Showing papers on "Thermal expansion published in 2007"

Electrical Conductivity and Thermal Expansion of Spinels at Elevated Temperatures

Abstract: Binary oxide spinels composed of Mg, Al, Cr, Mn, Fe, Co, Ni, Cu, and Zn were synthesized. Electrical conductivity was measured in air at 500°–800°C. Thermal expansion was measured from room temperature to 1000°C. Ferrite spinels have thermal expansion coefficients of 11–12 ppm/K, compared with 7–9 ppm/K for other spinels except Cu–Mn and Co–Mn which show anomalous behavior. The highest electrical conductivity among transition metal spinels was found for MnCo2O4 (60 S/cm at 800°C) and Cu1.3Mn1.7O4 (225 S/cm at 750°C).

Equations of state of MgO, Au, Pt, NaCl-B1, and NaCl-B2: Internally consistent high-temperature pressure scales

Abstract: Semiempirical equations of state (EoS) of Au, Pt, MgO, NaCl-B1, and NaCl-B2 based on expanded Mie–Gruneisen–Debye approach, which are consistent both with the Mie–Gruneisen–Bose–Einstein approach and the thermochemical, X-ray, ultrasonic and shock-wave data in a wide pressure-temperature range, have been constructed. It is shown that to determine the volume dependence of the Gruneisen parameter, not only shock-wave and static compression data, but also experimental information on heat capacity, bulk moduli, volume, and thermal expansion coefficient at zero pressure need to be taken into account. Intrinsic anharmonicity is of great importance at construction of EoS at high temperatures and x=V/V 0>1. Cross-comparison of the current equations of state with independent measurements shows that these EoS may be used as the internally consistent and independent pressure scales in a wide range of temperatures and pressures.

Resistance to Thermal Shock and to Oxidation of Metal Diborides–SiC Ceramics for Aerospace Application

Abstract: Two SiC-containing metal diborides materials, classified in the ultra-high-temperature ceramics (UHTCs) group, were fabricated by hot-pressing. SiC, sinterability apart, promoted resistance to oxidation of the diboride matrices. Both the compositions, oxidized in air at 1450°C for 1200 min, had mass gains lower than 5 mg/cm2. Slight deviations from parabolic oxidation kinetics were seen. The resistance to thermal shock (TSR) was studied through the method of the retained flexure strength after water quenching (20°C of bath temperature). Experimental data showed that the (ZrB2+HfB2)–SiC and the ZrB2–SiC materials retained more than 70% of their initial mean flexure strength for thermal quenchs not exceeding 475° and 385°C, respectively. Certain key TSR properties (i.e., fracture strength and toughness, elastic modulus, and thermal expansion coefficient) are very similar for the two compositions. The observed superior critical thermal shock of the (ZrB2+HfB2)–SiC composite was explained in terms of more favorable heat transfer parameters conditions that induce less severe thermal gradients across the specimens of small dimensions (i.e., bars 25 mm × 2.5 mm × 2 mm) during the quench down in water. The experimental TSRs are expected to approach the calculated R values (196° and 218°C for ZrB2+HfB2–SiC and ZrB2–SiC, respectively) as the specimen size increases.

Cellular solids with tunable positive or negative thermal expansion of unbounded magnitude

Abstract: Material microstructures are presented with a coefficient of thermal expansion larger in magnitude than that of either constituent. Thermal expansion can be large positive, zero, or large negative. Three-dimensional lattices with void space exceed two-phase bounds but obey three-phase bounds; lattices and normal materials have a trend of expansion decreasing with modulus. Two-phase composites with a negative stiffness phase exceed bounds that assume positive strain energy density. The author determined Young’s modulus and its relation to thermal expansion. Behavior of these composites is compared with that of homogeneous solids in expansion-modulus maps.

