X-ray topographic study of solid-state polymerized poly-[1,2-bis-(p tolylsulphonyloxymethylene)-1-butene-3-inylene]

Local extinction contrast effects have been observed in macroscopic polymer crystals, using Berg-Barrett X-ray topography. Results from as-polymerized crystals indicate that the crystals are perfect enough for defect regions to be imaged. As-polymerized crystals exhibited apparent defects lying along [021]. [010] slip traces in the (100) surface of a crystal were imaged; the slip system here appears to be consistent with (h01) [¯10h], forh, l 0 and [010] the chain direction.     

Microscopic deformation in a heated unidirectional graphite-epoxy composite

Out-of-plane displacements caused by heating of a unidirectional P75S/1962-ERLX graphite-epoxy composite were measured on a surface cut normal to the fibres after thermal cycling. On heating to 44° C from room temperature, the epoxy confined within clusters of three fibres sank in a trough beneath the plane of the fibre ends by 50–120 nm. The sense of the deformation was the opposite of what one would expect from differential thermal expansion. The role of residual stresses in this deformation was studied by measuring the depth of the trough for several different thermal histories applied before the free surface was cut. When the maximum temperature achieved in the prior thermal cycles exceeded 100° C, the depth of the trough increased. However, if cycles exceeding 100° C were followed by thermal cycles of decreasing amplitude, chosen to induce interfacial stress relaxation, the depth of the trough decreased, as expected. The experiments illustrate the feasibility of deducing quantitative information about local deformation in the interior of a specimen from high spatial resolution strain measurements on cut surfaces.     

Electric resistance and magnetoresistance of 30-nm-Thick La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> films elastically compressed in the (110) plane

We have studied the structure, electric resistance, and magnetoresistance of 30-nm-thick (110)La_0.67Ca_0.33MnO₃ (LCMO) films grown by laser deposition on (001)-oriented LaAlO₃ substrates. The unit cell parameters a and b (along the [100] and [010]LCMO axes, respectively) of these manganite films are significantly (by ∼1.2%) increased as compared to the corresponding values in the pseudocubic unit cell of bulk stoichiometric LCMO crystals. At T < 150 K, the temperature dependence of the resistivity of LCMO films is well described by the relation ρ = ρ₁ + ρ₂ (H) T ^4.5. The value of ρ 2 decreases with increasing magnetic field and is close to the analogous coefficient for manganite films grown on substrates with small lattice misfit.     

High temperature X-ray studies on barium and strontium zirconium phosphate based low thermal expansion materials

Stoichiometric compounds of the chemical formula Ba_1.5−xSr_xZr₄P₅SiO₂₄, x being 0, 1.0, 1.25, and 1.5 have been synthesized by the ceramic procedure. The compounds show low thermal expansion behavior with x=1.0 and 1.25 showing almost 0 thermal expansion up to 800°C. High temperature X-ray diffraction studies have been carried out on these samples and the axial thermal expansion coefficients calculated at each temperature by using the lattice parameters. The thermal expansion behavior is attributed to the unique crystal structure and the ionic substitutions of Ba and Sr in [M] site.     

Growth of cubic and hexagonal InN nanorods

InN nanorods with polyhedral ends were grown by evaporating indium in the flow of ammonia gas on Si substrates coated with Au catalyst. The samples were characterized by scanning electron microscopy, transmission electron microscopy (TEM), and X-ray diffraction (XRD). XRD shows that the samples contain both cubic and hexagonal phases. From the TEM results, it can be found that the cubic nanorods grow along [010] direction with a lattice constant of 0.497 nm, while the hexagonal nanorods grow along [112¯0] direction. The growth mechanism of the obtained nanostructures is discussed.     

Correlation of hardness and microstructure in unoriented lamellar polyethylene

The local deformation of the lamellar microstructure of isothermally melted crystallized unoriented polyethylene has been investigated using microindentation hardness (MH). The polymer can be visualized as a composite material consisting of hard and weak elements. The former, the lamellae, are considered to consist of mosaic blocks with liquid-like lattice distortions (paracrystallites). The latter are the interlamellar amorphous regions and the mosaic block lateral grain boundaries. The deformation mechanisms beneath the indenter are discussed in the light of current models of plastic deformation. MH is shown to depend on the packing density of the macromolecules in both phases and, as a result, it can be clearly correlated with the macroscopic density of the material. The unit cell expansion and lattice distortions increase in parallel as a consequence of increasing incorporation of chain defects within the lattice. This provokes a conspicuous decrease in the microhardness of the crystals. The increase in lattice distortions is consistent with the concurrent decrease of lamellar thickness and, hence, of the coherently diffracting lattice volume. These results unambiguously emphasize the physical significance of the mosaic block character of the lamellae in determining the micromechanical properties of the material. Finally it is shown that the strain boundary which defines the zone of crystal destruction under the indenter also depends on the average volume of the paracrystallites and on the volume fraction of crystalline material.     

