Effect of equal channel angular pressing on grain refinement and texture evolution in a biomedical alloy Ti-13Nb-13Zr

Evolution of texture and concomitant grain refinement during Equal Channel Angular Pressing (ECAP) of Ti - 13Nb - 13Zr alloy has been presented. Sub-micron sized equiaxed grains with narrow grain size distribution could be achieved after eight pass at 873 K. A characteristic ECAP texture evolved in alpha phase till four passes while the evolution of characteristic ECAP texture in the beta phase could be observed only beyond the fourth pass. On increasing the deformation up to eight passes, the texture in alpha phase weakens while the beta phase shows an ideal ECAP texture. A weaker texture, low dislocation density and high crystallite size values in alpha phase suggest the occurrence of dynamic recrystallization. The absence of texture evolution in beta phase till four passes can be attributed to local lattice rotations. The characteristic ECAP texture in the eight pass deformed sample is attributed to delayed dynamic recrystallization in the beta phase. (C) 2013 Elsevier Inc. All rights reserved.

Deformation field heterogeneity in punch indentation

Plastic heterogeneity in indentation is fundamental for understanding mechanics of hardness testing and impression-based deformation processing methods. The heterogeneous deformation underlying plane-strain indentation was investigated in plastic loading of copper by a flat punch. Deformation parameters were measured, in situ, by tracking the motion of asperities in high-speed optical imaging. These measurements were coupled with multi-scale analyses of strength, microstructure and crystallographic texture in the vicinity of the indentation. Self-consistency is demonstrated in description of the deformation field using the in situ mechanics-based measurements and post-mortem materials characterization. Salient features of the punch indentation process elucidated include, among others, the presence of a dead-metal zone underneath the indenter, regions of intense strain rate (e. g. slip lines) and extent of the plastic flow field. Perhaps more intriguing are the transitions between shear-type and compression-type deformation modes over the indentation region that were quantified by the high-resolution crystallographic texture measurements. The evolution of the field concomitant to the progress of indentation is discussed and primary differences between the mechanics of indentation for a rigid perfectly plastic material and a strain-hardening material are described.

Orientation dependent mechanical properties of commercially pure (cp) titanium

The present investigation is an attempt at correlating the crystallographic orientation and mechanical properties of hexagonal commercially pure titanium (cp-titanium). Annealed cp-titanium sheets are subjected to tensile deformation along the rolling direction, along 45 degrees to the rolling direction and along 90 degrees to the rolling direction respectively. Crystallographic textures and mechanical properties of these cp-titanium samples are investigated in the present study. The hardness of different grains/orientations is estimated through nanoindentation, grain average misorientation, orientation estimated elastic stiffness and Taylor factor measurements. It is observed that the hardness of the grains close to basal orientation is higher compared to non-basal orientations. It is further observed that the estimated bulk mechanical properties of cp-titanium have a direct relationship with the volume fraction of basal grains/orientations. (C) 2014 Elsevier Inc All rights reserved.

Role of deformation temperature on the evolution and heterogeneity of texture during equal channel angular pressing of magnesium

Investigations on texture evolution and through-thickness texture heterogeneity during equal channel angular pressing (ECAP) of pure magnesium at 200 degrees C, 150 degrees C and room temperature (RT) was carried out by neutron, high energy synchrotron X-ray and electron back-scatter diffraction. Irrespective of the ECAP temperature, a distinctive basal (B) and pyramidal (C-2) II type of fibers forms. The texture differs in the bottom 1 mm portion, where the B-fiber is shifted similar to 55 degrees due to negative shear attributed to friction. (C) 2015 Elsevier Inc. All rights reserved.