Effects of Doping on Thermal Conductivity of Pyrochlore Oxides for Advanced Thermal Barrier Coatings

Abstract: Pyrochlore oxides of general composition, A2B2O7, where A is a 3(+) cation (La to Lu) and B is a 4(+) cation (Zr, Hf, Ti, etc.) have high melting point, relatively high coefficient of thermal expansion, and low thermal conductivity which make them suitable for applications as high-temperature thermal barrier coatings. The effect of doping at the A site on the thermal conductivity of a pyrochlore oxide La2Zr2O7, has been investigated. Oxide powders of various compositions La2Zr2O7, La(1.7)Gd(0.3)Zr2O7, La(1.7)Yb(0.3)Zr2O7 and La(1.7)Gd(0.15)Yb(0.15)Zr2O7 were synthesized by the citric acid sol-gel method. These powders were hot pressed into discs and used for thermal conductivity measurements using a steady-state laser heat flux test technique. The rare earth oxide doped pyrochlores La(1.7)Gd(0.3)Zr2O7, La(1.7)Yb(0.3)Zr2O7 and La(1.7)Gd(0.15)Yb(0.15)Zr2O7 had lower thermal conductivity than the un-doped La2Zr2O7. The Gd2O3 and Yb2O3 co-doped composition showed the lowest thermal conductivity.

Mechanochemical mechanism for fast reaction of metastable intermolecular composites based on dispersion of liquid metal

Abstract: An unexpected mechanism for fast reaction of Alnanoparticles covered by a thin oxide shell during fast heating is proposed and justified theoretically and experimentally. For nanoparticles, the melting of Al occurs before the oxide fracture. The volume change due to melting induces pressures of 1–2 GPa and causes dynamic spallation of the shell. The unbalanced pressure between the Al core and the exposed surface creates an unloading wave with high tensile pressures resulting in dispersion of atomic scale liquid Al clusters. These clusters fly at high velocity and their reaction is not limited by diffusion (this is the opposite of traditional mechanisms for micron particles and for nanoparticles at slow heating). Physical parameters controlling the melt dispersion mechanism are found by our analysis. In addition to an explanation of the extremely short reaction time, the following correspondence between our theory and experiments are obtained: (a) For the particle radius below some critical value, the flame propagation rate and the ignition time delay are independent of the radius; (b) damage of the oxide shell suppresses the melt dispersion mechanism and promotes the traditional diffusive oxidation mechanism; (c) nanoflakes react more like micron size (rather than nanosize) spherical particles. The reasons why the melt dispersion mechanism cannot operate for the micron particles or slow heating of nanoparticles are determined. Methods to promote the melt dispersion mechanism, to expand it to micron particles, and to improve efficiency of energetic metastable intermolecular composites are formulated. In particular, the following could promote the melt dispersion mechanism in micron particles: (a) Increasing the temperature at which the initial oxide shell is formed; (b) creating initial porosity in the Al; (c) mixing of the Al with a material with a low (even negative) thermal expansion coefficient or with a phase transformation accompanied by a volume reduction; (d) alloying the Al to decrease the cavitationpressure; (e) mixing nano- and micron particles; and (f) introducing gasifying or explosive inclusions in any fuel and oxidizer. A similar mechanism is expected for nitridation and fluorination of Al and may also be tailored for Ti and Mg fuel.

An improved thermal model for machine tool bearings

Abstract: Thermal model for machine tool spindle is of great importance to machine tool design. Traditionally, the thermal contact resistance between solid joints and the change of the heat generation power with the bearing temperature are often ignored when thermal characteristics of a machine tool spindle are analyzed. This has caused inaccuracies in the thermal model. With the heat source models and the heat transfer models from Bossmanns and Tu [Journal of Manufacturing Science and Engineering 123 (2001) 495–501, International Journal of Machine Tools & Manufacture 39 (1995) 1345–1366], a model including the thermal contact resistance at solid joints based on a fractal model and the change of the heat generation power, viz. the amount of the heat generation per second, with the bearing temperature increases is developed. The complete thermal model is used to simulate the temperature distribution in grinding machine housing with a conventional spindle bearing. Compared with experiment, it is shown that the completed model is much more accurate than the traditional model which ignores the two important factors above. The thermal expansion of the housing system is analyzed.

