heating and cooling rates employed.
|
2017 |
|
Effect of Microwave Processing on the Crystallization and Energy Density of BaO‐Na2O‐Nb2O5‐SiO2‐B2O3 Glass‐Ceramics
C Davis III, AL Pertuit, JC Nino
Journal of the American Ceramic Society 100 (1), 65-73
Barium sodium niobate (BNN) glass-ceramics were successfully synthesized through a controlled crystallization method, using both a conventional and a microwave hybrid heating process. The dielectric properties of glass-ceramics devitrified at different temperatures and conditions were measured. It was found that the dielectric constant increased with higher crystallization temperature, from 750°C to 1000°C, and that growth of the crystalline phase above 900°C was essential to enhancing the relative permittivity and overall energy storage properties of the material. The highest energy storage was found for materials crystallized conventionally at 1000°C with a discharge energy density of 0.13 J/cm3 at a maximum field of 100 kV/cm. Rapid microwave heating was found to not give significant enhancement in dielectric properties, and coarsening of the ferroelectric crystals was found to be critical for higher energy storage.
|
2017 |
|
Potentiostatic deposition of Cu2O films as p-type transparent conductors at room temperature
MM Moharam, EM Elsayed, JC Nino, RM Abou-Shahba, MM Rashad
Thin Solid Films 616, 760-766
Single phase Cu 2O films have been prepared via an electrodeposition technique onto ITO glass substrates at room temperature. Likewise, Cu 2O films were deposited using a potentiostatic process from an alkaline electrolyte containing copper (II) nitrate and 1 M sodium citrate. Single phase Cu 2O films were electrodeposited at a cathodic deposition potential of 500 mV for a reaction period of 90 min, and pH of 12 to yield a film thickness of 0.49 μm. The mechanism for nucleation of Cu 2O films was found to vary with deposition potential. Applying the Scharifker and Hills model at − 500 and − 600 mV to describe the mechanism of nucleation for the electrochemical reaction, the nucleation mechanism consisted of a mix between instantaneous and progressive growth mechanisms at − 500 mV, while above − 600 mV the growth mechanism was instantaneous. Using deposition times from 30 to 90 min at − 500 mV deposition potential, pure Cu 2O films with different microstructures were electrodeposited. Changing the deposition time from 30 to 90 min varied the microstructure from cubic to more complex polyhedra. The transmittance of electrodeposited Cu 2O films ranged from 20 to 70% in visible range, and samples exhibited a 2.4 eV band gap. The electrical resistivity for electrodeposited Cu 2O films was found to decrease with increasing deposition time from 0.854 × 10 5 Ω-cm at 30 min to 0.221 × 10 5 Ω-cm at 90 min without any thermal treatment following the electrodeposition process.
|
2016 |
|
Kinetic Analysis of Crystallization in Li1.3Al0.3Ti1.7(PO4)3 Glass Ceramics
C Davis III, AL Pertuit, JC Nino
Journal of the American Ceramic Society 99 (10), 3260-3266
The crystallization mechanisms for Li1.3Al0.3Ti1.7(PO4)3 (LATP) glass ceramics were studied using thermophysical property characterization techniques. Differential scanning calorimetry (DSC) revealed two separate exothermic events that were ascribed to the initial growth and growth to coherency of a dendritic phase. It was found that the commonly used Johnson‐Mehl‐Avrami is not a suitable kinetic model for this material. Rather, the Sestak‐Berggren (SB) autocatalytic kinetic model was used to analyze the DSC data and the activation energy for initial growth (259 kJ/mol) and coherency (272 kJ/mol) was calculated using isoconversional methods. The calculated parameters for the SB model were used to compare experimental and calculated values for heat flow during the crystallization of LATP and good fits were found for both exothermic events.
|
2016 |
|
Enhanced gamma ray sensitivity in bismuth triiodide sensors through volumetric defect control
PM Johns, JE Baciak, JC Nino
Applied Physics Letters 109 (9), 092105
Some of the more attractive semiconducting compounds for ambient temperature radiation detector applications are impacted by low charge collection efficiency due to the presence of point and volumetric defects. This has been particularly true in the case of BiI3, which features very attractive properties (density, atomic number, band gap, etc.) to serve as a gamma ray detector, but has yet to demonstrate its full potential. We show that by applying growth techniques tailored to reduce defects, the spectral performance of this promising semiconductor can be realized. Gamma ray spectra from >100 keV source emissions are now obtained from high quality Sb:BiI3 bulk crystals with limited concentrations of defects (point and extended). The spectra acquired in these high quality crystals feature photopeaks with resolution of 2.2% at 662 keV.
|
2016 |
|
Internal barrier layer capacitor, nearest neighbor hopping, and variable range hopping conduction in BaSrTiO nanoceramics.