Role of Si modification on the compressive flow behavior of Al-Si based alloy

The flow characteristics of a near-eutectic heat-treated Al-Si based cast alloy have been examined in compression at strain rates varying from 3 x 10(-4) to 10(2) s(-1) and at three different temperatures, i.e., room temperature (RT), 100 degrees C and 200 degrees C. The dependence of flow behavior on modification is examined by testing the alloy in both the unmodified and modified conditions. Modification has strong influence on strain rate sensitivity (SRS), strength and work hardening behavior of the alloy. The strength of the alloy is found to increase with increase in strain rate for both the conditions. The increase is more rapid above the strain rate of 10(-1) s(-1) for the unmodified alloy at all the temperatures. This rapid increase is observed at 1 s(-1) at RT and 100 degrees C, and at 10(-2) s(-1) at 200 degrees C for the modified alloy. The thermally dependent process of the Al matrix is rate controlling in the unmodified alloy. On the other hand, the thermally dependent process of both Al matrix and Si particles are rate controlling, which is responsible for the higher strain rate sensitivity (SRS) in the modified alloy. The unmodified alloy exhibits a larger work hardening rate than the modified alloy during the initial stages of straining due to fiber loading of unmodified Si particles. However, the hardening rate decreases sharply at higher strains for the unmodified alloy due to a higher rate of Si particle fracture. Thermal softening is observed for both alloys at 200 degrees C due to precipitate coarsening, which leads to a decrease in SRS at higher temperatures. Stress simulations by microstructure based finite element method support the experimentally observed particle and matrix fracture behavior. Negative SRS and serrated flow are observed at lower strain rate regime (3 x 10(-4) to 10(-2) s(-1)) at RT and 100 degrees C, in both alloys. The critical onset strain is found to be lower and the magnitude of serration is found to be higher for the modified alloy, which suggests that, in addition to dynamic strain aging, Si particle size and morphology also play a role in serrated flow. (C) 2015 Elsevier Inc All rights reserved.

Quantum Mechanics Studies of Fuel Cell Catalysts and Proton Conducting Ceramics with Validation by Experiment

We carried out quantum mechanics (QM) studies aimed at improving the performance of hydrogen fuel cells. This led to predictions of improved materials, some of which were subsequently validated with experiments by our collaborators.

In part I, the challenge was to find a replacement for the Pt cathode that would lead to improved performance for the Oxygen Reduction Reaction (ORR) while remaining stable under operational conditions and decreasing cost. Our design strategy was to find an alloy with composition Pt3M that would lead to surface segregation such that the top layer would be pure Pt, with the second and subsequent layers richer in M. Under operating conditions we expect the surface to have significant O and/or OH chemisorbed on the surface, and hence we searched for M that would remain segregated under these conditions. Using QM we examined surface segregation for 28 Pt₃M alloys, where M is a transition metal. We found that only Pt₃Os and Pt₃Ir showed significant surface segregation when O and OH are chemisorbed on the catalyst surfaces. This result indicates that Pt₃Os and Pt3Ir favor formation of a Pt-skin surface layer structure that would resist the acidic electrolyte corrosion during fuel cell operation environments. We chose to focus on Os because the phase diagram for Pt-Ir indicated that Pt-Ir could not form a homogeneous alloy at lower temperature. To determine the performance for ORR, we used QM to examine all intermediates, reaction pathways, and reaction barriers involved in the processes for which protons from the anode reactions react with O₂ to form H₂O. These QM calculations used our Poisson-Boltzmann implicit solvation model include the effects of the solvent (water with dielectric constant 78 with pH 7 at 298K). We found that the rate determination step (RDS) was the O_ad hydration reaction (O_ad + H₂O_ad -> OH_ad + OH_ad) in both cases, but that the barrier for pure Pt of 0.50 eV is reduced to 0.48 eV for Pt₃Os, which at 80 degrees C would increase the rate by 218%. We collaborated with the Pu-Wei Wu’s group to carry out experiments, where we found that the dealloying process-treated Pt2Os catalyst showed two-fold higher activity at 25 degrees C than pure Pt and that the alloy had 272% improved stability, validating our theoretical predictions.

We also carried out similar QM studies followed by experimental validation for the Os/Pt core-shell catalyst fabricated by the underpotential deposition (UPD) method. The QM results indicated that the RDS for ORR is a compromise between the OOH formation step (0.37 eV for Pt, 0.23 eV for Pt_2ML/Os core-shell) and H₂O formation steps (0.32 eV for Pt, 0.22 eV for Pt_2ML/Os core-shell). We found that Pt_2ML/Os has the highest activity (compared to pure Pt and to the Pt₃Os alloy) because the 0.37 eV barrier decreases to 0.23 eV. To understand what aspects of the core shell structure lead to this improved performance, we considered the effect on ORR of compressing the alloy slab to the dimensions of pure Pt. However this had little effect, with the same RDS barrier 0.37 eV. This shows that the ligand effect (the electronic structure modification resulting from the Os substrate) plays a more important role than the strain effect, and is responsible for the improved activity of the core- shell catalyst. Experimental materials characterization proves the core-shell feature of our catalyst. The electrochemical experiment for Pt_2ML/Os/C showed 3.5 to 5 times better ORR activity at 0.9V (vs. NHE) in 0.1M HClO₄ solution at 25 degrees C as compared to those of commercially available Pt/C. The excellent correlation between experimental half potential and the OH binding energies and RDS barriers validate the feasibility of predicting catalyst activity using QM calculation and a simple Langmuir–Hinshelwood model.

