Showing papers on "Thermal expansion published in 2005"

Giant negative thermal expansion in Ge-doped anti-perovskite manganese nitrides

Abstract: We report the discovery of a large negative thermal expansion (NTE) up to α=−25×10−6K−1 (α: coefficient of linear thermal expansion) in Ge-doped anti-perovskite manganese nitrides Mn3AN (A=Cu,Zn,Ga). This gigantic NTE is several to ten times higher than that of commercially available NTE materials. The discontinuous lattice expansion seen in pure Mn3AN is broadened by Ge substitution over a wide temperature window, at widest ΔT∼100K, around room temperature. Such a large, isotropic and nonhysteretic NTE is desirable for practical applications.

Negative thermal expansion

Abstract: There has been substantial renewed interest in negative thermal expansion following the discovery that cubic ZrW2O8 contracts over a temperature range in excess of 1000 K. Substances of many different kinds show negative thermal expansion, especially at low temperatures. In this article we review the underlying thermodynamics, emphasizing the roles of thermal stress and elasticity. We also discuss vibrational and non-vibrational mechanisms operating on the atomic scale that are responsible for negative expansion, both isotropic and anisotropic, in a wide range of materials.

Measurement of thermo-optic properties of Y3Al5O12, Lu3Al5O12, YAIO3, LiYF4, LiLuF4, BaY2F8, KGd(WO4)2, and KY(WO4)2 laser crystals in the 80–300K temperature range

Abstract: Thermo-optic materials properties of laser host materials have been measured to enable solid-state laser performance modeling The thermo-optic properties include thermal diffusivity (β), specific heat at constant pressure (Cp), thermal conductivity (κ), coefficient of thermal expansion (α), thermal coefficient of the optical path length (γ) equal to (dO∕dT)∕L, and thermal coefficient of refractive index (dn∕dT) at 1064nm; O denotes the optical path length, which is equal to the product of the refractive index (n) and sample length (L) Thermal diffusivity and specific heat were measured using laser-flash method Thermal conductivity was deduced using measured values of β, Cp, and the density (ρ) Thermal expansion was measured using a Michelson laser interferometer Thermal coefficient of the optical path length was measured at 1064nm, using interference between light reflected from the front and rear facets of the sample Thermal coefficient of the refractive index was determined, using the measured val

Thermo-physical properties of plasma electrolytic oxide coatings on aluminium

Abstract: Plasma electrolytic oxide coatings appear to offer attractive combinations of hardness, wear resistance, corrosion resistance and interfacial adhesion. In order to optimise such characteristics, however, more basic thermo-physical property data are required, together with an understanding of how they are affected by processing conditions and microstructure. In the present study, coatings were produced on 6082 aluminium and characterised using profilometry, scanning electron microscopy, X-ray diffraction and nanoindentation. The in-plane thermal expansivity of detached coatings was measured by dilatometry to be about 8 microstrain K −1 . There is thus a rather substantial mismatch between the expansivities of coating and substrate, amounting to about 15 microstrain K −1 . The global in-plane Young's modulus was estimated using cantilever bending of sandwich coated substrates and also by measuring the curvature generated in a bi-material beam on cooling to low temperature. It was found to lie in the approximate range of 10–40 GPa. Values of this order, which are low compared with the figure of around 370 GPa expected for fully dense polycrystalline alumina, are thought to be associated with the presence of a network of microcracks and voids. A low value is expected to be beneficial in terms of conferring good strain tolerance, and hence resistance to spallation driven by differential thermal expansion.

Lattice thermal conductivity of silicon from empirical interatomic potentials

Abstract: We present calculations of the lattice thermal conductivity of silicon that incorporate several commonly used empirical models of the interatomic potential Second- and third-order force constants obtained from these potentials are used as inputs to an exact iterative solution of the inelastic phonon Boltzmann equation, which includes the anharmonic three-phonon scattering as well as isotopic defect and boundary scattering Comparison of the calculated lattice thermal conductivity with the experiment shows that none of these potentials provides satisfactory agreement Calculations of the mode Gr\"uneisen parameters and the linear thermal expansion coefficient help elucidate the reasons for this We also examine a set of parameters for one of these empirical potentials that produces improved agreement with both the measured lattice thermal conductivity and the thermal expansion data