S Sulekar, J Kim, H Han, P Dufour, C Tenailleau, J Nino, E Cordoncillo, …
Journal of Materials Science 51 (16)
The dielectric properties of the solid solution Ba1-xSrxTiO3-d (0 B x B 1) have been investigated. The nanopowders were prepared via a coprecipitation reaction followed by a calcination treatment. Spark plasma sintering allowed to obtain dense nanocrystalline ceramics. Broadband impedance spectroscopy revealed colossal permittivity (e0 = 105) associated with low losses (tan d = 0.03) in the most favorable case. The bulk conductivity data was analyzed using
Jonscher’s universal dielectric response model. In the Ba-rich compound, conduction process followed variable range hopping conduction model while the Sr-rich BST compound showed the nearest neighbor hopping conduction mechanism associated with displacements of space charges. These two different conduction mechanisms might be able to explain superior temperature–frequency-independent dielectric properties in Sr-rich BST compound compared to Ba-rich BST compound.
|
2016 |
|
Internal barrier layer capacitor, nearest neighbor hopping, and variable range hopping conduction in Ba 1− x Sr x TiO 3− δ nanoceramics
S Sulekar, JH Kim, H Han, P Dufour, C Tenailleau, JC Nino, E Cordoncillo, …
Journal of Materials Science 51 (16), 7440-7450
The dielectric properties of the solid solution Ba1−x Sr x TiO3−δ (0 ≤ x ≤ 1) have been investigated. The nanopowders were prepared via a coprecipitation reaction followed by a calcination treatment. Spark plasma sintering allowed to obtain dense nanocrystalline ceramics. Broadband impedance spectroscopy revealed colossal permittivity (ε′ = 105) associated with low losses (tan δ = 0.03) in the most favorable case. The bulk conductivity data was analyzed using Jonscher’s universal dielectric response model. In the Ba-rich compound, conduction process followed variable range hopping conduction model while the Sr-rich BST compound showed the nearest neighbor hopping conduction mechanism associated with displacements of space charges. These two different conduction mechanisms might be able to explain superior temperature–frequency-independent dielectric properties in Sr-rich BST compound compared to Ba-rich BST compound.
|
2016 |
|
Fabrication and testing of antimony doped bismuth tri-iodide semiconductor gamma-ray detectors
SS Gokhale, HS Han, O Pelaez, JE Baciak, JC Nino, KA Jordan
Radiation Measurements 91, 1-8
Antimony (Sb) doped bismuth tri-iodide (BiI 3) radiation detectors were fabricated from large single crystals that were grown using the modified vertical Bridgman technique. Detectors were prepared by subjecting the crystal surfaces to different mechanical and chemical treatments. Surface quality of the detectors was evaluated using optical microscopy. The influence of surface quality on detector performance was analyzed by measuring the leakage current for each of the detectors. The radiation response of the detectors was measured using an Americium (241 Am) gamma-ray source at room temperature. The first successful use of BiI 3 detectors for gamma-ray spectroscopy is reported here with energy resolution of 7.5% at 59.5 keV. The mobility-lifetime product for electrons was also estimated to be about 5.2× 10− 4 cm 2/V.
|
2016 |
|
Role of composition and structure on the properties of metal/multifunctional ceramic interfaces
FY Lin, A Chernatynskiy, JC Nino, JL Jones, R Hennig, SB Sinnott
Journal of Applied Physics 120 (4), 045310
The formation of intermetallic secondary phases, such as Pt3Pb, has been observed experimentally at PbTiO3/Pt and Pb(Zr,Ti)O3/Pt, or PZT/Pt, interfaces. Density functional theory calculations are used here to calculate the work of adhesion of these interfacial systems with and without the secondary intermetallic phase. The charge density maps of the interfaces reveal the electronic interactions at the interface and the impact of the secondary phase. In addition, Bader charge analysis provides a quantitative assessment of electron transfer from the perovskites to the Pt. Analysis of the band diagrams indicates an increase of the potential barrier associated with electron transfer due to the formation of the Pt3Pb at PZT/Pt interfaces.
