Low Temperature Elastic Constants of Polycrystalline MgB2

We have used a resonant ultrasound spectroscopy technique to measure the bulk and shear modulus of fully dense, polycrystalline MgB₂ between 4 and 300 K. Both moduli show good agreement with published first principle calculations. The internal friction shows a broad maximum around 40 K.     

Elastic constants of AlB2-type compounds from first-principles calculations

Elastic constants (C_ij’s) of 24 compounds in the AlB₂-type diborides have been calculated by first-principles with the generalized gradient approximation and compared with the available experimental data. Values of all independent elastic constants as well as bulk modulus in a and c directions (B_a and B_c, respectively) were predicted. The elastic modulus of the AlB₂-type compounds were calculated according to the theoretical elastic constants by Voigt-Reuss-Hill averaging scheme. Ductility and anisotropy in these compounds were further analyzed based on their B/G ratio and elastic constants. It is founded that AlB₂ is more ductile while ScB₂ is more brittle, and AlB₂ has a highest elastic anisotropy in the 24 AlB₂-type compounds.     

Elastic constants of porous silver compacts after acid assisted consolidation at room temperature

Acid assisted consolidation (AAC) is a technique that provides elevatedlevels of cohesiveness to silver powder compacts without recourse to a high temperature treatment. Ultrasonic techniques were used to measure the elastic constants of high purity silver compacts as a function of several processing variables. The elastic moduli of untreated (NT) samples were found to be significantly lower than those of samples that had undergone AAC and compacted at the same pressure. Post compaction sintering increases the elastic constants of both AAC and NT samples. The results indicate that the elastic constants are dependent not only on the density that was attained but also on the processing route that was followed. The elastic constant of a porous metal, M, can be expressed as M = M ₀ g, where M ₀ is the elastic modulus of the bulk metal, g is a geometrical factor that reflects the interparticle contact area and is a quality factor that depends on the nature of the interparticle interfaces. The results suggest that sound wave velocity is a parameter more appropriate than density for predicting the elastic moduli of porous metallic compacts.     

Elastic constants of MoSi2 and WSi2 single crystals

Single crystals of MoSi₂ and WSi₂ with a body-centred-tetragonal C 1 1_b structure were fabricated using a floating-zone method. The elastic wave velocity was measured for samples with various orientations using a simple pulse echo method at room temperature, and six elastic stiffness constantsc _ij were calculated. The stiffness constants were a little higher for WSi₂ than for MoSi₂.c ₁₁ andc ₃₃ of these compounds were approximately equal toc ₁₁ of tungsten and molybdenum, respectively, althoughc _ij (i j) was a little higher for these compounds than for molybdenum and tungsten. Young's modulus 1/s ₁₁ was the highest in the <0 0 1> direction, and the lowest in the <1 0 0> direction. The shear modulus 1/s ₆₆ was high on the {0 0 1} plane and independent of shear direction. It was generally low on the close-packed {1 1 0} plane and largely dependent on shear direction. The elastic constants for the polycrystalline materials were estimated fromc _ij ands _ij . Poisson's ratiov was 0.15 for MoSi₂ and for WSi₂, and these values were much lower than for ordinary metals and alloys. The Debye temperature _D was estimated using the elastic-wave velocity of the polycrystalline materials via the elastic constants such as Young's modulus and shear modulus: it was 759 K for MoSi₂ and 625 K for WSi₂.     

Finite element modeling of nano porous sintered silver material using computed tomography images

Limitations in the packaging industry require improvements in lead‐based microelectronics, because regulations restricting hazardous substances have ended the use of conventional lead‐based solders. Sintered silver paste presents an alternative method for bonding chips to a substrate because it has low sensitivity to the oxidation, high melting point, and improved thermal and electrical conductivity. Due to the sintering process, however, the silver exhibits a significant pore fraction that substantially reduces the density of the material compared to bulk silver. The traits of the sintered silver pastes are affected by the pore distribution; hence, it can be considered as a significant factor in modelling the behaviour of the whole electronic system under operating conditions. This study defines the influence of the morphology and pore distribution on the response of the silver material. The quality of tomographic images was improved through coding a program in MATLAB®. Finite element software ABAQUS® was employed to evaluate the elastic properties of the material. To validate the model, the results have gone through several studies to determine changes of the material properties.     