Chemically Induced Expansion of La2NiO4+δ-Based Materials

Abstract: The equilibrium chemical strains induced by the oxygen hyperstoichiometry variations in mixed-conducting La2Ni1-xMxO4+δ (M = Fe, Co, Cu; x = 0−0.2) with K2NiF4-type structure, were studied by controlled-atmosphere dilatometry at 923−1223 K in the oxygen partial pressure range 5 × 10-4 to 0.7 atm. In combination with the oxygen content measured by coulometric titration and thermogravimetry, the results reveal a very low chemical expansivity, favorable for high-temperature electrochemical applications. Under oxidizing conditions, the isothermal expansion relative to atmospheric oxygen pressure (eC) is less than 0.02%. The ratio between these values and the corresponding nonstoichiometry increment varies from −3 × 10-3 to 6 × 10-3, which is much lower compared to most permeable mixed conductors derived from perovskite-like cobaltites and ferrites. Consequently, the chemical contribution to apparent thermal expansion coefficients at a fixed oxygen pressure, (13.7−15.1) × 10-6 K-1, does not exceed 5%. The high...

High Thermal Conductivity of Gallium Nitride (GaN) Crystals Grown by HVPE Process

Abstract: A relatively large sample of gallium nitride (GaN) was grown as a single crystal using the hydride vapor phase epitaxy (HVPE) process. The thermal diffusivity of the single crystal has been measured using a vertical-type laser flash method. The thermal expansion was measured using a dilatometer in order to estimate the thermal diffusivity with sufficient reliability. The effect of sample thickness and temperature on thermal diffusivity was evaluated. The specific heat capacity of GaN was also measured by using a differential scanning calorimeter. The thermal properties of single-crystal GaN have been compared with the measured thermal properties of single-crystal silicon carbide (SiC). The thermal conductivity of single-crystal GaN at room temperature is found to be 253 � 8:8% W/mK, which is approximately 60% of the value obtained for SiC. The excellent thermal property that is obtained in this study clearly indicates that GaN crystals are one of the promising materials for use in high-power-switching devices. [doi:10.2320/matertrans.MRP2007109]

Stress-induced large-area lift-off of crystalline Si films

Abstract: A new implantation-free lift-off process is presented. We deposit a layer with mismatched thermal expansion coefficient with respect to the substrate. Upon cooling, the differential contraction induces a large stress field which is released by the initiation and the propagation of a crack parallel to the surface. The principle is demonstrated on both single and multi-crystalline silicon. Films with an area of 25 cm2 and a thickness of 30–50 μm have been obtained. Some Si layers were further processed into solar cells. An energy conversion efficiency of 9.9% was reached on a 1 cm2 sample.

Thermal Expansion and Defect Chemistry of MgO-Doped Sm2Zr2O7

Abstract: A series of solid solutions of (Sm2 - xMgx)Zr2O7 - x/2 pyrochlores were prepared by a solid-state reaction and characterized by X-ray diffraction, high-temperature dilatometry, Raman spectroscopy, and X-ray photoelectron spectroscopy. X-ray diffraction and Raman spectroscopy reveal that MgO doping does not break down the pyrochlore structure of Sm2Zr2O7 for all of the samples, though it increases the degree of structure disorder. However, the thermal expansion coefficient is remarkably increased through doping up to x = 0.075 with a maximum value around 11.94 × 10-6 K-1 (room temperature to 1000 °C), which can mitigate the mismatches of thermal expansion in the high-temperature applications of Sm2Zr2O7. A new solid−solution mechanism that is different from previous research was proposed and confirmed by analyzing the variation of the lattice parameters, experimental density, and X-ray photoelectron spectroscopy of samples. It involves the transformation of the solid−solution model about Mg2+ interstitial ...