|
2016 |
|
Hydrothermal crystal growth, piezoelectricity, and triboluminescence of KNaNbOF5
KB Chang, BW Edwards, L Frazer, EJ Lenferink, TK Stanev, NP Stern, …
Journal of Solid State Chemistry 236, 78-82
Single crystals of the noncentrosymmetric KNaNbOF5 polymorph were grown for piezoelectric and triboluminescent measurements. Piezoelectric measurements yielded a d33 value of ±6.3 pCN−1 and an effective electromechanical coupling coefficient of up to 0.1565 in the frequency range 1960–2080 kHz. Crystals of KNaNbOF5 were found to exhibit a strong triboluminscence effect visible to the naked eye as blue sparks when crystals are crushed. This triboluminescence effect is uncommon in that it is likely independent from both the piezoelectric effect and atmospheric electrical discharge. Instead, triboluminescence may originate from crystal defects or be related to an electroluminescence effect. |
2016 |
|
Colossal permittivity and low losses in Ba1–xSrxTiO3–δ reduced nanoceramics
S Dupuis, S Sulekar, JH Kim, H Han, P Dufour, C Tenailleau, JC Nino, …
Journal of the European Ceramic Society 36 (3), 567-575
The oxalate route offers a controlled approach to synthesize pure Ba 1–xSr xTiO 3 (BST) (0 ≤ x ≤ 1) nanoparticles ( ϕ ≈ 150 nm in diameter). Reduced BST dense nanoceramics were obtained by spark plasma sintering (SPS) and then annealed for a short time to reach colossal permittivity ( <span id=”MathJax-Element-1-Frame” class=”MathJax_SVG” style=”margin: 0px;padding: 0px;font-style: normal;font-weight: normal;line-height: normal;font-size: 16.2px;text-indent: 0px;text-align: left;text-transform: none;letter-spacing: normal;float: none;direction: ltr;max-width: none;max-height: none;min-width: 0px;min-height: 0px;border: 0px” role=”presentation” data-mathml=”“> = 10 5) with low dielectric losses (tan δ = 0.03) at 1 kHz and 300 K. The effects of Ba–Sr substitution on structural, microstructural and electrical properties were analyzed. Comprehensive analysis of the electrical properties indicates that polaron hopping, mediated by Ti 3+ ions and oxygen vacancies is the main contributing mechanism to colossal permittivity in Ba-rich BST compounds. Substitution of Ba by Sr reduced the contribution of polaron hopping and led to a decrease of real and imaginary parts of permittivity, while preserving interfacial polarization and yielding better temperature stability. The lowest temperature coefficient of capacitance, or TCC (variation of capacitance between 310 K and 450 K) value, i.e., 44 ppm K −1, is obtained for SrTiO 3.
|
2016 |
|
Three-dimensional quantification of composition and electrostatic potential at individual grain boundaries in doped ceria
DR Diercks, J Tong, H Zhu, R Kee, G Baure, JC Nino, R O’Hayre, …
Journal of Materials Chemistry A 4 (14), 5167-5175
Despite typically comprising a small volume fraction, grain boundaries often limit many properties of ceramics for energy technologies. The localized charges present at grain boundaries affect local structure and composition which thus impact the ionic conductivities along with thermal, electrical, optical, magnetic, and mechanical properties. While theory regarding grain boundary effects has progressed, direct knowledge of the local chemistry and corresponding potential around individual boundaries remains elusive. Some model bicrystal systems have been well-characterized experimentally; however, the complexities of random grain boundary structures in bulk-prepared polycrystalline ceramics have prevented quantified analysis of the most commonly occurring types. Here, the three-dimensional quantification of oxygen and cation compositions around arbitrarily selected high-angle grain boundaries in a polycrystalline material as measured by atom probe tomography are used to extract nm-scale, quantitative values of the three-dimensional space-charge potentials around grain boundaries and are related to the observed macro-scale conductivities. We focus specifically on Nd-doped ceria, a well-known ion conducting oxide with significant energy applications. However, the techniques employed here are directly applicable to other technologically-relevant polycrystalline ceramics and create opportunities for correlating nano-scale composition with macro-scale properties for optimizing materials design, expanding progress in ionic chemistry theory, and refining simulations for “real-world” polycrystalline materials. |
