Residual Mismatch Research Articles

Effects of particle size, particle volume fraction and matrix composition on the fatigue crack growth resistance of Al alloy/Al alloy + SiCp bimaterials

The fatigue crack growth resistance of Al alloy/Al alloy + SiCp bimaterials for crack growth perpendicular to the interface is affected by thermal residual stresses, elastic mismatch, plastic mismatch and direction of crack approach to the interface. When the crack approaches the interface from the composite side, the crack growth resistance is mainly controlled by the compressive residual stress near to the interface. Conversely, when the crack grows from the aluminium side towards the composite, the crack is shielded primarily by the elastic/plastic mismatch. In this work, the effects of particle size, particle volume fraction and matrix composition on the fatigue crack growth resistance of Al alloy/Al alloy + SiCp bimaterials have been assessed. These parameters can affect both the thermal residual stress profile and the elastic/plastic mismatch, and hence the effective crack tip driving force for crack extension is also affected.

Welded plate and T-stub tests and implications on structural behavior of moment frame connections

A series of tests on simple-welded plate specimens (SWPS) and T-stub tension specimens simulating some of the joint details in moment frame connections were conducted in this investigation. The effects of weld strength mismatch and weld metal toughness on structural behavior of these specimens were considered under both static and dynamic loading conditions. Finite element analyses were performed by taking into account typical weld residual stress distributions and weld metal strength mismatch conditions to facilitate the interpretation of the test results. The major findings are as follows: (a) Sufficient specimen size requirements are essential in simulating both load transfer and constraint conditions that are relevant to moment frame connections, (b) Weld residual stresses can significantly elevate stress triaxiality in addition to structural constraint effects, both of which can significantly reduce the plastic deformation capacity in moment frame connections, (c) Based on the test results, dynamic loading within a loading rate of 0.02 in/in/sec, as used in this study, premature brittle fractures were not seen, although a significant elevation of the yield strength can be clearly observed. However, brittle fracture features can be clearly identified in T-stub specimens in which severe constraint effects (stress triaxiality) are considered as the primary cause, (d) Based on both the test and FEA results, T-stub specimens provide a reasonable representation of the joint conditions in moment frame connections in simulating both complex load transfer mode and constraint conditions.

Use of TiN(O)/Ti as an effective intermediate stress buffer and diffusion barrier for Cu/parylene-n interconnects

Copper and parylene-n (Pa-n) are studied for ultralarge scale integration circuits because of their low electrical resistivity, resistance to electromigration and low dielectric constant, chemical inertness, and compatibility with current integrated circuit manufacturing, respectively. Copper diffusion observed at and above 300 °C in Pa-n correlates to an increase in the crystallinity of the α phase and subsequent transformation to the more open structure of β parylene. Titanium nitride (oxygen) [TiN(O)]/titanium (Ti) bilayers are successfully implemented as a diffusion barrier. TiN is proven to be a very good diffusion barrier up to 500 °C for copper due to its large negative heat of formation and hence its thermal stability. Incorporation of an intermediate titanium layer reduced the residual stress and thermal mismatch between Pa-n and TiN. Without the Ti layer thermal cracking of TiN occurred. The presence of the buffer layer had no detrimental effects on the overall resistivity. The effectiveness of the barrier is attributed to stuffing of the grain boundaries with oxygen and nitrogen. This results in the elimination of rapid diffusion paths. This work provides the foundation for future implementation of Cu/Pa-n for higher temperature microelectronics.

Redundant Exonuclease Involvement in Escherichia coli Methyl-directed Mismatch Repair