Elastic constants identification of symmetric angle-ply laminates via a two-level optimization approach

A two-level optimization method for elastic constants identification of symmetric angle-ply laminates is presented. Measured axial and lateral strains of two symmetric angle-ply laminates with different fiber angles are used in the proposed method to identify four elastic constants of the composite laminates. In the first-level optimization process, the theoretically and experimentally predicted axial and lateral strains of a [(45°/−45°)₂]_s laminate are used to construct the error function which is a measure of the differences between the experimental and theoretical predictions of the axial and lateral strains. The identification of the material constants is then formulated as a constrained minimization problem in which the best estimates of the shear modulus and Poisson’s ratio of the laminate are determined by making the error function a global minimum. The problem of this level of optimization is then solved using a multi-start global minimization algorithm. In the second-level optimization process, the shear modulus and Poisson’s ratio determined in the previous level of optimization are kept constant while the Young’s moduli of the second angle-ply laminate with fiber angles other than 45° are identified using the same minimization technique that has been used in the previous level. The accuracy of the proposed method are studied by means of a number of numerical examples on the material constants identification of symmetric angle-ply laminates made of different composite materials. Finally, static tensile tests of [(45°/−45°)₂]_s and [(30°/−30°)₂]_s laminates made of Gr/ep composite material are performed to measure the strains of the laminates. The experimental data are then used to identify the elastic constants of the laminates. The excellent results obtained in the experimental investigation have demonstrated the feasibility and applications of the proposed method.     

Strength and fracture of porous ceramic sintered from spherical particles

Mechanical properties of ceramics obtained by the sintering of stabilized zirconia microspheres are investigated. Strength at compression and tension, elastic deformation and modulus of elasticity at compression, specific works of fracture, of fracture initiation, and stress intensity factor are determined. An expression is proposed to establish the dependence of strength on macrostructure parameters of brittle material sintered from microspheres, and its analysis is given.     

Numerical modeling of the tensile deformation process of sintered 316L based on microtomography of porous mesostructures

This paper concerns the numerical modeling of the tensile deformation process of porous sinters prepared from 316L powder. In the research specimens with 41%, 33% and 26% porosity were used. To take into account the porosity of materials (at a mesoscale) in the numerical modeling, X-ray microtomography was used. Based on the micro-CT images three-dimensional models were generated which mapped porous structures of the materials. Then, the surfaces of the models were subjected to triangulation and saved as finite element meshes. Numerical calculations were performed using the finite element method (FEM). To define the material nonlinearity the true stress–strain curve of solid 316L was used. Two approaches were employed for modeling the deformation process. The essence of the applied methods was the reduction of the impact of the non-mapped geometries (of a size less than the micro-CT accuracy) on mechanical properties of the materials. As a result of the calculations carried out by two methods, stress and strain fields were obtained and nominal stress–strain curves of the porous materials were determined. Based on the results of numerical calculations the influence of material discontinuities at the mesoscopic scale on macromechanical properties was investigated.     

The Elastic Properties, Generalized Stacking Fault Energy and Dissociated Dislocations in MgB2 Under Different Pressure

The $\langle11\overline{2}0\rangle$ perfect dislocation in MgB₂ is suggested to dissociate into two partial dislocations in an energy favorable way $\langle11\overline{2}0\rangle\rightarrow\frac{1}{2}\langle11\overline{2}0\rangle +\mathrm{SF}+\frac{1}{2}\langle11\overline{2}0\rangle$ . This dissociation style is a correction of the previous dissociation $\langle1000\rangle\rightarrow\frac{1}{3}\langle1\overline{1}00\rangle+\mathrm{SF}+\frac{1}{3}\langle2100\rangle$ proposed by Zhu et al. to model the partial dislocations and stacking fault observed by transmission electron microscopy. The latter dissociation results in a maximal stacking fault energy rather than a minimal one according to the generalized stacking fault energy calculated from first-principles methods. Furthermore, the elastic constants and anisotropy of MgB₂ under different pressure are investigated. The core structures and mobilities of the $\langle11\overline{2}0\rangle$ dissociated dislocations are studied within the modified Peierls–Nabarro (P–N) dislocation theory. The variational method is used to solve the modified P–N dislocation equation and the Peierls stress is also determined under different pressure. High pressure effects on elastic anisotropy, core structure and Peierls stress are also presented.     

用非单质硼源直接合成MgB2超导粉末

用非单质硼源直接合成了MgB2超导粉末，X射线衍射分析表明，除了MgB2主相外，还形成了副产物Mg28205，低温段保温的工艺控制是该制备方法中形成MgB2的关键，非单质硼源直接合成法是研制MgB2新型超导体的一种很有前途的技术．     

Enhanced critical current density of ex situ MgB2 via control of Mg assisted sintering

Journal of Materials Science: Materials in Electronics - In this work,&nbsp;ex situ&nbsp;MgB2&nbsp;was mixed with 0.5&nbsp;mol of Mg and sintered. The sintering conditions were...     