Guest-Dependent Negative Thermal Expansion in Nanoporous Prussian Blue Analogues MIIPtIV(CN)6·x{H2O} (0 ≤ x ≤ 2; M = Zn, Cd)

Abstract: The guest-dependent thermal expansion behavior of the nanoporous Prussian Blue analogues MIIPtIV(CN)6.x{H2O} (0

Thermal and Chemical Expansion of Sr-Doped Lanthanum Cobalt Oxide (La1-xSrxCoO3-δ)

Abstract: The equilibrium thermal expansivity (βT) and oxygen-vacancy chemical expansivity (βC) of polycrystalline La1-xSrxCoO3-δ (LSC; x = 0.2, 0.4, and 0.7) have been measured at 600 °C < T < 900 °C and 10-4 atm < PO2 < 0.21 atm using controlled-atmosphere dilatometry. These measurements show only a moderate dependence of βT on temperature, suggesting that increases often observed in the coefficient of thermal expansion when measured at constant PO2 are primarily thermally induced chemical expansion associated with changes in oxygen stoichiometry. The dependences of βT and βC on the temperature, oxygen-vacancy concentration, and Sr content (x) were characterized and found to follow a consistent nonlinear trend, which may result from the relaxation of lattice strain with increasing defect concentration. A slowly relaxing secondary expansion effect (and/or expansion hysteresis) was also discovered. Possible causes of this behavior, including phase transition/segregation at high vacancy concentration, are discussed.

Direct Observation of a Transverse Vibrational Mechanism for Negative Thermal Expansion in Zn(CN) 2 : An Atomic Pair Distribution Function Analysis

Abstract: The instantaneous structure of the cyanide-bridged negative thermal expansion (NTE) material Zn(CN)2 has been probed using atomic pair distribution function (PDF) analysis of high energy X-ray scattering data (100−400 K) The temperature dependence of the atomic separations extracted from the PDFs indicates an increase of the average transverse displacement of the cyanide bridge from the line connecting the ZnII centers with increasing temperature This allows the contraction of non-nearest-neighbor Zn···Zn‘ and Zn···C/N distances despite the observed expansion of the individual direct Zn−C/N and C−N bonds Thus, this analysis provides definitive structural confirmation that an increase in the average displacement of bridging atoms is the origin of the NTE behavior The lattice parameters reveal a slight reduction in the NTE behavior at high temperature from a minimum coefficient of thermal expansion (α = dl/ldT) of −198 × 10-6 K-1 below 180 K, which is attributed to interaction between the doubly interp

An analytical model of rumpling in thermal barrier coatings

Abstract: Multilayer thermal barrier coatings (TBCs) deposited on superalloy turbine blades provide protection from combustion temperatures in excess of 1500 °C. One of the dominant failure modes comprises cracking from undulation growth, or rumpling, of the highly compressed oxide layer that grows between the ceramic top coat and the intermetallic bond coat. In this paper, a mechanistic model providing an analytical approximation of undulation growth is presented for realistic cyclic thermal histories. Thickening, lateral growth straining and high temperature yielding of the oxide layer are taken into account. Undulation growth in TBC systems is highly nonlinear and characterized by more than 20 material and geometric parameters, highlighting the importance of a robust yet computationally efficient model. At temperatures above 600 °C, the bond coat creeps. Thermal expansion mismatch occurs between the superalloy substrate and the oxide layer and, in some systems, the bond coat. In addition, some bond coats, such as PtNiAl, exhibit a martensitic phase transformation accompanied by nearly a 1% linear expansion, giving rise to a large effective mismatch. These two mismatches promote undulation growth. Nonlinear interaction between the stress in the bond coat induced by the constraining effect of the thick substrate and normal tractions applied at the surface of the bond coat by the compressed, undulating oxide layer produces an increment of undulation growth during each thermal cycle, before the stress decays by creep. A series of problems for systems without the ceramic top coat are used to elucidate the mechanics of undulation growth and to replicate trends observed in a series of experiments and in prior finite-element simulations. The model is employed to study for the first time the effect on undulation growth of a shift in the temperature range over which the transformation occurs, as well as the relative importance of the transformation compared to thermal expansion mismatch. The role of the top coat and other viable ways of reducing undulation growth are considered.