Residual stress and thermal cycling of planar solid oxide fuel cells

Abstract: The published literature relating to damage to planar solid oxide fuel cells caused by thermally induced stresses and thermal cycling is reviewed. This covers reported studies of thermal cycling performance and stresses induced by temperature gradients and differences in thermal expansion coefficients in typical planar SOFC configurations, namely electrolyte supported; anode supported and inert substrate supported cells. Generally good agreement is found between electrolyte residual stresses measured by X-ray diffraction or cell curvature and stresses calculated from simple thermo-elastic analysis. Finite element modelling of temperature distributions in cells and stacks in steady state operation are well advanced and capable of being extended to compute stress distributions. Failure criteria are then discussed for laminated cell structures based on critical energy release rate fracture mechanics models developed originally for coatings. However, in most cases the data required to apply the models...

Pressure enhancement of negative thermal expansion behavior and induced framework softening in zinc cyanide.

Abstract: The pressure-dependent structure and functionality of the coordination framework material zinc cyanide, Zn(CN)2, has been explored using in situ neutron powder diffraction. A third-order Birch−Murnaghan equation of state fit to variable pressure (0−0.6 GPa) data collected at ambient temperature (K0 = 34.19(21) GPa, K0‘ = −6.0(7)) shows that, contrary to behavior observed for typical materials, the Zn(CN)2 framework becomes more compressible at higher pressures. Variable temperature (50−300 K) data collected at 0.2 and 0.4 GPa indicate that the negative thermal expansion effect in Zn(CN)2 becomes more pronounced at pressure with the coefficient of thermal expansion (α = dT/𝓁d𝓁) varying by ca. −1 × 10-6 K-1 per 0.2 GPa applied pressure up to an average (50−300 K) value of −19.42(23) × 10-6 K-1 at 0.4 GPa. Both these unusual phenomena have been linked to increased framework flexibility at high pressure.

Structural and Electrical Characterization of the Novel SrCo0.9Sb0.1O3–δ Perovskite: Evaluation as a Solid Oxide Fuel Cell Cathode Material

Abstract: A novel perovskite oxide with the title composition has been prepared by soft-chemistry procedures followed by thermal treatments at 1000 °C. This polycrystalline sample has been characterized by temperature-dependent neutron powder diffraction (NPD), thermal analysis, electrical conductivity, and thermal expansion measurements, in order to evaluate its potential use as a mixed electronic-ionic conductor in intermediate-temperature solid oxide fuel cells (IT-SOFCs). At room temperature (RT), the sample adopts a tetragonal superstructure of perovskite with a = a 0 , c = 2a 0 (a 0 ≈ 3.9 A) defined in the P4/mmm space group. Co and Sb are distributed at random over the octahedral positions of the perovskite; flattened and elongated (Co,Sb)O 6 octahedra alternate along the c axis, sharing corners in a three-dimensional array (3C-like structure). The refinement of the oxygen occupancy factors yields the crystallographic formula SrCo 0.9 Sb 0.1 O 2.73(4) ; the oxygen vacancies are located at the equatorial 02 and 03 atoms, in alternating layers with different occupancy. 03 atoms exhibit, at RT, large thermal factors of 5.3 A 2 , suggesting a considerable mobility. This structure is stable up to 500 °C; between 500 and 700 °C, an order-disorder phase transition takes place to give a fully disordered simple-cubic perovskite with a = a 0 (space group Pm3m); this structure is shown to be stable up to 940 °C from NPD data. This is a second-order nonreconstructive transition, which is not observed at the differential thermal analysis curves, although it is probably responsible for a subtle change of slope at 650 °C in the thermal expansion curve. The thermal evolution of the electrical conductivity exhibits a maximum of 300 S·cm -1 at 400 °C; above this electronic transition, the conductivity regularly decreases, but it is still well above the required 100 S·cm -1 in the temperature region 650-850 °C corresponding to the working regime of a IT-SOFC.