2016 |
|
Superheating suppresses structural disorder in layered BiI3 semiconductors grown by the Bridgman method
PM Johns, S Sulekar, S Yeo, JE Baciak, M Bliss, JC Nino
Journal of Crystal Growth 433, 153-159
The susceptibility of layered structures to stacking faults is a problem in some of the more attractive semiconductor materials for ambient-temperature radiation detectors. In this work, Bridgman-grown BiI 3 layered single crystals are investigated to understand and eliminate structural disorder, which reduces radiation detector performance. The use of superheating gradients has been shown by others to improve crystal quality in non-layered semiconductor crystals (Rudolph et al., 1996) [26]; thus the technique was explored to improve the growth of BiI 3. When investigating the homogeneity of non-superheated crystals, highly geometric void defects were found to populate the bulk of the crystals. Applying a superheating gradient to the melt prior to crystal growth improved structural quality and decreased defect density from the order of 4600 voids per cm 3 to 300 voids per cm 3. Corresponding moderate improvements to electronic properties also resulted from the superheat gradient method of crystal growth. Comparative measurements through infrared microscopy, etch-pit density, X-ray rocking curves, and sheet resistivity readings show that superheat gradients in BiI 3 growth led to higher quality crystals. |
2016 |
|
Diffusion Across M/Pb(Zr,Ti)O3 Interfaces (M=Pt3Pb or Pt) Under Different System Conditions
FY Lin, A Chernatynskiy, J Nikkel, R Bulanadi, JL Jones, JC Nino, …
Journal of the American Ceramic Society 99 (1), 356-362
Interfaces between functional ceramics, such as Pb(Zr0.5Ti0.5)O3 or PZT, and metal electrodes, such as Pt, are important for many devices. Maintaining an interface that is free of secondary phases is necessary for the efficient transfer of electrons and device function. However, there are instances where unstable transient phases form at the interface due to atomic diffusion, such as Pt3Pb. Here, we investigate the migration barriers for the diffusion of Pb across the PZT/Pt and PZT/Pt3Pb interfaces using density functional theory (DFT) and the climbing image nudge elastic band (c-NEB) method. Our calculation models take into account the influence of atmospheric conditions on Pb diffusion through the preferential stabilization of defects near the interface as a result of changes to the Pb and O chemical potentials. In addition, the PZT structures that are stable above and below the Curie temperature are considered. The migration barriers are predicted to be strongly dependent on atmospheric conditions and the phase of the PZT, tetragonal or cubic. In particular, an inversion of the Pb diffusion direction at the PZT/Pt interface is predicted to take place as the oxygen partial pressure increases. This prediction is confirmed by experimental in situ X-ray diffraction measurements of a PZT/Pt interface. |
2016 |
|
Epoxy interface method enables enhanced compressive testing of highly porous and brittle materials
M Mehr, C Davis, K Sadman, RJ Hooper, MV Manuel, JC Nino
Ceramics International 42 (1), 1150-1159
The compressive mechanical characterization of highly porous ceramics is problematic due to sensitivity to stress concentrations and localized fractures. In this paper a review of the methods used by the scientific community is done and their typical results, advantages, and shortcomings are discussed. Here, a new methodology that can address some of the problems associated with the current measurement procedures is presented. The proposed method is applied to highly porous barium titanate foams. This procedure produces more consistent data that can be analyzed and interpreted objectively. We show that properties such as the crushing or collapse stress as well as the compressive modulus cannot be extracted correctly by the conventional methods. By contrast, the proposed method collects data that is more accurate, consistent with what is expected of porous ceramics and that yields smaller measurement to measurement variation.