Thermally activated rotation of Co1−xO particles within zirconia grains

Yttria partially stabilized zirconia (Y-PSZ) and Co_1−xO powders in 4:1 molar ratio were sintered and then annealed at 1300 and 1600°C to investigate the orientation change of Co_1−xO particles within Y-PSZ grains. Transmission electron microscopic observations indicated the Co_1−xO particles remained nonepitaxy in Y-PSZ grains after annealing at 1300°C for 300 h. When fired at 1600°C for 1–100 h, submicro-sized Co_1−xO particles (denoted as C) reached parallel epitaxy relationship, i.e. [100]_C//[100]_Z, [010]_C//[010]_Z and another relationship, i.e. [111]_C//[100]_Z, //[011]_Z with respect to the host zirconia grain (denoted as Z) nearly free of tetragonal precipitates. On the other hand, larger intragranular Co_1−xO particles (>1 μm in diameter) failed to reach epitaxial orientations even subject to prolonged annealing (100 h) at 1600°C. The temperature and size dependence of orientation change of the intragranular particle is in accordance with theoretical consideration of Brownian type rotation of the particle above a critical temperature for anchorage release at interface.     

Anomalous thermal expansion of silver chlorate in the tetragonal and cubic phases

Silver chlorate (AgClO₃) undergoes a phase transition from a tetragonal to a cubic phase at 139° C. The temperature dependence of lattice parameters and the coefficient of thermal expansion in the tetragonal phase between 23° C and 130° C and in the cubic phase between 140° C and 184° C have been investigated. The lattice parameters in the tetragonal phase and the lattice parameter in the cubic phase increase with temperature. In the tetragonal phase the coefficient of expansion ( _c) increases with temperature whereas the coefficient of expansion along a perpendicular direction ( _a) decreases with increasing temperature. In the cubic phase the coefficient of expansion shows a pronounced decrease with increasing temperature. These results have been discussed in the light of the known structure.     

Transmission electron microscopic observation of a metastable phase on the thermal decomposition process of Ca-deficient hydroxyapatite

Calcium-deficient hydroxyapatite (Ca-def HAp) decomposes to stoichiometric hydroxyapatite (HAp) and β-tricalcium phosphate (β-TCP) at high temperature. In a previous study, we reported that a metastable phase with a high Ca/P molar ratio appeared in the temperature range from 700 to 800°C. In the present study, the formation process of a metastable phase and the crystallographic relationship between the Ca-rich metastable phase and HAp matrix were investigated by high-resolution transmission electron microscopy (HRTEM). Ca-def HAp was annealed at 600–850°C for 2 or 6 h in air. TEM observations were performed before and after annealing Ca-def HAp. Based on analysis of image of Ca-def HAp before annealing, several HAp crystals with different aspect ratios agglomerated. The metastable phases grew thicker by long-term annealing. HRTEM image suggested that the Ca-rich metastable phase was formed by migration to the interface and continuous accumulation of calcium ions from HAp crystals with a small aspect ratio. From HRTEM images and results of the analysis of selected area electron diffraction patterns along the [010], [110] and [001] zone axes, lattice constants of the metastable phases were determined to be a = 2.86 nm, b = 0.94 nm, and c = 0.69 nm with orthorhombic crystals system.     

Orientation and temperature dependence of the tensile behavior of GaN nanowires: an atomistic study

Gallium nitride (GaN) is a high-temperature semiconductor material of considerable interest. It emits brilliant light and has been considered as a key material for the next generation of high frequency and high power transistors that are capable of operating at high temperatures. Due to its anisotropic and polar nature, GaN exhibits direction-dependent properties. Growth directions along [001], [1?10] and [110] directions have all been synthesized experimentally. In this work, molecular dynamics simulations are carried out to characterize the mechanical properties of GaN nanowires with different orientations at different temperatures. The simulation results reveal that the nanowires with different growth orientations exhibit distinct deformation behavior under tensile loading. The nanowires exhibit ductility at high deformation temperatures and brittleness at lower temperature. The brittle to ductile transition (BDT) was observed in the nanowires grown along the [001] direction. The nanowires grown along the [110] direction slip in the {010} planes, whereas the nanowires grown along the [1?10] direction fracture in a cleavage manner under tensile loading.     

