Reactivation kinetics of boron acceptors in hydrogenated silicon during zero bias anneal

The reactivation kinetics of passivated boron accepters in hydrogenated silicon during zero bias annealing in the temperature range of 65-130 degrees C are reported, For large annealing times and high annealing temperatures, the reactivation process follows second-order kinetics and is rate limited by a thermally activated <(H)over tilde (2)> complex formation process, For short annealing times and low annealing temperatures, the reactivation rate is found to be larger than that due to <(H)over tilde (2)> complex formation alone. We conclude that the faster reactivation is caused by the diffusion of the liberated hydrogen atoms into the bulk as well as <(H)over tilde (2)> complex formation. The effective diffusion coefficient of hydrogen is measured and found to obey the Arrhenius relation with an activation energy (1.41 +/- 0.1) eV. (C) 1997 American Institute of Physics.

ZnFe(2)O(4): Rapid and sub-100 degrees C synthesis and anneal-tuned magnetic properties

Nanocrystalline zinc ferrite (ZFO) has been synthesized from metal acetylacetonates by microwave irradiation for 5 min in the presence of a surfactant. The as-prepared material is ZFO and has been subjected in air to conventional furnace annealing and to rapid annealing at different temperatures. Both annealing protocols lead to well-crystallized ZFO, with crystallite sizes in the range similar to 8-20 nm, which is ferrimagnetic, even at room temperature, with magnetization attaining saturation. While the magnetization M(S) of conventionally annealed ZFO varies with crystallite size in the expected manner, rapid annealing leads to high M(S) even when the crystallite size is relatively large. The coercivity is greater in the conventionally annealed ZFO. Thermal and magnetic measurements suggest that the inhomogeneous site cationic distribution within each crystallite caused by rapid annealing can be used to tailor the magnetic behaviour of nanocrystalline ferrites.

Influence of cobalt doping on superconducting transition in as-grown YBCO single crystals

The role of a charge buffer layer in the superconductivity of high-T-c materials is best studied by cationic substitutions. In this work, the chain copper in YBCO single crystals is substituted by Co3+ ion and consequent effect on superconducting transition temperature (T-c) studied. The T-c is measured using non-resonant Microwave Absorption technique, which is a highly sensitive and contactless method. It is seen that T-c of as-grown crystals is considerably enhanced by cobalt doping in low concentration regime. In contrast, higher T-c is achieved in undoped crystals only after extended oxygen anneal. When dopant concentration increases beyond an optimal value, T-c decreases and the system does not show superconductivity when cobalt content is high (x > 0.5 in YBa2Cu3-xCOxO7+/-delta). This behaviour consequent to cobalt substitution is discussed with reference to the apical oxygen model. Optimal cobalt doping can be thought of as an alternative to extended oxygen anneal in as-grown crystals of YBCO.

Hole Pairing In Antiferromagnetically Correlated Layered Oxides

We have considered a two-band Hubbard model having interlaced Cu-3d(x2−y2) and O-2p(x, y) orbitals representing the CuO2 square planes. Simple CuO2 -cluster calculation suggests that the additional holes created by doping stay mainly on oxygen. Motion of an oxygen hole interlacing with the antiferromagnetically correlated background of copper spins, creates a string of high energy spin configuration of finite length giving mass renormalization. Another hole of opposite spin can now anneal this string tension providing a triangular pairing potential for large pair momentum. The latter implies unusual Bose condensation of the wake-bound compact Bose-like pairs on a non-zero momentum shell. Effect of disorder favouring condensation at the mobility edge is pointed out.

Toughness of as-cast and partially crystallized composites of a bulk metallic glass

The deformation and fracture response of a bulk metallic glass (BMG) post-annealing above the glass transition temperature is examined. The toughness of the glass-matrix composite exhibits a sharp transition beyond a critical volume fraction of crystallization to values as low as that of brittle silicate glass. Instrumented indentation tests supplemented by impact tests were used to study this ductile to brittle transition exhibited by the partially crystallized samples. Indentation on the anneal-embrittled specimens shows lateral cracks in addition to cracks along the corners. The applicability of the Poisson's ratio-toughness correlation with respect to partially crystallized samples is also investigated.

Structural Relaxation and Crystallization in a Pd40Cu30Ni10P20 Bulk Metallic Glass

The stability of a Pd40Cu30Ni10P20 bulk metallic glass (BMG) against structural relaxation is investigated by isothermal and isochronal annealing heat treatments below and above its glass transition temperature, Tg, for varying periods. Differential scanning calorimetry (DSC) of the annealed samples shows an excess endotherm at Tg, irrespective of the annealing temperature. This recovery peak evolves exponentially with annealing time and is due to the destruction of anneal-induced compositional short range ordering. The alloy exhibits a high resistance to crystallization on annealing below Tg and complex Pd- and Ni-phosphides evolve on annealing above Tg.