In part II, we used QM calculations to study methane stream reforming on a Ni-alloy catalyst surfaces for solid oxide fuel cell (SOFC) application. SOFC has wide fuel adaptability but the coking and sulfur poisoning will reduce its stability. Experimental results suggested that the Ni4Fe alloy improves both its activity and stability compared to pure Ni. To understand the atomistic origin of this, we carried out QM calculations on surface segregation and found that the most stable configuration for Ni₄Fe has a Fe atom distribution of (0%, 50%, 25%, 25%, 0%) starting at the bottom layer. We calculated that the binding of C atoms on the Ni4Fe surface is 142.9 Kcal/mol, which is about 10 Kcal/mol weaker compared to the pure Ni surface. This weaker C binding energy is expected to make coke formation less favorable, explaining why Ni₄Fe has better coking resistance. This result confirms the experimental observation. The reaction energy barriers for CHx decomposition and C binding on various alloy surface, Ni₄X (X=Fe, Co, Mn, and Mo), showed Ni₄Fe, Ni₄Co, and Fe₄Mn all have better coking resistance than pure Ni, but that only Ni₄Fe and Fe₄Mn have (slightly) improved activity compared to pure Ni.

In part III, we used QM to examine the proton transport in doped perovskite-ceramics. Here we used a 2x2x2 supercell of perovskite with composition Ba₈X₇M₁(OH)₁O₂₃ where X=Ce or Zr and M=Y, Gd, or Dy. Thus in each case a 4⁺ X is replace by a 3⁺ M plus a proton on one O. Here we predicted the barriers for proton diffusion allowing both includes intra-octahedron and inter-octahedra proton transfer. Without any restriction, we only observed the inter-octahedra proton transfer with similar energy barrier as previous computational work but 0.2 eV higher than experimental result for Y doped zirconate. For one restriction in our calculations is that the O_donor-O_acceptor atoms were kept at fixed distances, we found that the barrier difference between cerates/zirconates with various dopants are only 0.02~0.03 eV. To fully address performance one would need to examine proton transfer at grain boundaries, which will require larger scale ReaxFF reactive dynamics for systems with millions of atoms. The QM calculations used here will be used to train the ReaxFF force field.

Paths Towards High Efficiency Silicon Photovoltaics

While photovoltaics hold much promise as a sustainable electricity source, continued cost reduction is necessary to continue the current growth in deployment. A promising path to continuing to reduce total system cost is by increasing device efficiency. This thesis explores several silicon-based photovoltaic technologies with the potential to reach high power conversion efficiencies. Silicon microwire arrays, formed by joining millions of micron diameter wires together, were developed as a low cost, low efficiency solar technology. The feasibility of transitioning this to a high efficiency technology was explored. In order to achieve high efficiency, high quality silicon material must be used. Lifetimes and diffusion lengths in these wires were measured and the action of various surface passivation treatments studied. While long lifetimes were not achieved, strong inversion at the silicon / hydrofluoric acid interface was measured, which is important for understanding a common measurement used in solar materials characterization.

Cryogenic deep reactive ion etching was then explored as a method for fabricating high quality wires and improved lifetimes were measured. As another way to reach high efficiency, growth of silicon-germanium alloy wires was explored as a substrate for a III-V on Si tandem device. Patterned arrays of wires with up to 12% germanium incorporation were grown. This alloy is more closely lattice matched to GaP than silicon and allows for improvements in III-V integration on silicon.

Heterojunctions of silicon are another promising path towards achieving high efficiency devices. The GaP/Si heterointerface and properties of GaP grown on silicon were studied. Additionally, a substrate removal process was developed which allows the formation of high quality free standing GaP films and has wide applications in the field of optics.

Finally, the effect of defects at the interface of the amorphous silicon heterojuction cell was studied. Excellent voltages, and thus efficiencies, are achievable with this system, but the voltage is very sensitive to growth conditions. We directly measured lateral transport lengths at the heterointerface on the order of tens to hundreds of microns, which allows carriers to travel towards any defects that are present and recombine. This measurement adds to the understanding of these types of high efficiency devices and may aid in future device design.