|
2016 |
|
Thermal properties of novel binary geopolymers based on metakaolin and alternative silica sources
MA Villaquirán-Caicedo, RM de Gutiérrez, S Sulekar, C Davis, JC Nino
Applied Clay Science 118, 276-282
The objective of this study was to evaluate the thermal properties of geopolymers based on metakaolin (MK) and alternative silica-based activators. These activators were produced by mixing rice husk ash (RHA) and silica fume (SF) with potassium hydroxide (KOH). In addition, boiler slag (BS) was used as partial substitute for the MK aluminosilicates precursor. The thermo-physical properties of the geopolymers prepared with alternative activators were compared to geopolymers activated with commercial potassium silicate (KS). In general, thermal conductivity, diffusivity and specific heat are related to porosity of the geopolymer. The values of thermal conductivity were between 0.170–0.353 W/m·K. The coefficient of thermal expansion varies between 1.06 and 1.13 × 10− 4 K− 1 and the thermal diffusivity was 0.16–0.27 mm2/s. The geopolymer with RHA (G-RHA) showed low bulk density (1338 kg/m3) and high porosity (35%). Additionally this material has the lowest thermal conductivity (0.17 W/m·K) and thermal diffusivity (0.16 mm2/s). The results obtained show that geopolymers here developed can be considered potentially useful for applications such as thermal insulation. In addition, they constitute a better ecological choice because of the type of source of alkaline activator making their production environmentally friendly.
|
2015 |
|
Investigation of Bismuth Triiodide (BiI3) for Photovoltaic Applications
RE Brandt, RC Kurchin, RLZ Hoye, JR Poindexter, MWB Wilson, …
The journal of physical chemistry letters 6 (21), 4297-4302
Guided by predictive discovery framework, we investigate bismuth triiodide (BiI3) as a candidate thin-film photovoltaic (PV) absorber. BiI3 was chosen for its optical properties and the potential for “defect-tolerant” charge transport properties, which we test experimentally by measuring optical absorption and recombination lifetimes. We synthesize phase-pure BiI3 thin films by physical vapor transport and solution processing and single-crystals by an electrodynamic gradient vertical Bridgman method. The bandgap of these materials is ∼1.8 eV, and they demonstrate room-temperature band-edge photoluminescence. We measure monoexponential recombination lifetimes in the range of 180–240 ps for thin films, and longer, multiexponential dynamics for single crystals, with time constants up to 1.3 to 1.5 ns. We discuss the outstanding challenges to developing BiI3 PVs, including mechanical and electrical properties, which can also inform future selection of candidate PV absorbers.
|
2015 |
|
Mechanical and thermal properties of low temperature sintered silicon carbide using a preceramic polymer as binder
M Mehr, DT Moore, JR Esquivel-Elizondo, JC Nino
Journal of materials science 50 (21), 7000-7009
Silicon carbide is used for a variety of applications, however, sintering still remains a challenge due to the high temperature and pressure required as well as the need for sintering aids. The use of preceramic polymers as binder is a promising technique for pressureless low-temperature sintering of SiC without sintering aids. However, the mechanical and thermophysical properties as well as the microstructure of bodies sintered through this technique has not been extensively documented. One of the main polymers which has gained attention in the past few years as a SiC preceramic is allylhydridopolycarbosilane (AHPCS). Here, using AHPCS as binder, silicon carbide pellets were sintered at temperatures as low as 930 °C, and the microstructural, mechanical, and thermophysical property characterization is presented. Compared to conventionally sintered SiC, the material shows similar fracture toughness, lower hardness, strength, and thermal conductivity. The observed properties are explained as a result of residual porosity combined with amorphous SiC at the grain boundaries.
|
2015 |
|
Sb-doped BiI3–Developing the Next Room Temperature Spectrometer
PM Johns, M Bliss, JC Nino
Transactions 113 (1), 498-501
|
2015 |
|
Microwave Processing for Improved Ionic Conductivity in Li2O–Al2O3–TiO2–P2O5 Glass‐Ceramics
C Davis III, JC Nino
Journal of the American Ceramic Society 98 (8), 2422-2427
Li1.4Al0.4Ti1.6(PO4)3 (LATP) was synthesized using a glass‐ceramics approach through crystallization in a conventional box furnace and a modified microwave furnace. The microstructure of samples that were microwave processed at 1000°C showed a larger average grain size (0.87 μm) when compared with the grain size of conventionally processed samples (0.30 μm) at the same temperature. Microwave processing led to significant enhancement of the conductivity when compared with conventional processing for all crystallization temperatures investigated. The highest total conductivity achieved was of glass microwave processed at 1000°C, with a conductivity of 5.33 × 10−4 S/cm. This conductivity was five times higher than that of LATP crystallized conventionally at the same temperature.
