Showing papers on "Thermal expansion published in 1998"

Phonon density of states and negative thermal expansion in ZrW2O8

Abstract: Thermal expansion of solids arises from anharmonic lattice dynamics. The contrasting phenomenon of negative thermal expansion (NTE)—where expansion occurs on cooling rather than heating—was discovered1 in ZrW2O8 in 1968. Recently, this material has attracted interest in the context of NTE for several reasons: the magnitude of the effect is relatively large (−9 p.p.m. K−1); the temperature range over which NTE occurs is also large (from close to absolute zero up to the decomposition temperature of about 1,050 K); and the NTE effect is isotropic2, evidenced by the fact that ZrW2O8 remains cubic at all temperatures. These characteristics make ZrW2O8 an important system in which to study unusual lattice dynamics of this type, and potentially well suited for application in composite materials with an engineered thermal expansion coefficient3. Here we report neutron-scattering measurements of ZrW2O8 that allow us to investigate its phonon spectrum, and hence determine the energy scale for the lattice motions governing NTE. We find that NTE can be modelled by several low-energy phonon modes, suggesting that the effect arises from the unusual crystal structure of ZrW2O8, which supports highly anharmonic vibrational modes.

Pressure-Induced Amorphization and Negative Thermal Expansion in ZrW2O8

Abstract: It has recently been shown that zirconium tungstate (ZrW2O8) exhibits isotropic negative thermal expansion over its entire temperature range of stability. This rather unusual behavior makes this compound particularly suitable for testing model predictions of a connection between negative thermal expansion and pressure-induced amorphization. High-pressure x-ray diffraction and Raman scattering experiments showed that ZrW2O8 becomes progressively amorphous from 1.5 to 3.5 gigapascals. The amorphous phase was retained after pressure release, but the original crystalline phase returned after annealing at 923 kelvin. The results indicate a general trend between negative thermal expansion and pressure-induced amorphization in highly flexible framework structures.

Simple Formulas for Thermophysical Properties of Liquid Water for Heat Transfer Calculations (from 0°C to 150°C)

Abstract: In this communication, simple formulas (based on the latest experimental tabulated data) for 11 physical properties of liquid ordinary water substance at saturation state—saturation pressure, density, volumetric thermal expansion coefficient, specific volume of saturated vapor, specific enthalpy, specific heal, latent heal of vaporization, thermal conductivity, dynamic viscosity, Prandtl number, and surface tension as a function of temperature (from 0 to 150°C)—which are used in heat transfer calculations for heat exchangers in heating systems and also for many other technological applications, are presented. Also, the uncertainties of these formulas are given. In most practical cases the pressure of liquid water is within the range from 1 to 10 absolute bar, which makes it possible to neglect the effect of pressure. All properties of saturated liquid water calculated with the recommended formulas are tabulated with a temperature increment of 5°.

Isotropic negative thermal expansion

Abstract: ▪ Abstract Materials that contract on heating are unusual and have important applications. Materials showing such negative thermal expansion behavior are usually anisotropic and usually exhibit this behavior over only a small temperature range. The zirconium tungstate family is unique in showing strong negative thermal expansion over a broad temperature range.

Thermal Expansion of Nickel‐Zirconia Anodes in Solid Oxide Fuel Cells during Fabrication and Operation

Abstract: The thermal expansion of NiO-8 mol % Y{sub 2}O{sub 3}-stabilized ZrO{sub 2} (YSZ) composites and Ni-YSZ cermets in air and hydrogen have been investigated in the temperature range from 50 to 1,000 C. The average linear thermal expansion coefficient (TEC) of NiO-YSZ composites in air increased with NiO content over the entire composition range. While NiO in the composites was changed to Ni in the H{sub 2} stream, their expansions were governed by the reduction of NiO. For reduced, Ni-YSZ cermets, the TEC increases significantly with Ni content in the composition range > 60 vol% Ni. The TEC increased gradually during repeated thermal cycles between room temperature and 1,000 C. When cermets were measured in air, the Ni particles were fully oxidized to NiO above 900 C, and many cracks appeared in the samples.