Desenvolvimento de um sistema de detecção fotoacústico utilizando dois microfones: aplicações em medidas de difusividade térmica

Neste trabalho, desenvolveu-se um sistema de detecção fotoacústico para medidas simultâneas e independentes dos sinais fotoacústicos dianteiro e traseiro, utilizando dois microfones e um único feixe de excitação. Utiliza-se a diferença de fase entre estes sinais para a determinação da difusividade térmica de materiais, com base na abordagem teórica da técnica da Diferença de Fase dos Dois Feixes (T2F). Na metodologia apresentada não há a necessidade de se alternar o feixe de excitação entre as faces da amostra. Esta característica torna mais rápido o procedimento de medida e simplifica o monitoramento automatizado de processos dinâmicos que afetam a difusividade térmica do material, como a cura de resinas poliméricas. É apresentado o procedimento utilizado para determinar a diferença entre as fases intrínsecas dos microfones e o método empregado para compensar tal diferença e, assim, obter a defasagem entre os sinais fotoacústicos dianteiro e traseiro. O sistema de detecção desenvolvido é avaliado em medidas de difusividade térmica de amostras metálicas (aço inoxidável AISI 304 e aço SAE 1020) e poliméricas (polipropileno e polietileno de baixa densidade). Os resultados obtidos concordam de forma satisfatória com dados disponíveis na literatura. Finalmente, a aplicação do sistema proposto ao monitoramento de cura de amostras de resina epóxi indicou sua potencialidade de acompanhar, em tempo real, este tipo de processo dinâmico.

Suppression of indium droplet formation by adding CCl4 during metalorganic chemical vapor deposition growth of InN films

In this work, the influences of CCl4 on the metalorganic chemical vapor deposition (MOCVD) growth of InN were studied for the first time. It was found that the addition of CCl4 can effectively suppress the formation of metal indium (In) droplets during InN growth, which was ascribed to the etching effect of Cl to In. However, with increasing of CCl4 flow, the InN growth rate decreased but the lateral growth of InN islands was enhanced. This provides a possibility of promoting islands coalescence toward a smooth surface of the InN film by MOCVD. The influence of addition of CCl4 on the electrical properties was also investigated.

有机／无机杂化材料的自组装表征及应用

杂化的纳米尺寸的有机／无机化合物开辟了材料科学的新纪元。作为新型功能材料，杂化的有机／无机杂化材料由于在光学、电子、机械、防护涂层、催化、传感器、生物等方面有潜在的应用前景而在材料科学领域引起广泛关注。杂多酸（POMs）作为一种重要的无机化合物，有特异的纳米尺寸结构，优越的物理化学性质和广泛应用价值，因此多酸化合物功能材料的研究引起人们足够的重视。最近，人们把更多的注意力集中到杂化的有机／无机杂化材料范围内，如何更有序的组装杂多酸，因为它们有着更有趣的电、磁、氧化还原和光学性质。1．在4-氨基苯甲酸修饰的玻碳电极表面，我们用层层组装方法制备了六钒取代的理杂多酸（PW6V6）多层膜。表面等离子（SP）技术和循环伏安（CV）法对多层膜进行了表征。结果表明，多层膜生长均匀，平均厚度为2.8nm，并且研究了多层膜对亚硝酸根伽02一）和浪酸根（BrO3-）的催化还原活性。2．研究了通过静电吸附作用，P2W17Fe和QPvP-Os交替组装多层膜。循环伏安（CV），电化学交流阻抗谱（EIS）和紫外一可见光谱（UV－vis）研究结果表明了这种多层膜是均一稳定的。多层膜对H2O2，BrO3-和NO2-有很高的催化活性。特别是EIS成功地监测多层膜的形成过程。随着层数增长，电子传输阻抗Rc，线性增加，氧化还原电对Fe（CN）63/4的电子传输受到抑制，这进一步证实了多层膜的均一性。3，首次在4-氨基苯甲酸修饰的玻碳电极表面成功地构建了含杂多酸P2W15V3的多层膜。值得注意的是，该多层膜对BrO3-和NO2-的还原有显著的电催化活性，而且由于过渡金属钒在P2W15V3起催化作用而有很低的过电位。另外，多层膜中QPVP-O5聚合物的Os中心在抗坏血酸氧化时的催化活性仍然存在，而且通过最外层的调整，多巴胺和抗坏血酸能被分别检测，所以在多巴胺存在时它可用作检测抗坏血酸的电化学传感器。4．迄今为止，人们把更多的注意力集中在杂多酸多层膜的制备，电化学和电催化活性的研究，对杂多酸多层膜功能材料的研究却很少，我们首次够构造了发光的杂多酸多层膜，紫外可见光谱，循环伏安，电化学阻抗技术和荧光光谱证明多层膜生长均匀，室温下，多层膜的发光性能显示Eu3+的特征发射峰是5D0→7Fj（j=1，2，3，4）的跃迁。电化学阻抗谱被成功得应用于监测多层膜的沉积过程。我们发现随多层膜的逐步形成，半圆直径有规律地长大。而且，电荷迁移电阻Rc，随双层数线性增长。可以认为多层膜在基底电极上以非常均匀规律的方式沉积，Fe（CN）63/4氧化还原电对的氧化还原反应逐步被多层膜抑制。5．我们用层层组装方法成功构造了金纳米粒子多层膜，紫外可见光谱，循环伏安和原子力显微技术表征了多层膜的形成，金纳米粒子粒径大约14nm，表面粗糙度6.43nm。