Texture evolution in an Al-Cu alloy during equal channel angular pressing: the effect of starting microstructure

In this article, the effect of initial microstructure on the texture evolution in 2014 Al alloy during equal channel angular pressing (ECAP) through route A has been reported. Three heat treatment conditions were chosen to generate the initial microstructures, namely (i) the recrystallization anneal (as-received), (ii) solution treatment at 768 K for 1 h, and (iii) solution treatment (768 K for 1 h) plus aging at 468 K for 5 h. Texture analyses were performed using orientation distribution function (ODF) method. The texture strength after ECAP processing was different for the three samples in the order, solutionised > solutionised plus aged condition > as-received. The prominent texture components were A (E) /(A) over bar (E) and B(E)/(B) over bar (E) in addition to several weaker components for the three materials. The strong texture evolution in solutionised condition has been attributed to higher strain hardening of the matrix due to higher amount of solute. In case of the as-received as well as solutionised plus aged alloy, the weaker texture could be due to the strain scattering from extensive precipitate fragmentation and dissolution during ECAP.

High-Performance Metal-Insulator-Metal Capacitors Using Europium Oxide as Dielectric

We report the first demonstration of metal-insulator-metal (MIM) capacitors with Eu2O3 dielectric for analog and DRAM applications. The influence of different anneal conditions on the electrical characteristics of the fabricated MIM capacitors is studied. FG anneal results in high capacitance density (7 fF/mu m(2)), whereas oxygen anneal results in low quadratic voltage coefficient of capacitance (VCC) (194 ppm/V-2 at 100 kHz), and argon anneal results in low leakage current density (3.2 x 10(-8) A/cm(2) at -1 V). We correlate these electrical results with the surface chemical states of the films through X-ray photoelectron spectroscopy measurements. In particular, FG anneal and argon anneal result in sub-oxides, which modulate the electrical properties.

Energy-efficient synthesis of ferrite powders and films

In recent years, there has been significant effort in the synthesis of nanocrystalline spinel ferrites due to their unique properties. Among them, zinc ferrite has been widely investigated for countless applications. As traditional ferrite synthesis methods are energy- and time-intensive, there is need for a resource-effective process that can prepare ferrites quickly and efficiently without compromising material quality. We report on a novel microwave-assisted soft-chemical synthesis technique in the liquid medium for synthesis of ZnFe2O4 powder below 100 °C, within 5 min. The use of β-diketonate precursors, featuring direct metal-to-oxygen bonds in their molecular structure, not only reduces process temperature and duration sharply, but also leads to water-soluble and non-toxic by-products. As synthesized powder is annealed at 300 °C for 2 hrs in a conventional anneal (CA) schedule. An alternative procedure, a 2-min rapid anneal at 300 °C (RA) is shown to be sufficient to crystallize the ferrite particles, which show a saturation magnetization (MS) of 38 emu/g, compared with 39 emu/g for a 2-hr CA. This signifies that our process is efficient enough to reduce energy consumption by ∼85% just by altering the anneal scheme. Recognizing the criticality of anneal process to the energy budget, a more energy-efficient variation of the reaction process was developed, which obviates the need for post-synthesis annealing altogether. It is shown that the process also can be employed to deposit crystalline thin films of ferrites.

Review: Overcoming the paradox of strength and ductility in ultrafine-grained materials at low temperatures

Ultrafine-grained (UFG) materials with grain sizes in the submicrometer or nanometer range may be prepared through the application of severe plastic deformation (SPD) to bulk coarse-grained solids. These materials generally exhibit high strength but only very limited ductility in low-temperature testing, thereby giving rise to the so-called paradox of strength and ductility. This paradox is examined and a new quantitative diagram is presented which permits the easy insertion of experimental data. It is shown that relatively simple procedures are available for achieving both high strength and high ductility in UFG materials including processing the material to a very high strain and/or applying a very short-term anneal immediately after the SPD processing. Significant evidence is now available demonstrating the occurrence of grain boundary sliding in these materials at low temperatures, where this is attributed to the presence of non-equilibrium grain boundaries and the occurrence of enhanced diffusion along these boundaries.