Theoretical investigation on the stability,mechanical and thermal properties of the newly discovered MAB phase Cr_4AlB_4

The nanolaminated MAB phases have attracted great research interests in recent years due to their similarities to MAX phases, which display both metallic and ceramic-like properties. In the present work, a newly discovered MAB phase Cr_4AlB_4 was investigated by first principles calculations. Energy evaluations indicate that Cr_4AlB_4 can be synthetized in Al lean condition, which can further transform to Cr_2AlB_2 in Al rich condition. The full set of elastic properties and their dependences on temperature, ideal strengths under different tensile and shear deformations, and thermal expansions of Cr_4AlB_4 were predicted. The results reveal that the properties of Cr_4AlB_4 are dominated by the layered crystal structure and weak bonding nature between Al and Cr_2B_2 layers, including low elastic stiffness and large thermal expansion along [010] direction(the stacking direction of Al and Cr_2B_2 layers), low shear resistances in(010) plane,and preferentially cleavage along and/or shear in(010) plane. Therefore, it suggests that Cr_4AlB_4 displays similar mechanical properties to MAX phases, including readily machinable, thermal shock resistant, and damage tolerant. In combination with the fact that Cr, Al and B all can form dense oxides to protect the material from further oxidation, Cr_4AlB_4 is regarded as a promising high temperature ceramic.     

EBSD characterization of an ECAP deformed Nb single crystal

A niobium single crystal was subjected to equal channel angular pressing (ECAP) at room temperature after orienting the crystal such that [1 −1 −1] ∥ ND, [0 1 −1] ∥ ED, and [−2 −1 −1] ∥ TD. Electron backscatter diffraction (EBSD) was used to characterize the microstructures both on the transverse and the longitudinal sections of the deformed sample. After one pass of ECAP the single crystal exhibits a group of homogeneously distributed large misorientation sheets and a well formed cell structure in the matrix. The traces of the large misorientation sheets match very well with the most favorably oriented slip plane and one of the slip directions is macroscopically aligned with the simple shear plane. The lattice rotation during deformation was quantitatively estimated through comparison of the orientations parallel to three macroscopic axes before and after deformation. An effort has been made to link the microstructure with the initial crystal orientation. Collinear slip systems are believed to be activated during deformation. The full constraints Taylor model was used to simulate the orientation evolution during ECAP. The result matched only partially with the experimental observation.     

The relationship between dislocations and molecular structure in aromatic crystals

Topographical studies of cleaved anthraquinone crystals provide evidence for twinning on (20¯1) in accordance with the predictions of the sense of angle (SA) mechanism. Supplementary studies on etched, as-grown and cleaved crystals of anthraquinone and p-terphenyl, and a comparison of the results with the available data on anthracene, reveal that the dislocation arrangements in the three isostructural solids can be interpreted in terms of the relative dimensions of the unit cells. In anthraquinone, the (100) [010] slip system is found to be of greater significance than the (001) [010] system, which is dominant in anthracene, whereas the near orthorhombic nature of the p-terphenyl structure results in slip being favoured on (1¯10). Twinning and glide are common to (20¯1) of anthraquinone, whereas glide alone has been observed on (20¯1) of anthracene and (201) of p-terphenyl. Possible partial dislocations and associated stacking faults are discussed.     

Grain orientation and electrical properties of Sr2Nb2O7 ceramics

Sr₂Nb₂O₇ ceramics sintered at 1753 K and the hot-pressed compacts annealed at 1073 K possess low density because of the formation of voids by large expansion. The expansion is attributed to cleavage and due to the preferential grain growth along the <010> direction. An increase in dielectric constant at temperatures over 973 K and a parabolic decrease of resistivity with increasing temperature are observed. The constant variation corresponds well to the decrease in thermal strain and the grain orientation factor along the <0K0> direction.     

Thermal expansion in Cr:LiSrGaF6

High-temperature behaviour of LiSrGaF₆ doped with 1.5 at.% of Cr³⁺ was studied with high-resolution synchrotron angle-dispersive X-ray powder diffraction in the temperature range 298-539 K. No phase transitions were detected. The origin of negative thermal expansion along the c axis is discussed based on the temperature dependencies of structural parameters and octahedral distortions obtained with the Rietveld method. The SrF₆ slab contracts with increasing temperatures because of the diminishing F-Sr-F octahedral angles without any changes in the F-F octahedral edges not only around strontium but also around lithium and gallium. At the same time, the angular distortions of the SrF₆ octahedra are largely diminished. Such a behaviour is discussed in comparison with the thermal expansion of LiCaAlF₆ and LiSrAlF₆.     