Singularity-free interpretation of the thermodynamics of supercooled water. II. Thermal and volumetric behavior

Abstract: According to the singularity-free interpretation of the thermodynamics of supercooled water, the isothermal compressibility, isobaric heat capacity, and the magnitude of the thermal expansion coefficient increase sharply upon supercooling, but remain finite. No phase transition or critical point occurs at low temperatures. Instead, there is a pronounced but continuous increase in volume and a corresponding decrease in entropy at low temperatures, the sharpness of which becomes more pronounced the lower the temperature and the higher the pressure. We investigate the behavior of the response functions, equation of state, and entropy of a schematic waterlike model that exhibits singularity-free behavior, and thereby illustrate the simplest thermodynamically consistent interpretation that is in accord with existing experimental evidence on water’s low-temperature anomalies. In spite of its simplicity, the model captures many nontrivial aspects of water’s thermodynamics semiquantitatively.

Synthesis and properties of the negative thermal expansion material cubic ZrMo2O8

Abstract: A new phase, cubic ZrMo2O8, has been prepared by the low-temperature dehydration of ZrMo2O7(OH)2·2H2O. This material displays isotropic negative linear thermal expansion (α = −5.0 × 10-6 K-1) over a large temperature range. Unlike the previously reported cubic ZrW2O8, it does not undergo any phase transformations on heating at atmospheric pressure, and it does not display any pressure-induced phase transformations below 0.6 GPa at room temperature.

Thermal expansion of LaAlO3 and (La,Sr)(Al,Ta)O3, substrate materials for superconducting thin-film device applications

Abstract: The thermal expansion for the perovskite (La,Sr)(Al,Ta)O3, i.e., LSAT, grown from the formulation 0.29(LaAlO3):0.35(Sr2AlTaO6), was determined by Rietveld refinement of neutron powder diffraction data over the temperature range of 15–1200 K. In comparison to LaAlO3 the relative volume thermal expansion is the same, although the cell volume of LSAT is slightly larger. Site occupation refinement for LSAT gives a structural formula of (La0.29(5)Sr0.71(5))A site(Al0.65(1)Ta0.35(1))B siteO3. At and below 150 K, LSAT shows a small distortion from cubic symmetry. Unlike the cubic-to-rhombohedral transition (800 K) observed in LaAlO3, the low temperature structural phase transition in LSAT appears to be cubic-to-tetragonal or cubic-to-orthorhombic. The rms displacement of the A site in LSAT is significantly larger than that for LaAlO3, and about half of the difference can be accounted for by a static displacement component.

Measurement of residual stress in plasma-sprayed metallic, ceramic and composite coatings

Abstract: Residual stresses in plasma-sprayed coatings were studied by three experimental techniques: curvature measurements, neutron diffraction and X-ray diffraction. Two distinct material classes were investigated: (1) single-material coatings (molybdenum) and (2) bi-material composites (nickel+alumina and NiCrAlY+yttria-stabilized zirconia), with and without graded layers. This paper deals with the effects of coating thickness and material properties on the evolution of residual stresses as a function of composition and thickness in both homogeneous and graded coatings. Mathematical analysis of the results allowed in some cases the separation of the quenching stress and thermal stress contributions to the final residual stress, as well as the determination of the through-thickness stress profile from measurements of different thickness specimens. In the ceramic–metal composites, it was found that the quenching stress plays a dominant role in the metallic phase, whereas the stress in the ceramic phase is mostly dominated by thermal mismatch. The respective thermal expansion coefficients and mechanical properties are the most important factors determining the stress sign and magnitude. The three residual stress measurement methods employed here were found to be complementary, in that each can provide unique information about the stress state. The most noteworthy outcomes are the determination of the through-thickness stress profile in graded coatings with high spatial resolution (curvature method) and determination of stress in each phase of a composite separately (neutron diffraction).

A miniature capacitance dilatometer for thermal expansion and magnetostriction

Abstract: A very small capacitive sensor for measuring thermal expansion and magnetostriction of small and irregular shaped samples has been developed. A capacitive method with tilted plates is used. The tilted plate capacitance formula is used for the calculation of the capacitor gap, the calibration is performed by measuring the signal of a standard material. The active length of the sample can be less than 1 mm. The absolute resolution is about 1 A. All mechanical connections of the dilatometer are carried out by tiny Cu–Be springs, enabling the small force on the sample to be adjusted (50–500 mN) and no additional sample fixing is necessary. The cell has been tested in the temperature range 0.3–200 K and in static magnetic fields up to 15 T. The zero signal of the dilatometer has been determined by measuring a silver sample. The correct operation and reproducibility has been verified by measuring the thermal expansion of Cu. The thermal expansion and magnetostriction of a DyCu2 single crystal has been determined. The advantage of this method compared to specific heat measurements is that a large temperature range can be covered with one equipment. This high static and dynamic range of sample length, temperature, and magnetic field suggests a number of possible applications, like the investigation of crystal field effects on the magnetoelastic properties of single crystals or structural phase transitions.