|
2015 |
|
Thin Films: Combined Experimental and Computational Methods Reveal the Evolution of Buried Interfaces during Synthesis of Ferroelectric Thin Films (Adv. Mater. Interfaces 10/2015)
JL Jones, JM LeBeau, J Nikkel, AA Oni, JH Dycus, C Cozzan, FY Lin, …
Advanced Materials Interfaces 2 (10)
Scanning transmission electron microscopy is utilized to reveal atomic-level interactions that occur during thermal processing at thin film oxide-metal interfaces, as described by JL Jones and co-workers in article 1500181. These observations, combined with in situ X-ray diffraction and theoretical techniques, are used to determine how solution-processed lead zirconate titanate (PZT) films nucleate on Pt electrodes, results that will guide future ferroelectric film processing.
|
2015 |
|
Combined experimental and computational methods reveal the evolution of buried interfaces during synthesis of ferroelectric thin films
JL Jones, JM LeBeau, J Nikkel, AA Oni, JH Dycus, C Cozzan, FY Lin, …
Advanced Materials Interfaces 2 (10), 1500181
Understanding interfaces between dissimilar materials is crucial to the development of modern technologies, for example, semiconductor–dielectric and thermoelectric–semiconductor interfaces in emerging electronic devices. However, the structural characterization of buried interfaces is challenging because many measurement techniques are surface sensitive by design. When interested in interface evolution during synthesis, the experimental challenges multiply and often necessitate in situ techniques. For solution-derived lead zirconate titanate (PZT) ferroelectric thin films, the evolution of buried interfaces during synthesis (including dielectric–metal and metal–metal) is thought to dramatically influence the resultant dielectric and ferroelectric properties. In the present work, multiple experimental and computational methods are combined to characterize interface evolution during synthesis of ferroelectric PZT films on platinized Si wafers—including in situ X-ray diffraction during thermal treatment, aberration-corrected scanning transmission electron microscopy of samples quenched from various synthesis states, and calculations using density functional theory. Substantial interactions at buried interfaces in the PZT/Pt/Ti/SiO x /Si heterostructure are observed and discussed relative to their role(s) in the synthesis process. The results prove that perovskite PZT nucleates directly from the platinum (111)-oriented bottom electrode and reveal the roles of Pb and O diffusion and intermetallic Pt3Pb and Pt3Ti phases.
|
2015 |
|
Prediction and characterization of heat-affected zone formation in tin-bismuth alloys due to nickel-aluminum multilayer foil reaction
RJ Hooper, CG Davis, PM Johns, DP Adams, D Hirschfeld, JC Nino, …
Journal of Applied Physics 117 (24), 245104
Reactive multilayer foils have the potential to be used as local high intensity heat sources for a variety of applications. Most of the past research effort concerning these materials have focused on understanding the structure-property relationships of the foils that govern the energy released during a reaction. To improve the ability of researchers to more rapidly develop technologies based on reactive multilayer foils, a deeper and more predictive understanding of the relationship between the heat released from the foil and microstructural evolution in the neighboring materials is needed. This work describes the development of a numerical model for the purpose of predicting heat affected zone size in substrate materials. The model is experimentally validated using a commercially available Ni-Al multilayer foils and alloys from the Sn-Bi binary system. To accomplish this, phenomenological models for predicting the variation of physical properties (i.e., thermal conductivity, density, and heat capacity) with temperature and composition in the Sn-Bi system were utilized using literature data.
|
2015 |
|
Growth, fabrication, and testing of bismuth tri-iodide semiconductor radiation detectors
SS Gokhale, H Han, JE Baciak, JC Nino, KA Jordan
Radiation Measurements 74, 47-52
Bismuth tri-iodide (BiI 3) is an attractive material for high energy resolution radiation detectors. For the purpose of this research, detectors were fabricated using single crystals grown from ultra-pure BiI 3 powder; synthesized by the Physical Vapor Transport (PVT) technique. This technique yielded powder with total impurity level of 7.9 ppm. Efforts were also made to purify commercial BiI 3 powder using a custom-built Traveling Zone Refining (TZR) system. Initial trial runs were successful in reducing the total impurity level of the commercial powder from 200 ppm to less than 50 ppm. Using the modified vertical Bridgman technique and a customized sharp tip ampoule, a large BiI 3 single crystal was grown. The crystal had a surface area of 2.2 cm 2 and a thickness of 0.8 cm, which corresponds to a volume of 1.78 cm 3. Radiation detectors were fabricated and then tested by measuring their electrical characteristics and radiation response. An alpha particle spectrum (using a 241Am α-source) was recorded at room temperature with a BiI 3 detector 0.09 cm thick and with a surface area of 0.16 cm 2. The electron mobility was estimated to be 433 ± 79 cm 2/V.