Previous biochemical analysis of Escherichia coli methyl-directed mismatch repair implicates three redundant single-strand DNA-specific exonucleases (RecJ, ExoI, and ExoVII) and at least one additional unknown exonuclease in the excision reaction (Cooper, D. L., Lahue, R. S., and Modrich, P. (1993) J. Biol. Chem. 268, 11823-11829). We show here that ExoX also participates in methyl-directed mismatch repair. Analysis of the reaction with crude extracts and purified components demonstrated that ExoX can mediate repair directed from a strand signal 3' of a mismatch. Whereas extracts of all possible single, double, and triple exonuclease mutants displayed significant residual mismatch repair, extracts deficient in RecJ, ExoI, ExoVII, and ExoX exonucleases were devoid of normal repair activity. The RecJ(-) ExoVII(-) ExoI(-) ExoX(-) strain displayed a 7-fold increase in mutation rate, a significant increase, but less than that observed for other blocks of the mismatch repair pathway. This elevation is epistatic to deficiency for MutS, suggesting an effect via the mismatch repair pathway. Our other work (Burdett, V., Baitinger, C., Viswanathan, M., Lovett, S. T., and Modrich, P. (2001) Proc. Natl. Acad. Sci. U. S. A. 98, 6765-6770) suggests that mutants are under-recovered in the exonuclease-deficient strain due to loss of viability that is triggered by mismatched base pairs in this genetic background. The availability of any one exonuclease is enough to support full mismatch correction, as evident from the normal mutation rates of all triple mutants. Because three of these exonucleases possess a strict polarity of digestion, this suggests that mismatch repair can occur exclusively from a 3' or a 5' direction to the mismatch, if necessary.

Step-by-step simulations of diamond nucleation and growth on a silicon (001) surface

Abstract To pursue single-crystal diamond growth on silicon substrates, the mechanism of diamond nucleation and growth must be well understood. For this purpose, step-by-step depositions of hydrocarbon species on a silicon (001)-(2×1) surface were simulated based on a series of calculations using semi-empirical molecular orbital PM3 and density-functional theories. Molecular mechanics was also used to optimize the interface structure of a large cluster model in order to reveal the interface strain and bonding. It is shown that diamond can be coincidentally built on the silicon (001) surface in spite of the 3:2 lattice mismatch. Issues such as residual lattice mismatch, nucleation mode and surface roughness relating to single-crystal diamond growth are discussed.

On GPS positioning and integrity monitoring

The paper reinvestigates the measurement model associated with Global Positioning System (GPS) signal processing. It is argued that the GPS positioning model is better formulated as a linear equation with errors in both the data matrix and measurement variables. Furthermore, depending on the nature of the measurement errors, the model is categorized into unstructured and structured perturbation cases. In the former, the total least squares method is proposed for position fixing and clock bias determination. In the latter, an iteration method is developed to search for the optimal solution. In addition to the position update, both the total least squares and optimization methods provide estimates of the model mismatch which leads to a measure of GPS receiver autonomous integrity monitoring. Two new GPS fault detection metrics are then proposed and discussed. The first integrity test statistic is the norm of the residual vector in the total least squares estimate. Statistical properties of this test statistic are obtained for integrity monitoring. The second metric is a two-dimensional vector that characterizes the norm of the residual vector and mismatch matrix, both are outcomes of the total least squares method or the optimization method. The positioning and integrity monitoring schemes are verified using simulated examples.

Accurate High-Speed Spatial Normalization Using an Octree Method

The goal of regional spatial normalization is to remove anatomical differences between individual three-dimensional (3-D) brain images by warping them to match features of a standard brain atlas. Full-resolution volumetric spatial normalization methods use a high-degree-of-freedom coordinate transform, called a deformation field, for this task. Processing to fit features at the limiting resolution of a 3-D MR image volume is computationally intensive, limiting broad use of full-resolution regional spatial normalization. A highly efficient method, designed using an octree decomposition and analysis scheme, is presented to resolve the speed problem while targeting accuracy comparable to current volumetric methods. Translation and scaling capabilities of octree spatial normalization (OSN) were tested using computer models of solid objects (cubes and spheres). Boundary mismatch between transformed and target objects was zero for cubes and less than 1% for spheres. Regional independence of warping was tested using brain models consisting of a homogenous brain volume with one internal homogenous region (lateral ventricle). Boundary mismatch improved with successively smaller octant-level processing and approached levels of less than 1% for the brain and 5% for the lateral ventricle. Five 3-D MR brain images were transformed to a target 3-D brain image to assess boundary matching. Residual boundary mismatch was approximately 4% for the brain and 8% for the lateral ventricle, not as good as with homogeneous brain models, but similar to other results. Total processing time for OSN with a 2563 brain image (1-mm voxel spacing) was less than 10 min.