The properties and microstructure of Al-based composites reinforced with ceramic particles

Abstract: Aluminum (Al)-based composite materials which were produced using both powder metallurgical and vacuum plasma spraying (VPS) techniques are presented. The objective was to produce materials with low coefficients of thermal expansion, tailored to approach those of steel (≃13×10 −6 K −1 ), and to improve the mechanical properties of the matrix. Composite materials based on Al are used in different fields where weight and thermal stability are key requirements. Typical applications include aerospace components, electronic packaging, high precision instrumentation, and automobile engine components. The nature, size and the relative quantities of the different reinforcing phases were considered in calculations involving the optimization of the main characteristics of the composites. Fine dispersed powders of Si 3 N 4 , AlN, TiB 2 , B 4 C (5–10 μm) and 3Al 2 O 3 ·2SiO 2 (20–75 μm) were used as the strengthening phases, while pure Al, 6061 and Al–27Si–6Ni alloys were used as the matrix. The dimensional and relaxation stability was investigated for several of the extruded composites. The mechanical properties of the extruded composites and the VPS produced composites were tested. Through a combination of plastic deformation and heat treatment (annealing or quenching with ageing) major improvements in the mechanical properties of the VPS composites were obtained. Varying levels of improvement were seen in the ductility of the VPS composite materials (elongation values ranging from two to ten times greater than the as-sprayed values). The ultimate tensile strengths were also improved by 30–75% compared to the as-sprayed values.

The role of defects on thermophysical properties : thermal expansion of V, Nb, Ta, Mo and W

Abstract: Thermophysical properties at high temperatures and pressures are difficult to measure. Many reviews have approximated experimental data with empirical polynomial functions. In the case of thermal expansion and molar volume, extensive results for refractory body centered cubic (BCC) metals have been published. A critical evaluation of these experimental data is essential for many other studies. We provide this evaluation in terms of models that interrelate the thermophysical properties, self diffusion, and high temperature thermal defects. Experimental and theoretical methods for measuring and representing thermal expansion and the limitations of such methods are also briefly reviewed. Results for V, Nb, Ta, Mo, and W fall into two distinct subgroups relating to their elemental positions in the periodic table. The thermal expansions for these elements are analyzed within the constraints of a simple vibrational model and its equation of state. This approach represented the thermal expansion as the contributions from a perfect crystal and the crystal's high temperature anharmonicity as well as its thermal defects. Quantitative expressions, neglecting electronic contributions, are provided for the coefficient of thermal expansion and the expansivities for these five BCC metals from near 20°K to their melting temperatures. Vacancy formation enthalpies and entropies are also estimated. Our vacancy thermodynamic results are compared with earlier predictions and results from positron annihilation, thermal expansion, and specific heat measurements.

Exceptional Negative Thermal Expansion in AlPO4-17

Abstract: Anhydrous AlPO4-17 has been found to experience a very strong negative thermal expansion over the temperature range 18−300 K. Powder synchrotron X-ray diffraction data taken at six temperatures over this range show an essentially linear decrease in cell volume, with a linear coefficient of thermal expansion of −11.7 × 10-6 K-1 which is significantly more negative than previously reported for any material. The contraction of the structure along the a and b cell edges is considerably greater than along the c edge of this hexagonal structure. Rietveld refinements of the framework as a function of temperature suggest that the negative thermal expansion is related to the harmonic transverse vibrations of bridging oxygen atoms, which lead to dynamic rocking of the essentially rigid tetrahedral building blocks of the structure.

Molecular-dynamics simulation of structural and thermodynamic properties of boron nitride

Abstract: Structural and thermodynamic properties of cubic boron nitride (c-BN) under pressure and for varying temperature are studied by molecular-dynamics (MD) simulation with the use of a well-tested Tersoff potential. Various physical quantities including the thermal expansion coefficient and heat capacity are predicted. Our simulation is extended to study liquid boron nitride at various densities.