|
2015 |
|
High‐efficiency solution‐processed planar perovskite solar cells with a polymer hole transport layer
D Zhao, M Sexton, HY Park, G Baure, JC Nino, F So
Advanced Energy Materials 5 (6), 1401855
A high-efficiency solution-processed inverted perovskite solar cell with poly [N, N′-bis (4-butylphenyl)-N, N′-bis (phenyl) benzidine](poly-TPD) as the hole transport layer is demonstrated. The perovskite forms large crystallites on poly-TPD, yielding devices with an average power conversion efficiency of 13.8% and a maximum of 15.3%.
|
2015 |
|
Across plane ionic conductivity of highly oriented neodymium doped ceria thin films
G Baure, RM Kasse, NG Rudawski, JC Nino
Physical Chemistry Chemical Physics 17 (18), 12259-12264
A methodology to limit interfacial effects in thin films is proposed and explained. The strategy is to reduce the impact of the electrode interfaces and eliminate cross grain boundaries that impede ionic motion. To this end, highly oriented Nd0.1Ce0.9O2−δ (NDC) nanocrystalline thin films were grown using pulsed laser deposition (PLD) on platinized single crystal a-plane sapphire substrates. High resolution cross-sectional transmission electron microscopy (HR-XTEM), scanning electron microscopy (SEM) and X-ray diffraction (XRD) verified the films were textured with columnar grains. The average widths of the columns were approximately 40 nm and not significantly changed by film thickness between 100 and 300 nm. HR-XTEM and XRD determined the {111} planes of NDC were grown preferentially on top of the {111} planes of platinum despite the large lattice mismatch between the two planes. From the XRD patterns, the out of plane strains on the platinum and NDC layers were less than 1%. This can be explained by the coincident site lattice (CSL) theory. Rotating the {111} ceria planes 19.11° with respect to the {111} platinum planes forms a Σ7 boundary where 1 in 7 cerium lattice sites are coincident with the platinum lattice sites. This orientation lowers interfacial energy promoting the preferential alignment of those two planes. The across plane ionic conductivity was measured at low temperatures (<350 °C) for the various film thicknesses. It is here shown that columnar grain growth of ceria can be induced on platinized substrates allowing pathways that are clear of blocking grain boundaries that cause conductivities to diminish as film thickness decreases.
|
2015 |
|
Biocompatibility evaluation of porous ceria foams for orthopedic tissue engineering
JP Ball, BA Mound, AG Monsalve, JC Nino, JB Allen
Journal of Biomedical Materials Research Part A 103 (1), 8-15
Ceria ceramics have the unique ability to protect cells from free radical-induced damage, making them materials of interest for biomedical applications. To expand upon the understanding of the potential of ceria as a biomaterial, porous ceria, fabricated via direct foaming, was investigated to assess its biocompatibility and its ability to scavenge free radicals. A mouse osteoblast (7F2) cell line was cultured with the ceria foams to determine the extent of the foams’ toxicity. Toxicity assessments indicate that mouse osteoblasts cultured directly on the ceria scaffold for 72 h did not show a significant (p > 0.05) increase in toxicity, but rather show comparable toxicity to cells cultured on porous 45S5 Bioglass®. The in vitro inflammatory response elicited from porous ceria foams was measured as a function of tumor necrosis factor alpha (TNF-α) secreted from a human monocytic leukemia cell line. Results indicate that the ceria foams do not cause a significant inflammatory response, eliciting a response of 27.1 ± 7.1 pg mL−1 of TNF-α compared to 36.3 ± 5.8 pg mL−1 from cells on Bioglass, and 20.1 ± 2.9 pg mL−1 from untreated cells. Finally, we report cellular toxicity in response to free radicals from tert-butyl hydroperoxide with and without foamed ceria. Our preliminary results show that the foamed ceria is able to decrease the toxic effect of induced oxidative stress. Collectively, this study demonstrates that foamed ceria scaffolds do not activate an inflammatory response, and show potential free radical scavenging ability, thus they have promise as an orthopedic biomaterial.
|
2015 |