MRI-SPET and SPET-SPET brain co-registration: evaluation of the performance of eight different algorithms.

The aim of this study was to assess the accuracy and computing time needed for MRI-SPET and SPET-SPET brain co-registration using eight different algorithms (Hermes software from Nuclear Diagnostics Ltd run on a SUN Ultra Sparc 2) to determine the clinically most suitable algorithm. MRI-SPET co-registration was evaluated using phantom studies. To approximate clinical dual-headed SPET studies, a Hoffman brain phantom was filled with 99Tcm. For MRI imaging (1.5 Tesla), the phantom was filled with water and doped with Gd-DTPA for contrast enhancement. For both modalities, phantom images were acquired and reconstructed using a routine clinical protocol. MRI and SPET images were matched by Downhill Simplex minimization of the sum of absolute Count Differences (CD), the sum of the Square Root of absolute count differences (SR), the Difference in Shape between the binary masks (SD), the number of Sign Changes in the subtracted image (SC), the Variance of intensities between corresponding pixels (VAR), the sum of absolute count differences between the 2D- and 3D-Gradient images (2DG-3DG) and, finally, the standard deviation of the Uniformity Index (UI), that is the intensity ratio between spatially corresponding voxels. Six degrees of freedom were allowed (three translation and three rotation parameters, three scaling parameters were constrained). The accuracy of the matching process with these different similarity measures was evaluated via the residual mismatch between external markers. We found that CD, SR, VAR nad UI give the most accurate registration compared with the other similarity measures. For the evaluation of SPET-SPET co-registration, five 99Tcm-ECD brain perfusion SPET scans were performed with a dual-headed gamma camera. These studies were then manually misaligned, and subsequently re-aligned using the methods outlined above. For this application, CD, SR and VAR were also found to give the most accurate registration. For all of these algorithms, the computing time required was clinically acceptable (i.e. less than 10 min).

Acoustic and mechanical properties of polyurethanes based on polybutadiene soft segment

Mechanical and acoustic properties of castable polyurethane elastomers comprising polybutadiene polyol, diphenyl methane diisocyanate, and 2-ethylhexane-1,3-diol chain extender are reported. Such elastomers are used to encapsulate sonar transducers. For this application the principal acoustic requirement is that a good impedance match to water be obtained by making both sound velocity and density similar to that of water. Perfect matching would give perfect transmission of the signal from the water into the material but due to the viscoelastic nature of polymers there is always some residual mismatch. Increasing chain extender content improves strength and increases hardness and can bring about formation of phase separated structures. The stronger materials have poorer acoustic performance due to increased shear modulus and damping. A compromise must be reached between acoustic properties and strength, which depends primarily on the layer thickness and the frequency of the sound energy in the particular application.

System configurations for nine-port network analyser employing symmetrical five-port junctions

A nine-port network analyser that requires only six (instead of eight or ten) power sensors to obtain the unknown scattering coefficients of a two-port device is proposed. Hardware imperfections will naturally affect the performance of this instrument. Monte Carlo simulations have been conducted in order to evaluate the deleterious effects due to the residual mismatches of the constituent components within the nine-port system. The results suggest that improvements in instrumentation accuracy may be achieved simply by modifying the system's overall configuration.

Microstructural Design of Silicon Nitride with Improved Fracture Toughness: II, Effects of Yttria and Alumina Additives

Significant improvements in the fracture resistance of self‐reinforced silicon nitride ceramics have been obtained by tailoring the chemistry of the intergranular amorphous phase. First, the overall microstructure of the material was controlled by incorporation of a fixed amount of elongated ß‐Si3N4 seeds into the starting powder to regulate the size and fraction of the large reinforcing grains. With controlled microstructures, the interfacial debond strength between the reinforcement and the intergranular glass was optimized by varying the yttria‐to‐alumina ratio in the sintering additives. It was found that the steady‐state fracture toughness value of these silicon nitrides increased with the Y:Al ratio of the oxide additives. The increased toughness was accompanied by a steeply rising R‐curve and extensive interfacial debonding between the elongated ß‐Si3N4 grains and the intergranular glassy phase. Microstructural analyses indicate that the different fracture behavior is related to the Al (and O) content in the ß´‐SiAlON growth layer formed on the elongated ß‐Si3N4 grains during densification. The results imply that the interfacial bond strength is a function of the extent of Al and Si bonding with N and O in the adjoining phases with an abrupt structural/chemical interface achieved by reducing the Al concentration in both the intergranular phase and the ß´‐SiAlON growth layer. Analytical modeling revealed that the residual thermal expansion mismatch stress is not a dominant influence on the interfacial fracture behavior when a distinct ß´‐SiAlON growth layer forms. It is concluded that the fracture resistance of self‐reinforced silicon nitrides can be improved by optimizing the sintering additives employed.