The Compact Pulsed Hadron Source: A Design Perspective

During the past. decades, large-scale national neutron sources have been developed in Asia, Europe, and North America. Complementing such efforts, compact hadron beam complexes and neutron sources intended to serve primarily universities and industrial institutes have been proposed, and some have recently been established. Responding to the demand in China for pulsed neutron/proton-beam platforms that are dedicated to fundamental and applied research for users in multiple disciplines from materials characterization to hadron therapy and radiography to accelerator-driven sub-critical reactor systems (ADS) for nuclear waste transmutation, we have initiated the construction of a compact, yet expandable, accelerator complex-the Compact Pulsed Hadron Source (CPHS). It consists of an accelerator front-end (a high-intensity ion source, a 3-MeV radio-frequency quadrupole linac (RFQ), and a 13-MeV drift-tube linac (DTL)), a neutron target station (a beryllium target with solid methane and room-temperature water moderators/reflector), and experimental stations for neutron imaging/radiography, small-angle scattering, and proton irradiation. In the future, the CPHS may also serve as an injector to a ring for proton therapy and radiography or as the front end to an ADS test facility. In this paper, we describe the design of the CPHS technical systems and its intended operation.

Aging behavior and mechanical properties of Mg-Gd-Ho alloys

The Mg-8Gd-0.6Zr-xHo (x = 1, 3 and 5, mass%) alloys were prepared by casting technology, and structures, aging strengthening mechanism and mechanical properties of the alloys were investigated. The age behaviors and the mechanical properties are improved by adding Ho addition. The structures of the alloys are characterized by the present of rosette-shaped equiaxed grains. The peak hardness value of the Mg-8Gd-0.6Zr-3Ho alloy is 100 Hv, which is about 30% higher than that of Mg-8Gd-0.6Zr alloy.

The effect of La or Ce on ageing response and mechanical properties of cast Mg-Gd-Zr alloys

Mg-8Gd-0.6Zr-1RE (RE = La or Ce, wt.%) alloys were prepared by casting. The microstructures, age hardening behavior and mechanical properties were investigated. The results show that the addition of 1 wt.% La or Ce to a Mg-8Gd-0.6Zr alloy reduces the dendrite arm spacing and slightly improves the mechanical properties and age hardening response.

Influences of Gd on the microstructure and strength of Mg-4.5Zn alloy

Microstructure and mechanical properties of peak-aged Mg-4.5Zn-xGd (x=0, 0.5, 1.0 and 1.5 wt.%) alloys have been investigated. The results showed that the grain size of the alloys was refined gradually with increasing Gd. Mg5Gd and Mg3Gd2Zn3 phases were found in the Gd-containing alloys. The strengths were greatly improved with Gd additions, and the highest strength level was obtained in the Mg-4.5Zn-1.5Gd alloy, in which the ultimate tensile strength and yield strength were 231 MPa and 113 MPa, respectively.

Modelling metal powder production by the gas atomisation process

Metal powder in the range of 10-100 microns is widely employed in the production of Raney nickel type catalysts for hydrogenation reactions and hydrogen fuel cell manufacture. In this presentation we examine the modelling of powder production in a gas atomisation vessel using CFD techniques. In a fully coupled Lagrangian-Eulerian two phase scheme, liquid meal particles are tracked through the vessel following atomisation of a liquid nickel-aluminium stream. There is full momentum, heat and turbulence transport between particles and surrounding argon gas and the model predicts the position of solidification depending on particle size and undercooled condition. Maps of collision probability of particles at different stages of solidification are computed, to predict the creation of satellite defects, or to initiate solidification of undercooled droplets. The model is used to support experimental work conducted under the ESA/EU project IMPRESS.