Average Linear Expansion Coefficient Research Articles

Pressure dependence of phase transformation, thermal expansion and barocaloric property in a polycrystalline Ni54Mn23Ga23 alloy

An experimental investigation was conducted to understand the influence of hydrostatic pressure on phase transition, thermal expansion and barocaloric property of a polycrystalline Ni54Mn23Ga23 alloy. Two pressure ranges of 0–1.28 GPa and 0–0.74 GPa were selected for thermal expansion and barocaloric property measurements due to the different test limits of instrumentals. The pressure dependence of fourteen parameters was studied systematically. It was found that the sample experiences a martensitic transformation (MT) near room temperature and the response of MT peak temperature (TM) to pressure is about 15.89 K/GPa. The phase transition strain (λtr), transformation width (ΔTtr) relative volume change (|ΔV/V|) and maximum adiabatic temperature change (ΔTad)max increase linearly with pressure and their shifting rates are about 0.071 %/GPa, 7.5 K/GPa, 0.211 %/GPa and 12.2 K/GPa, respectively. Simultaneously, applying hydrostatic pressure can also enhance the barocaloric property of the sample, leading to a large increase in the absolute value of the maximum entropy change (|ΔS|max), the refrigerating capacity (RC) and their reversible values. On cooling, the sensitivities of the reversible RC and its proportion to pressure are around 185.52 J/kg GPa and 16.7 %/GPa, respectively. Finally, the average linear expansion coefficient (αave) of both martensite and austenite phases keeps unchanged while in the MT region it decreases from -99.84×10–6/K to -86.04×10–6/K with increasing pressure from 0 to 1.28 GPa. These findings indicate that applying hydrostatic pressure can effectively adjust the phase transformation, thermal expansion and barocaloric property of Ni-Mn-Ga alloys, which are meaningful for their application in solid-state refrigeration.

Effect of Cu Doping on Phase Transition, Thermal Strain, and Thermal Expansion Property in Polycrystalline Ni50Mn23−xCuxGa27 Alloys

The influence of Cu doping on phase transition, thermal strain, and thermal expansion property of polycrystalline Ni50Mn23−xCuxGa27(x = 1, 3, 5, 6 and 7) alloys is investigated. The results show that with increasing Cu concentration, martensitic transformation gradually gets close to Curie transition. A magnetostructural transition (MST) is observed in samples with x = 6 and 7. Such MST can enhance magnitude of transformation strain. Besides, Ni50Mn23−xCuxGa27 alloys possess an isotropic, recoverable, and thermally stable thermal strain. Excitedly, Cu doping can effectively tune the phase transition, thermal strain, and thermal expansion properties of Ni50Mn23−xCuxGa27 alloys. Adjustable coefficients of thermal expansion from positive to negative are obtained by Cu‐doped Ni50Mn23−xCuxGa27 alloys. The average linear expansion coefficient (α) between −140.44 and −207.70 ppm K−1 is observed in samples with x = 6 and 7 for a narrow temperature span of 11–14 K. When the temperature span is about 72 K (x = 6) and 73 K (x = 7), which is the largest temperature span observed in Ni–Mn–Ga‐based alloys recently, the α values decrease to about −36 ppm K−1. These findings are beneficial for manipulating the thermal expansion property of Ni–Mn–Ga–Cu alloys and their multifunctional applications.

Microscopic Formation Mechanism and Physical Properties of Mo/Cu Composites with High Densification

The formation mechanism and physical properties of high-densification Mo/Cu composites are studied by analyzing materials' microstructure, atom diffusion near the phase interface and physical properties. In the liquid phase sintering, the atomic diffusion occurs at the interface of molybdenum and copper, mainly the diffusion of copper atoms into molybdenum phase. Copper atoms in the material diffuse into the molybdenum phase to form a micron sized Cu-Mo solid solution, and no compound phase is found in the material structure, which forms a good interface bonding effect and makes it have high densification. The average linear expansion coefficients, thermal conductivities, electrical conductivities and tensile strengths of high-densification Mo/Cu composites with different copper content are linearly correlated with copper content. Mo80Cu20 is organized as a connected molybdenum skeleton and a small amount of copper phase in the voids. The tensile fracture of Mo80Cu20 is mainly exhibited as brittle fracture of the sintering neck of the molybdenum phase. The copper phase in Mo70Cu30, Mo60Cu40 or Mo50Cu50 is in a connected state, with plasticity significantly increased. Under the action of tensile stress, the ductile fracture of copper phase and the brittle fracture of sintering neck of the molybdenum phase occur simultaneously in these materials.