Stellar models for very low-mass main-sequence stars: the role of model atmospheres

We present very low-mass stellar models as computed using non-grey model atmospheres for selected assumptions about the stellar metallicities. The role of atmospheres is discussed and the models are compared with models based on the Eddington approximation, and with similar models that have appeared in the recent literature. Theoretical predictions concerning both the HR diagram location and the mass-luminosity relation are presented and discussed in terms of expectations in selected photometric bands. Comparison with available observational data concerning both galactic globular clusters and dwarfs in the solar neighbourhood reveals a satisfactory agreement together with the existence of some residual mismatches.

HLA-DRB1 amino acid disparity is the major stimulus of interleukin-2 production by alloreactive helper T-lymphocytes.

The generation of interleukin-2 (IL-2)-mediated helper activity is a central step in the immune response induced by allogeneic histocompatibility antigens, and IL-2-producing helper T-lymphocyte precursor (HTLp) frequencies have been proposed as a measure of alloreactivity in transplant recipients. We analyzed the influence of HLA-matching on the alloresponse of HTLp in limiting dilution assays derived from healthy individuals. Mean HTLp frequencies were significantly higher in HLA-DR antigen-mismatched than HLA-DR-matched combinations. Significant differences in the effect of one or two mismatched HLA-DR antigens on mean HTLp frequencies were also detected. Mean HLA class I (HLA-A, -B, -Cw) mismatches were not significantly different in each group and had no apparent influence on HTLp frequencies. Analysis of HLA protein sequence disparities revealed no significant difference in the number of mismatched amino acid residues at the HLA-DRB1 locus between one and two HLA-DR antigen-mismatched combinations but correlated strongly with HTLp frequency. The positive correlation was evident with mismatched residues in the beta sheet and alpha helical regions of the HLA-DRB1 molecule, suggesting a predominant influence of bound peptides in the stimulation of alloreactive helper cells. This finding was supported by analysis of the location of individual residue mismatches. Evidence of an effect of polymorphism in the CD4-binding region in the beta-2 domain of HLA-DRB1 molecules was also found. Our results demonstrate the major influence of HLA-DR amino acid sequence mismatching on alloreactive HTLp frequencies but also suggest that additional genetic or environmental influences affect the alloreactive helper T-cell repertoire.

Ligation independent cloning irrespective of restriction site compatibility.

Here we report the use of exonuclease to expose complementary DNA between an insert and vector such that annealing becomes independent of restriction site compatibility. We demonstrate that unusual and, in some cases, previously impossible cloning strategies can be readily and efficiently achieved as long as the flanking sequences of the linear vectors are highly related. Furthermore, we show that the bacterial repair system resolves the residual mismatches, overhangs or gaps in a predictable fashion to generate excisable inserts. This approach facilitates cloning regardless of restriction site compatibility and overcomes an important limitation in current cloning techniques.

Strain determination in heteroepitaxial GaN

Residual strains have been evaluated in a variety of GaN layers grown on sapphire or 6H-SiC from wafer curvature at 293 K, which avoids needing to know the unstrained lattice parameters or energy gap of GaN in advance. Estimated strains at 1.7 K are correlated with the energy of the A free exciton to determine its strain dependence. We find that strain-free GaN has an A exciton energy of 3.468±0.002 eV at 1.7 K, and 293 K lattice parameters a=3.1912 Å and c=5.1836 Å. These values imply that GaN on SiC is frequently under net biaxial compressive stress due to residual lattice mismatch stress, and that several hundred μm thick GaN layers on sapphire and homoepitaxial layers grown on bulk platelets grown at high pressure are both under about 1×10−3 in-plane compressive strain. These conclusions conflict with most previous assumptions.