Deformation structure in ductile B2-type Zr–Co–Ni alloys with martensitic transformation

Microstructural evolution during tensile deformation in ternary Zr–Co–Ni alloys were investigated using transmission electron microscopy to clarify the mechanism of the enhancement of ductility observed in these alloys. In Zr₅₀Co₃₉Ni₁₁ alloy deformed at room temperature, lenticular martensite is observed in the B2 parent phase immediately after yielding, in addition to dislocations with the <100>_B2-type Burgers vector. The orientation relationship between the B2 parent phase and B33 martensite is determined to be [001]_B2//[100]_B33, (010)_B2//(021)_B33, and (110)_B2//(010)_B33. A midrib-like contrast is observed at the center of the lenticular martensite variant, and it is found to be a (021)_B33 twin. A trace analysis indicates that this contrast is nearly parallel to the {100}_B2, which may correspond to the habit plane of the martensite. The martensite variants grow into the B2 parent phase along the {100}_B2 with increasing tensile loading, and then grid-shaped martensite variants are formed at the failure of the specimen. The martensite would be dominantly formed and grow in the regions where the stress concentration occurs during tensile deformation. It is likely that the plastic deformation mainly proceeds in the untransformed B2 parent phase because this martensite is harder than the B2 parent phase. Consequently, the authors conclude that the remarkable enhancement of ductility can be attributed to a transformation-induced plasticity associated with deformation-induced martensite.     

Thermal expansion of bismuth telluride

The results of X-ray and dilatometric measurements of the thermal expansion of bismuth telluride in the temperature range of 4.2–850 K have been critically analyzed. The joint statistical processing of the experimental data has been performed by the least squares method and the most reliable temperature dependences of the linear thermal expansion coefficients along the principal crystallographic axes α _a and α _c and the average linear coefficient [`(a)] _L\bar \alpha _L , as well as the density of Bi₂Te₃, have been recommended. The results indicate that the linear thermal expansion coefficients along the directions parallel and perpendicular to the cleavage planes decrease with an increase in the temperature in a narrow temperature range. At temperatures below 298 K, the character of the temperature dependences of the linear thermal expansion coefficients of Bi₂Te₃ has been analyzed in terms of the anharmonicity of the chemical bonding forces in the layer structure and anisotropy of the elastic constants. In terms of the deviations of the composition of the Bi₂Te₃ compound from the stoichiometric one and the real (defect) structure of the compound, a model has been proposed to explain the minimum in the (α _a T) dependence of Bi₂Te₃ near the melting temperature of bismuth. A method for calculating the temperature dependence of the linear thermal expansion coefficients of the anisotropic layer crystals using the data on the specific heat has been discussed, which provides good agreement of the calculated linear thermal expansion coefficients with the experimental data within the accuracy of the measurements of the linear thermal expansion coefficients.     

The Thermophysical Properties (Heat Capacity and Thermal Expansion) of Single-Crystal Silicon

Fragmentary investigations of the heat capacity and of the thermal expansion coefficient of single crystals of high-purity silicon are reported. The results of these investigations are compared with the entire body of data on these properties available to date. Generalized equations expressing the heat capacity and thermal expansion coefficient of silicon as functions of temperature are obtained for the temperature ranges of 298–1690 and 100–1400 K, respectively. The Debye temperature of crystalline silicon and the root-mean-square dynamic displacement of atoms from the equilibrium position in its crystal lattice are calculated using the available data on thermal expansion.     

Thermal expansion of Bi2.2Sr1.8CaCu2Ox superconductor single crystals

Using a low-temperature stage X-ray diffractometer, we have investigated the temperature dependence of the lattice parameters of single crystal superconductors Bi₂Sr_1.8CaCu₂O_x. The experimental results show that (i) the lattice constants increase linearly with increasing temperature above about 100 K; (ii) thec-axis lattice parameter shows a kink in the superconducting transition region, while thea, b-axis parameters do not show any anomalous behavior in this region; (iii) botha, b, andc show negative thermal expansion coefficients below 40–50 K, which may be related to the characteristics of the Cu-O bond.     

热电材料CoSi和CrSi_2的晶格热膨胀性

利用动态高温X射线衍射技术分别对立方CoSi和六方CrSi2化合物在298~973 K温度范围内的晶格热膨胀性进行了研究。结果表明:化合物CoSi的点阵参数随温度升高呈线性增长关系,其平均线热膨胀系数aα和平均体热膨胀系数αV分别为1.14×10-5K-1和3.42×10-5K-1,两者之间符合立方晶系关系式,即3αa=αV;化合物CrSi2的点阵参数随温度升高而显著增大,其中沿a轴和c轴的平均线热膨胀系数及平均体热膨胀系数分别为αa=0.96×10-5K-1,cα=0.73×10-6K-1和αV=2.45×10-5K-1,三者之间符合六方晶系关系式,即2αa+cα=αV;化合物CrSi2沿a轴方向的线热膨胀系数远大于沿c轴方向的,呈较强的各向异性。