A simple method for preparing ThO2 ceramics with high density

AbstractA simple and effective method for preparing ThO2 ceramics with high density has been proposed. It mainly includes a positive precipitation approach and a conventional sintering process. The SEM images and laser particle size analysis show that the prepared powder exhibits a round‐plated shape with a uniform and narrow particle size distribution, which is the key to the densification sintering of ThO2 ceramics. The XRD analyses confirm that the ThO2 ceramics sintered at 1590°C were composed of cubic fluorite phase. The effect of sintering temperature on the density of ThO2 ceramics was also investigated, and a relative density higher than 95% were achieved after sintering at 1590°C for 6 h in an air atmosphere. Moreover, the Vickers hardness was up to 8.49 GPa, the average linear expansion coefficient within 40–800°C was low to 10.97 × 10−6/K, and the thermal conductivity and the thermal diffusivity were both higher than most of traditional ceramic materials (16.94–5.86 W/m K and 7.28–1.28 mm2/s, respectively, within the temperature range of 25–800°C). The previous great performances are closely related to its high density, which enables it to be a great candidate for nuclear fuels and other potential application fields.

Mechanical and thermal properties of Mo for packaging HgCdTe IRFPA detector at cryogenic temperatures

Molybdenum (Mo), as an electronic packaging material with excellent comprehensive performance, is widely used in the lead substrate and cold platform of HgCdTe infrared focal plane detector assembly. The thermodynamic parameters of molybdenum in the temperature ranging from 300 K down to 4.2 K are obtained by the thermodynamic tests in this paper. Mechanical properties and microstructure of Mo at 300 K, 77 K and 4.2 K are analyzed with the use of tensile tester and scanning electron microscope. Results show that: the average linear expansion coefficient of molybdenum between 300 K and 4.2 K is 3.21 × 10-6 /K, and molybdenum has the properties of high thermal conductivity and low specific heat capacity at cryogenic temperature. The elastic modulus of molybdenum increases, and the tensile strength and elongation decrease with decreasing temperature in the temperature ranging from 300 K down to 4.2 K. According to the macroscopic and microscopic morphology of tensile fracture, the fracture mode at 300 K is the ductile–brittle mixed fracture mode, and the material turns from ductile at 300 K to brittle at 77 K and 4.2 K. The fracture behaviors are transgranular fracture at 77 K and intergranular fracture at 4.2 K, respectively.

Effect of magnetic field on thermal strain, thermal expansion and negative thermal expansion property in a polycrystalline Ni55Mn18Ga27 alloy

The magnetic field effect on phase transition and thermal expansion properties were investigated in Ni55Mn18Ga27 alloy. The results show that the alloy has an excellent isotropy, reproducibility and thermostability in thermal strain and exhibits a large negative thermal expansion (NTE) effect in the direct and reverse martensitic transformations (MTs). For the direct MT, the average linear expansion coefficient α is about −225.21 ppm/K and the corresponding temperature span is around 8.2 K. Additionally, the sensitivities of the transformation strain and peak temperatures to field are about 88.5 ppm/T and 1.31 K/T, respectively. The rates of α and temperature spans of NTE to magnetic field are about 8.88 ppm/K T and 0.82 K/T, indicating that the magnetic field can improve the NTE property of the Ni–Mn-Ga alloy. Finally, the NTE mechanism has also been discussed. Our findings are beneficial to improving the thermal expansion and NTE properties of Ni–Mn-Ga alloys.

Research on Fundamental Characteristics of Nuclear Grade 316H Stainless Steel at Ultra High Temperature

A fundamental feature of the fourth-generation-reactor is that most of the designed operating temperature are between 500℃ to 800℃, while the traditional material system and data of the PWR are below 350℃, which cannot meet the requirements. In this paper, 316H is selected as the research object through demonstration and analysis, which is suitable for most reactors and closest to engineering application. The experimental study on mechanical properties, specific heat capacity, average linear expansion coefficient, intergranular corrosion characteristics and low cycle fatigue at 800℃ was carried out. the result shows that the measured data are significantly higher than the standard values. It is recommented that the temperature limit for the long-term operation shall not exceed 700℃, and the temperature limit for the short-term operation shall not exceed 800℃. This study provides a basis for the selection and evaluation of the structural materials of the fourth-generation-reactor.