Elastic constants of the high-temperature ceramic superconductor YBa2Cu3O7−x

Journal of Materials Science Letters -     

Correction to: Pinning Behavior in Bulk MgB2 Prepared Using Boron Powder Refined via High-Energy Ultra-Sonication

Journal of Superconductivity and Novel Magnetism -     

Correction to: Enhanced critical current density of ex situ MgB2 via control of Mg assisted sintering

Journal of Materials Science: Materials in Electronics -     

Synthesis of porous Mg-doped CeO2 powders via self-propagating high-temperature synthesis route

The aim of this study was synthesis of Mg-doped porous cerium oxide powder by self-propagating high-temperature synthesis reaction of Mg-xCeO₂-O₂ system. Results indicated that doping Mg in the CeO₂ crystal structure increased its lattice parameter slightly, and shifted the Ce-O band in the FTIR spectra from 490 to 574 cm⁻¹. Moreover, the EDS results revealed the distribution of Mg in the CeO₂ microstructure. There were a high volume fraction of interconnected macro-pores in the microstructure of cerium oxide after the synthesis process. Increase in the x-value from 0.05 to 0.25 mol decreased the mean size and volume fraction of pores from 2.4 to 1 µm and 50 to 30 vol%, respectively. In addition, the BET surface area of porous CeO₂ varied between 0.3 and 1.57 m²/g. Finally, it was inferred that the SHS technique can be introduced as a rapid and novel method for synthesis of Mg-doped porous CeO₂ powders.     

Synthesis and characterisation of a porous Al scaffold sintered from NaAlH<Subscript>4</Subscript>

A simple and cost-effective method for the synthesis of a porous Al scaffold has been optimised using only NaAlH₄ and TiCl₃. The starting materials were compacted into a pellet and sintered under dynamic vacuum to remove the Na and H₂. The sintering conditions, such as vacuum level, temperature, and time, were the key factors that influenced both the extraction of Na and H₂ from the pellet and the overall porosity. Quantitative phase analysis by X-ray diffraction revealed that after the sintering process, the as-prepared porous Al scaffold consisted primarily of Al. Morphological observations conducted by scanning electron microscopy showed that the scaffold exhibited an open network of pores with a small number of mesopores and no formation of micropores. The specific surface area of the scaffold was determined to be 7.9 ± 0.1 and 6.0 ± 0.5 m²/g by the Brunauer–Emmet–Teller method and from small-angle X-ray scattering measurements, respectively. The total porosity of the Al scaffold was 44.6%.     

Dielectric properties of sintered materials prepared from glass-ZrO2-SrTiO3 mixtures

(Ca,Sr,Ba)O-Al₂O₃-SiO₂-TiO₂ glass powder was mixed with 6.7 mol % ZrO₂ and 0–11.96 mol % SrTiO₃. The mixtures were sintered at 850 °C, 900 °C or 950 °C. Most of the glass phase changed to crystalline phases of (Ca,Sr,Ba)₂TiSi₂O₆TS, (Ca,Sr,Ba)Si₂O₆AS, ZrO₂ and SrTiO₃ during sintering. The dielectric properties of samples sintered at 900 °C were measured at 1 MHz using silver electrodes. With increasing SrTiO₃ content, the dielectric constant increased from 12 to 19 and the temperature coefficient of dielectric constant, TCD, changed from -30 ppm ^C^-1 to -400ppm^C^-1. Using mixture equations, dielectric constants and TCD values were estimated for the samples and these values were compared with experimental results.     

Preparation via gelling of porous Li2ZrO3 for fusion reactor blanket material

Crystalline powders of Li₂ZrO₃ were prepared by gelling ZrCl₄ and CH₃COOLi with NH₄OH at 50 °C, at different pH values (5, 7 and 8) and for different times (3, 12 and 24 h), then drying and calcining. Compacts from these powders were found sinter to higher densities than a commercial Li₂ZrO₃; their porosity at 1200°C was close to 3 m and was suitable for blanket material applications. Tritium release as HTO begins at 300 °C and reaches 73% at 400 °C, a temperature much lower than that required for Li₂O.     

Preparation of barium titanate ultrafine particles from amorphous titania by a hydrothermal method and specific dielectric constants of sintered discs of the prepared particles

Barium titanate ultrafine particles were synthesized from amorphous titania by a hydrothermal method. The mean size of the barium titanate particles prepared at a hydrothermal treatment time of 4 h, was nearly equal to 0.04 5 m in the range of barium-to-titanium molar ratio (BT) 2, and approximately agreed well with the crystallite size. At a BT molar ratio of 1.0, the mean particle size increased to 0.2 m, while the crystallite size remained constant at 0.045 m. When the particle size ranged from 0.12–0.20 m, prepared for the BT molar ratio of 1.0–1.4, the specific dielectric constant for a sintered disc composed of these particles attained a value of 5000 or more. As the BT molar ratio increased to exceed 1.5, when the mean particle size decreased from 0.13 m to 0.045 m, the specific dielectric constant for the sintered disc was decreased greatly. The specific dielectric constant for the sintered disc can be correlated well with the size of the composing particles.