Optical investigation of strain and defects in (100) CdTe/Ge/Si and ZnTe/Ge/Si grown by molecular beam epitaxy

Low temperature (1.7 K) reflectance and photoluminescence (PL) have been used to assess residual strain and impurities in molecular beam epitaxial (100) CdTe/Ge/Si and (100) ZnTe/Ge/Si. Both types of samples exhibit residual biaxial tensile thermal strain as expected from differences in previous thermal expansion data, but the measured magnitudes (0.72×10−3 for CdTe/Ge/Si and 1.5×10−3 for ZnTe/Ge/Si) are smaller than predicted. The results are consistent with either residual lattice mismatch stress or partial relaxation of the thermal strain during cooling. Residual acceptors in the CdTe include Cu and a frequently observed 49 meV level, whose PL peaks lack the previously reported linear polarization. However, a new series of linearly polarized bound exciton lines is reported in CdTe.

Quantitative Characterization of Indentation Crack Path in a Cubic Zirconia—10 vol% Alumina Composite

Stereological measurements were performed to characterize the indentation crack path in a cubic zirconia—10 vol% alumina (c‐ZrO2‐10 vol% Al2O3) composite. Cracks were generated using Vickers indentation, and the crack propagation behavior was characterized as a function of the indentation loading/unloading rates. Cracks that were produced by Vickers indentation formed at higher crack velocities as the loading/unloading rates increased. The amount of contact between the crack and the Al2O3 particles increased as the indentation rate decreased. The total number of crack—particle interactions per unit crack length also increased as the indentation rate decreased, because of an increase in the number of particles that were fractured per unit crack length, whereas the number of particles that were debonded remained relatively constant as the indentation rate changed. These results suggest that residual thermal mismatch stresses have predominant control of the crack path at lower crack velocities (low indentation loading/unloading rate), whereas elastic mismatch stresses predominate at higher crack velocities (high indentation loading/unloading rate).

The stress and spalling behavior of the oxide scale formed on polycrystalline Ni3Al

The residual stress in the oxide scale formed on polycrystalline Ni3Al in air in the range of 900–1200°C was determined at room temperature using optical fluorescence piezospectroscopy. The stress was found to be almost independent of time or temperature of oxidation. In contrast, the spalling behavior was strongly dependent on the oxidation temperature, not occurring below 1100°C and profusely at 1200°C. In the intermediate temperature regime (∼ 1100°C), spallation of the oxide scale occurred preferentially in the vicinity of grain boundaries in the Ni3Al. This is attributed to a buckling instability driven by the residual thermal mismatch stress and induced by both void formation and a non-planar interface structure resulting from preferential diffusion along the Ni3Al grain boundaries.

Structural modeling of the possible growth of oriented textured single-crystal diamond film on a silicon (111) surface

A model based on semi-empirical molecular orbital calculations and molecular mechanics is proposed for oriented textured diamond film growth on the Si(111) surface. The diamond <111≳ axis in our model is parallel to the silicon substrate <111≳ while the diamond <11̄0≳ is rotated 10.89° around the <111≳ axis referenced to the <11̄0≳ of the silicon substrate. The atomic density of the grown diamond in one dimension perpendicular to the <111≳ has a ratio 1.003:1 to that of a natural crystal, and thus is much better than the 1:1.015 of the heteroepitaxial diamond on the silicon (001) surface. We conjecture that this improvement in lattice match is important to achieve single-crystal growth. A model for heteroepitaxial growth with a diamond <111≳//Si<111≳ and diamond <11̄0≳//Si<11̄0≳ relationship is also studied but is found to have poorer bonding to the substrate and has a larger residual mismatch of 1:1.015.

O877 - Alloreactive helper T-lymphocyte precursor frequencies correlate with HLA-DR antigen amino acid residue mismatches

O877 - Alloreactive helper T-lymphocyte precursor frequencies correlate with HLA-DR antigen amino acid residue mismatches