Measurement and analysis of thermal expansion coefficients of mortar and limestone cooled to cryogenic temperatures

The force caused by the difference of thermal expansion coefficient between concrete aggregates at ultra-low temperature will inevitably affect the structure and should be considered in the structural design. The coefficient of thermal expansion of cement mortar and stone was measured by resistance strain gage test technology in ultra-low temperature environment, and the variation law of average linear expansion coefficient with temperature was obtained. The results show that the thermal expansion coefficients of cement mortar and stone have similar variation rules with the change of temperature. The stress state of cement mortar and stone under ultra-low temperature does not affect their thermal expansion coefficient. Thermal expansion coefficient of cement mortar under ultra-low temperature is affected by size effect.

Research on the Performance of Carbon Fiber Reinforced Inorganic Polymer Composite

This paper introduced a novel carbon fiber reinforced inorganic polymer non-sintering ceramic composite. It is composed of a low temperature curing material system, and can be manufactured by a traditional process for resin matrix composites. According to relevant test standards, the performances including the mechanical properties, the coefficient of linear expansion and the ablation ration were studied. The results show that the tensile strength is more than 150MPa, the bending strength is more than120MPa, the average coefficient of linear expansion is less than 0.213×10-6/°C(between room temperature and 800°C), the ablation ratio is less than 0.0715mm/s, the mass retention ratio after ablation at 1000°C for 600s is less than98%. The composite could be used for the micro ablative in large area thermal protection such as national defense, aircraft and so on.

Role of Anion Site Disorder in the Near Zero Thermal Expansion of Tantalum Oxyfluoride

Materials with the cubic ReO3-type structure are, in principle, excellent candidates for negative thermal expansion (NTE). However, many such materials, including TaO2F, do not display NTE. It is proposed that local distortions away from the ideal structure, associated with the need to accommodate the different bonding requirements of the disordered O/F, contribute to the occurrence of near zero thermal expansion rather than NTE. The local structure of TaO2F is poorly described by an ideal cubic ReO3-type model with O and F randomly distributed over the available anion sites. A supercell model featuring −Ta–O–Ta–O–Ta–F– chains along ⟨1 0 0⟩, with different Ta–O and Ta–F distances and O/F off-axis displacements, gives much better agreement with pair distribution functions (PDFs) derived from total X-ray scattering data for small separations (<8 Å). Analyses of PDFs derived from variable temperature measurements (80 to 487 K), over different length scales, indicate an average linear expansion coefficient of close to zero with similar contributions from the geometrically distinct Ta–O—Ta and Ta–F—Ta links in TaO2F.

Growth and thermal properties of tetragonal double tungstate KLa(WO4)2 crystal

An uncracked single crystal of double tungstate KLa(WO4)2 with the tetragonal scheelite structure has been successfully grown by the modified Czochralski method. For the first time, the thermal properties of this crystal, including the specific heat, thermal expansion, and thermal conductivity, were systematically investigated. The specific heat of the KLa(WO4)2 crystal changes slightly from 0.333Jg−1K−1 to 0.394Jg−1K−1 over the temperature range from 25°C to 500°C. The average linear expansion coefficients are α1=14.3×10−6K−1 and α3=29.3×10−6K−1. The anisotropy of the thermal expansion is explained from the point of view of crystal structure. The calculated thermal conductivities are 1.056Wm−1K−1 and 1.174Wm−1K−1 along the a- and c-axes, respectively. Compared with other double tungstate crystals, the KLa(WO4)2 crystal has a relatively low anisotropy of thermal expansion, as well as good thermal characteristics, and is suitable as a promising disordered laser host.

Effect of investment composition ratio for pure titanium crown and bridge on some mechanical properties of mould

To determine the optimal composition of a self-developing investment material by measuring physical and mechanical properties of mould. L(9) (3(4)) orthogonal design was adopted. One hundred and fifty specimens with the size of 80 mm × 20 mm × 20 mm were prepared to measure the atmospheric temperature bending strength, high temperature bending strength and residual bending strength. Nine specimens with the size of 5 mm diameter 25 mm heigh were prepared to survey the thermal expansion curve from ambient temperature to 1150°C. Strengths were greatly affected by fine powder proportion in refractory and water/powder ratio. When the content of fine powder was 35% and water/powder ratio was 1:7.5, adequate atmospheric temperature strength and high temperature strength could be achieved. Moreover, the residual strength was moderate. The thermal extension curves of specimens in experiment group were almost similar. And the average linear expansion coefficient was (4 ∼ 5) × 10(-6)/°C. The three kinds of bending strength of self-developing investment material are compared with commercialized investment material for titanium casting when water/powder ratio and the content of fine powder are carefully controlled.