Structure-rheology relationship in a sheared lamellar fluid

The structure-rheology relationship in the shear alignment of a lamellar fluid is studied using a mesoscale model which provides access to the lamellar configurations and the rheology. Based on the equations and free energy functional, the complete set of dimensionless groups that characterize the system are the Reynolds number (rho gamma L-2/mu), the Schmidt number (mu/rho D), the Ericksen number (mu(gamma)/B), the interface sharpness parameter r, the ratio of the viscosities of the hydrophilic and hydrophobic parts mu(r), and the ratio of the system size and layer spacing (L/lambda). Here, rho and mu are the fluid density and average viscosity, (gamma) over dot is the applied strain rate, D is the coefficient of diffusion, B is the compression modulus, mu(r) is the maximum difference in the viscosity of the hydrophilic and hydrophobic parts divided by the average viscosity, and L is the system size in the cross-stream direction. The lattice Boltzmann method is used to solve the concentration and momentum equations for a two dimensional system of moderate size (L/lambda = 32) and for a low Reynolds number, and the other parameters are systematically varied to examine the qualitative features of the structure and viscosity evolution in different regimes. At low Schmidt numbers where mass diffusion is faster than momentum diffusion, there is fast local formation of randomly aligned domains with ``grain boundaries,'' which are rotated by the shear flow to align along the extensional axis as time increases. This configuration offers a high resistance to flow, and the layers do not align in the flow direction even after 1000 strain units, resulting in a viscosity higher than that for an aligned lamellar phase. At high Schmidt numbers where momentum diffusion is fast, the shear flow disrupts layers before they are fully formed by diffusion, and alignment takes place by the breakage and reformation of layers by shear, resulting in defects (edge dislocations) embedded in a background of nearly aligned layers. At high Ericksen number where the viscous forces are large compared to the restoring forces due to layer compression and bending, shear tends to homogenize the concentration field, and the viscosity decreases significantly. At very high Ericksen number, shear even disrupts the layering of the lamellar phase. At low Ericksen number, shear results in the formation of well aligned layers with edge dislocations. However, these edge dislocations take a long time to anneal; the relatively small misalignment due to the defects results in a large increase in viscosity due to high layer stiffness and due to shear localization, because the layers between defects get pinned and move as a plug with no shear. An increase in the viscosity contrast between the hydrophilic and hydrophobic parts does not alter the structural characteristics during alignment. However, there is a significant increase in the viscosity, due to pinning of the layers between defects, which results in a plug flow between defects and a localization of the shear to a part of the domain.

Structure-function studies of serine hydrolases: synthesis of a gene for ∝-lytic protease and the purification and characterization of a mutant β-lactamase

The author has constructed a synthetic gene for ∝-lytic protease. Since the DNA sequence of the protein is not known, the gene was designed by using the reverse translation of ∝-lytic protease's amino acid sequence. Unique restriction sites are carefully sought in the degenerate DNA sequence to aid in future mutagenesis studies. The unique restriction sites are designed approximately 50 base pairs apart and their appropriate codons used in the DNA sequence. The codons used to construct the DNA sequence of ∝-lytic protease are preferred codons in E-coli or used in the production of β-lactamase. Codon usage is also distributed evenly to ensure that one particular codon is not heavily used. The gene is essentially constructed from the outside in. The gene is built in a stepwise fashion using plasmids as the vehicles for the ∝-lytic oligomers. The use of plasmids allows the replication and isolation of large quantities of the intermediates during gene synthesis. The ∝-lytic DNA is a double-stranded oligomer that has sufficient overhang and sticky ends to anneal correctly in the vector. After six steps of incorporating ∝-lytic DNA, the gene is completed and sequenced to ensure that the correct DNA sequence is present and that no mutations occurred in the structural gene.

β-lactamase is the other serine hydrolase studied in this thesis. The author used the class A RTEM-1 β- lactamase encoded on the plasmid pBR322 to investigate the roll of the conserved threonine residue at position 71. Cassette mutagenesis was previously used to generate all possible amino acid substitutions at position 71. The work presented here describes the purification and kinetic characterization of a T71H mutant previously constructed by S. Schultz. The mutated gene was transferred into plasmid pJN for expression and induced with IPTG. The enzyme is purified by column chromatography and FPLC to homogeneity. Kinetic studies reveal that the mutant has lower k_(cat) values on benzylpenicillin, cephalothin and 6-aminopenicillanic acid but no changes in k_m except for cephalothin which is approximately 4 times higher. The mutant did not change siginificantly in its pH profile compared to the wild-type enzyme. Also, the mutant is more sensitive to thermal denaturation as compared to the wild-type enzyme. However, experimental evidence indicates that the probable generation of a positive charge at position 71 thermally stabilized the mutant.

一种制作掺Yb相移光纤光栅激光器的实验方案

采用遮挡法引入相移制作了掺Yb相移光纤光栅(PS-FBG)。在制作光栅的过程中,将其作为激光器的谐振腔,通过监测激光器的输出功率来确定相移大小。当激光器的输出功率开始下降时,停止曝光,此时引入的相移为π/2。为了使光栅的特性尽快稳定下来需要对光栅进行退火,这将导致引入的相移小于π/2。为了弥补退火过程中引起的相移降低,需要对退火后的光栅进行二次曝光,以使光栅的相移恢复π/2。利用该方法制作了一只光纤光栅激光器。当抽运功率为100 mW时,获得了25 mW的输出功率,信噪比(SNR)为60 dB。在1 h内

不同退火气氛下γ-LiAlO2形貌和结构研究

研究了温度梯度法生长的γ-LiAlO2晶体在1100℃下富Li气氛和空气中退火处理后的表面形貌、表面结构以及吸收光谱。发现γ-LiAlO2晶体的抛光面在1100℃空气中退火后变为粗糙面，X射线衍射(XRD)分析表明此粗糙面为单相的LiAl5O8。而γ-LiAlO2晶体的抛光面在1100℃富Li气氛中处理后几乎没有变化。同时对不同气氛下热处理的γ-LiAlO2晶体进行了光谱分析，确认了晶片中196nm的吸收峰是由Li空位引起的。