Negative thermal expansion in beta-quartz

Abstract: Computer modelling and theoretical analysis are used to explain the nearly zero and slightly negative coefficients of thermal expansion in β-quartz well above the α-β phase transition temperature. Quartz was selected for study as an archetypal material with a framework structure of stiff units, namely SiO4 tetrahedra, linked through shared oxygen atoms as very flexible hinges. The contributions of the soft mode, the Vallade mode, the TAz phonon branch and the phonon spectrum as a whole are discussed in detail. The results fully support and illustrate a recent theory of the negative contribution to thermal expansion in framework structures. It is a geometrical effect due to the rotation of the tetrahedral units, folding together as they vibrate. The very rapid increase in the lattice parameters for about 20 K above the transition temperature is well accounted for within quasiharmonic theory, and is therefore not evidence for critical fluctuations or fluctuating patches of α +, α − structure.

The temperature dependence of the local free volume in polyethylene and polytetrafluoroethylene: A positron lifetime study

Abstract: Positron lifetime measurements, performed in the temperature range 80-300 K, are reported for polyethylene (PE) and polytetrafluoroethylene (PTFE). The lifetime spectra have been analyzed using the data processing routines LIFSPECFIT and MELT. Two long-lived components appear, which are attributed to pick-off annihilation of ortho-positronium in crystalline regions and at holes in the amorphous phase. The ortho-positronium lifetimes, T 3 and T 4 , are used to estimate the crystalline packing density and the size of local free volumes in the crystalline and amorphous phases. The interstitial free volume in the crystals exhibits a weak linear increase with the temperature which is attributed to thermal expansion of the crystal unit cell. In the amorphous phase, the hole volume varies between 0.053 and 0.188 nm 3 (PE) and between 0.152 and 0.372 nm 3 (PTFE). Its temperature variation may be fitted by two straight lines, the intersection of which is used to estimate a glass transition temperature of T g = 195 K for both PE and PTFE. The slopes of the free volume in the glassy and crystalline phases with the temperature correlate well with each other. The coefficients of thermal expansion of the hole volume are compared with the macroscopic volume change below and above the glass transition. From this comparison a fractional hole volume at T g of 4.5 (PE) and 5.7% (PTFE) and a number of 0.73 (PE) and 0.36 (PTFE) x 10 27 holes/m 3 is estimated. Finally, it is found that the intensity of o-Ps annihilation in crystals shows a different temperature dependence to that in the amorphous phase.

First-principles theory of vibrational effects on the phase stability of Cu-Au compounds and alloys

Abstract: The importance of vibrational effects on the phase stability of Cu-Au alloys is investigated via a combination of first-principles linear response calculations and a statistical mechanics cluster expansion method. We find that (i) the logarithmic average of the phonon density of states in ordered compounds is lower than in the pure constituents, thus leading to positive vibrational entropies of formation and to negative free energies of formation, stabilizing the compounds and alloys with respect to the phase separated state. (ii) The vibrational free energy is lower in the configurationally random alloy than in ordered ground states, which leads to lower order-disorder transition temperatures. (iii) The random alloys have larger thermal expansion coefficients than ordered ground states, and therefore the vibrational entropy difference between the random and ordered states is a strongly increasing function of temperature. However, (iv) due to the associated increase in the static internal energy, the effect of thermal expansion on the free energy (and thus on the phase diagram) is only half that of the entropy alone.

Bulk Amorphous Alloys : preparation and Fundamental Characteristics

Abstract: 1. History of Bulk Amorphous Alloys 2. High Glass-Forming Ability and its Dominant Factors 3. Compositional Dependence of the Dominant Factors for GlassFormation, Tx and Tg/Tm. 3.1. Ln-Al-(Ni,Cu) Systems 3.2. Mg-Ln-(Ni,Cu) Systems 3.3. Zr-Al-TM Systems 3.4. Pd-Cu-Ni-P System 4. Continuous Cooling Transformation Behavior of SupercooledLiquid and Critical Cooling Rate for Glass Formation 5. Preparation Methods of Bulk Amorphous Alloys. 5.1. High-Pressure Die Casting Method. 5.2. Water Quenching Method 5.3. Copper Mold Casting Method 5.4. Arc Melting Method 5.5. Unidirectional Zone Melting Method 5.6. Suction Casting Method 6. Compositional Dependence of Maximum Sample Thickness (tmax). 6.1. La-Al-(Co,Ni,Cu) Systems 6.2. Zr-Al-(Co,Ni,Cu) Systems 6.3. Zr-Al-Ni-Cu-TM (TM = Transition Metals) Systems 7. Structure of Bulk Amorphous Alloys. 7.1. Mg-La-Ni System 7.2. La-Al-Ni System 7.3. Zr-Al-Ni System 7.4. Zr-Y-Al-Ni System 8. Structural Relaxation and Thermal Stability. 8.1. Structural Relaxation 8.2. Glass Transition 8.3. Two-Stage Glass Transition 8.4. Crystallization Behavior 9. Physical Properties. 9.1. Density 9.2. Specific Heat 9.3. Viscosity 9.4. Atomic Diffusivity 9.5. Electrical Resistivity 9.6. Thermal Expansion