Speeds of Sound, Densities, Isobaric Thermal Expansion, Compressibilities, and Internal Pressures of Heptan-1-ol, Octan-1-ol, Nonan-1-ol, and Decan-1-ol at Temperatures from (293 to 318) K and Pressures up to 100 MPa

Abstract: The speeds of sound in heptan-1-ol, octan-1-ol, and nonan-1-ol at pressures up to 101 MPa and in decan-1-ol at pressures up to 76 MPa have been measured within the temperature range of (293 to 318) K. The densities have been measured in the same temperature range under atmospheric pressure. The densities, isobaric heat capacities, isentropic and isothermal compressibilities, isobaric thermal expansions, and internal pressures as functions of temperature and pressure have been calculated using the experimental results and the literature isobaric heat capacities for the atmospheric pressure. A modified method, based on the suggestion of Davis and Gordon (J. Chem. Phys. 1967, 46, 2650−2660), has been applied. The effect of pressure and temperature on the isothermal compressibility, isobaric thermal expansion, isobaric heat capacity, and internal pressure is discussed.

Low Dielectric Loss Polytetrafluoroethylene/TeO2 Polymer Ceramic Composites

Abstract: TeO2 particle-filled PTFE composites were prepared by the powder processing technique. The structure and microstructure of the composites were investigated by X-ray diffraction and scanning electron microscopic methods. The effect of the ceramic content (0–0.6 volume fraction) of TeO2 on the dielectric properties of the composites was studied at 1 MHz and 7 GHz. The dielectric constant and dielectric loss increased with an increase in the TeO2 content. For 60 vol% of TeO2, the composite has a dielectric constant of 5.4 and a loss tangent of 0.006 at 7 GHz. The measured dielectric constant (er) is compared with the effective dielectric constant calculated using different theoretical models. The observed dielectric constants are in agreement with that calculated using effective medium theory. The coefficient of thermal expansion of the composites decreases with the TeO2 content, reaching a minimum of 32 ppm/1C for 60 vol% loading.

Equation of state of cubic boron nitride at high pressures and temperatures

Abstract: We report accurate measurements of the equation of state (EOS) of cubic boron nitride by x-ray diffraction up to 160 GPa at 295 K and 80 GPa in the range 500-900 K. Experiments were performed on single crystals embedded in a quasihydrostatic pressure medium (helium or neon). Comparison between the present EOS data at 295 K and literature allows us to critically review the recent calibrations of the ruby standard. The full P-V-T data set can be represented by a Mie-Grueneisen model, which enables us to extract all relevant thermodynamic parameters: bulk modulus and its first pressure derivative, thermal expansion coefficient, and thermal Grueneisen parameter and its volume dependence. This equation of state is used to determine the isothermal Grueneisen mode parameter of the Raman TO band. A formulation of the pressure scale based on this Raman mode, using physically constrained parameters, is deduced.

A system with adjustable positive or negative thermal expansion

Abstract: We analyse the anisotropic thermal expansion properties of a two-dimensional structurally rigid construct made from rods of different materials connected together through hinges to form triangular units. In particular, we show that this system may be made to exhibit negative thermal expansion coefficients along certain directions or thermal expansion coefficients that are even more positive than any of the component materials. The end product is a multifunctional system with tunable thermal properties that can be tailor-made for particular practical applications.

Nanostructured crystals: unique hybrid semiconductors exhibiting nearly zero and tunable uniaxial thermal expansion behavior.

Abstract: A unique family of II−VI based nanostructured inorganic−organic hybrid semiconductors exhibit nearly zero uniaxial thermal expansion in the temperature range of 95−295 K. Both their optoelectronic and thermal expansion properties are systematically tunable. The diamine molecules show a strong negative thermal expansion effect, and the extent of such an effect increases as the length of the organic molecules increases.