Unusual Non-Bulk Properties in Nanoscale Materials: Thermal Metal−Metal Bond Contraction of γ-Alumina-Supported Pt Catalysts

Variable-temperature X-ray absorption spectroscopy measurements of sub-nanometer Pt nanoparticles on a high-surface-area gamma-alumina support reveal that the Pt-Pt bonds exhibit contraction upon heating [i.e., negative thermal expansion (NTE)]. Bare clusters under a He environment show an average linear expansion coefficient of -2.5 x 10-5 K-1 over the temperature range studied. Adsorption of hydrogen results in an overall bond relaxation and less dramatic Pt-Pt thermal bond-length contraction. From the temperature effect on bond length, disorder parameters, and the X-ray absorption near edge structure (XANES) spectra, temperature-dependent charge transfer between the support and the Pt clusters was concluded to be responsible for the observed behavior.

Phase transition and the soft lattice mode of a perovskite crystal studied by Raman scattering and thermal measurements

The paraelectric–ferroelectric structural transition of potassium lithium tantalite niobate has been studied by both Raman scattering and thermal measurements. A condensed soft lattice vibrational mode at the phase transition has been analyzed. It originates from the symmetric O2/O3–Nb/Ta–O3/O2 in-plane bending of the Nb/TaO6group. The soft optical phonon mode concerns the extraordinary transverse optical phonons propagating along the [110] direction. The thermal expansion experiments show a displacive phase transition and a big thermal contraction in thecdirection of the crystal, with an average linear expansion coefficient αc= −4.52 × 10−5K−1. The phase transition temperature and enthalpy are 358 K and 0.50 J/g, respectively. Curie temperature measured by four methods is within 353 and 360 K.

Thermal expansion in the F-doped Bi(Pb)-2:2:2:3 superconductor studied by positron annihilation.

Results of positron lifetime measurements as a function of temperature from 80 to 300 K for the F-doped Bi(Pb)-2:2:2:3 superconductor (${\mathit{T}}_{\mathit{c}}$=118 K) are presented. The lifetime ${\mathrm{\ensuremath{\tau}}}_{1}$ is about 200 ps and nearly temperature independent. The lifetime ${\mathrm{\ensuremath{\tau}}}_{2}$ fluctuates around 310 ps but its intensity ${\mathit{I}}_{2}$ increases slightly and the mean lifetime ${\mathrm{\ensuremath{\tau}}}_{\mathit{m}}$ increases continuously from 212 to 217 ps as the temperature is increased from 80 to 300 K. We discussed the possible effect of fluorine doping on the structural properties in this system and interpreted the temperature dependence of ${\mathrm{\ensuremath{\tau}}}_{\mathit{m}}$ in terms of a simple thermal expansion. An average linear expansion coefficient \ensuremath{\alpha}\ensuremath{\sim}35\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}6}$ for this system is obtained.

Structural characteristics of Bi-based superconductors studied by positron annihilation

We have measured the positron lifetime spectra as a function of temperature from 80 to 300 K and Doppler-broadening annihilation radiation (DBAR) spectra in normal and superconducting states for three kinds of Bi-based superconductors: Bi2212, Pb- and F-doped Bi2223. For Pb-doped Bi2223 superconductor, anomalies in the temperature dependence of positron lifetimes indicate the structural instabilities. However, a continuous increasing in the mean lifetime is observed for F-doped Bi2223 system and interpreted in terms of a simple thermal expansion with an average linear expansion coefficient alpha is similar to 35*10(-6). The difference spectra of DBAR spectra after deconvolution between two states show an evidence for charge transfer between the Cu-O and Bi-O planes in Bi-based superconductors. The role of oxygen defects in charge transfer was discussed.

Thermal Expansion Measurements on Thoria and Hafnia to 4500°F

Equipment for measuring thermal expansion to 4500°F is described. Linear thermal expansion to 4500°F was measured on thoria and hafnia. Average coefficient of linear expansion from 75° to 4500°F for thoria was 6.05 x 10‐3perOF; for stabilized hafnia with 10 wt% Y2O3 additions it was 6.33 × 10−6 per OF, and with 15 wt% Y2O3 additions it was 6.27 × 10−6 per °F.