Investigation of Thermal High Cycle and Low Cycle Fatigue Mechanisms of Thick Thermal Barrier Coatings

Abstract: Ceramic thermal barrier coatings have attracted increased attention for diesel engine applications. The advantages of using the ceramic coatings include a potential increase in efficiency and power density and a decrease in maintenance cost. Zirconia-based ceramics are the most important coating materials for such applications because of their low thermal conductivity, relatively high thermal expansivity and excellent mechanical properties. However, durability of thick thermal barrier coatings (TBCS) under severe temperature cycling encountered in engine conditions, remains a major question. The thermal transients associated with the start/stop and no-load/full-load engine cycle, and with the in-cylinder combustion process, generate thermal low cycle fatigue (LCF) and thermal high cycle fatigue (HCF) in the coating system. Therefore, the failure mechanisms of thick TBCs are expected to be quite different from those of thin TBCs under these temperature transients. The coating failure is related not only to thermal expansion mismatch and oxidation of the bond coats and substrates, but also to the steep thermal stress gradients induced in the coating systems. Although it has been reported that stresses generated by thermal transients can initiate surface and interface cracks in a coating system, the mechanisms of the crack propagation and of coating failure under the complex LCF and HCF conditions are still not understood. In this paper, the thermal fatigue behavior of an yttria partially stabilized zirconia coating system under simulated LCF and HCF engine conditions is investigated. The effects of LCF and HCF on surface crack initiation and propagation are also discussed.

Lattice model calculation of the strain energy density and other properties of crystalline licoo2

Abstract: The strain energy densities for various crystalline planes of LiCoO2 were calculated from the stiffness tensors obtained from lattice model calculations using the program GULP. In addition to Coulomb and Buckingham potentials, it was necessary to include shell models for the oxygen and cobalt ions in order to obtain acceptable agreement between the observed and calculated structural parameters and high frequency dielectric constant. The strain energy densities u due to differential thermal expansion were calculated using the theoretical stiffness tensors and estimated values for the thermal expansion coefficients of LiCoO2. For a temperature change of 675 °C, these ranged from 0.5 to 1.3×108 erg/cm3 or 5 to 13 J/m2 for 1-μm-thick films on alumina substrates. In particular, the energies for the (003), (101), and (104) planes were ordered as u(003)≫u(104)>u(101). This suggests that the strong (101) preferred orientation of LiCoO2 films (⩾1 μm thick) is due to the tendency to minimize volume strain energy that arises from differential thermal expansion between the film and the substrate. Additional properties obtained from the GULP calculations include the free energy, heat capacity, and the k=0 vibrational modes.

Combustion synthesis of aluminium titanate

Abstract: Initial interest in aluminium titanate was due to its low thermal expansion coefficient and high thermal shock resistance, but further research was soon discouraged following the discovery of the expansion anisotropy and the instability of the compound over a specific range of temperatures. The development of a suitable active precursor powder could provide a possible solution to the fabrication difficulties (microcracking and decomposition). The scarce available thermodynamic data for the formation of Al2TiO5from its constituent oxides indicate that the reaction is endothermic and only possible at high temperature because of the titanate being entropy stabilised. The present work describes a straightforward combustion synthesis technique to prepare submicron Al2TiO5 powders, using the corresponding metal precursors-urea mixtures, at low temperature and short reaction times. A thermodynamic interpretation of the reaction is provided and the characteristics of the powder produced, like morphology, specific surface area and grain size, are discussed. The thermal behaviour of the combustion powder is compared with that of Al2TiO5 produced via the conventional ceramic solid state route.