Crystal structure and thermal expansion of La1-xSrxFeO3-δ materials

Abstract: Crystal structure and thermal properties of La 1-x Sr x FeO 3-δ (x = 0, 0.1, 0.3, 0.4, 0.5, and 0.75) have been studied by high-temperature X-ray diffraction and thermal analysis in air and nitrogen (p(O 2 ) 10 -3 atm) atmosphere. The first-order phase transition from orthorhombic-to-rhombohedral La 1-x -Sr x FeO 3-δ (x = 0, 0.1) was strongly shifted to lower temperatures with increasing Sr content. The phase-transition temperature was observed significantly lower in polycrystalline ceramics compared with fine powders. The temperature depression of the phase transition in the ceramics was qualitatively explained by stresses induced both by the anisotropic thermal expansion of LaFeO3 and the observed volume contraction of the phase transition. Rhombohedral La 1-x Sr x -FeO 3-δ (x = 0.3, 0.4, 0.5) were observed to transform to the cubic perovskite structure during heating. The second-order phase-transition temperature decreased with increasing Sr content and decreasing partial pressure of oxygen. On the basis of the present findings, a pseudobinary phase diagram of the LaFeO 3 -SrFeO 3-δ system is presented. Finally, a severely nonlinear thermal expansion was observed for the Sr-rich materials at high temperature. The high thermal expansion in this region is due to a chemical expansion resulting from a reduction of the valence state of Fe.

Thermophysical properties and thermal cycling behavior of plasma sprayed thick thermal barrier coatings

Abstract: Thick yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs) of 1.5 mm thickness were produced using atmospheric plasma spraying (APS). The microstructures, thermophysical properties and failure behaviour of the coatings were studied. The coating sprayed at high substrate temperature contained a fine segmentation crack network consisting of circular segments with a diameter of 0.7 mm. This coating had a thermal conductivity of up to 1.75 W/mK at 1200 °C. The thermal expansion appeared to be insensitive to the coating microstructure, and a thermal expansion coefficient of 11.2 K−1 was determined. The segmentation crack network was stable during thermal shock testing, and high segmentation crack density gave rise to improve the thermal shock resistance. The coating with a segmentation crack density of 2.7 mm−1 had a lifetime of more than 1770 cycles at a surface temperature of 1238 °C and approximately 300 cycles at 1335 °C. Failure of the TBCs occurred by spalling and delamination cracking, while oxidation of the bond coat did not play a significant role as failure mechanism. Additionally, phase transformation of the YSZ coating was not observed.

Assessments of molar volume and thermal expansion for selected bcc, fcc and hcp metallic elements

Abstract: The molar volume and thermal expansion of selected metallic elements with the bcc, fcc and hcp structures were studied by means of the Calphad approach. Experimental data were critically assessed, and model parameters were obtained yielding reasonable descriptions of all experimental data on molar volume and thermal expansion. The descriptions are valid between room temperature and the melting points at atmospheric pressure.

Thermal and Electrical Properties of New Cathode Material Ba0.5Sr0.5Co0.8Fe0.2O3 − δ for Solid Oxide Fuel Cells

Abstract: A newly developed cathode material, Ba0.5Sr0.5Co0.8Fe0.2O3-delta, was synthesized and characterized. X-ray diffraction results identified it as a cubic perovskite structure. Thermogravimetric results showed that lattice oxygen loss was about 2% in weight between 50 and 1000 degrees C. Thermal expansion coefficient results in air and argon were extraordinarily high, with slope changes occurring at about 382 and 360 degrees C, respectively. Electrical conductivity values were sufficient for cathode application with the maximum of about 28.1 S cm(-1) at 500 degrees C in air and 17.3 S cm(-1) at 900 degrees C in N-2, correspondingly. Both thermal and electrical properties were closely associated with lattice oxygen behaviors and oxygen partial pressure. (c) 2005 The Electrochemical Society.

Spherical Aluminum Nitride Fillers for Heat‐Conducting Plastic Packages

Abstract: It is necessary for encapsulants to have not only a suitable coefficient of thermal expansion (CTE) compatible to IC devices and a low dielectric constant to reduce the device propagation delay, but also a high thermal conductivity to dissipate large amounts of heat from power-hungry, high-speed IC and high-density packages. Fillers such as silica have been mixed with polymers to improve their properties. Aluminum nitride (AlN) is considered as an alternative one, because it has a higher theoretical thermal conductivity of ∼320 W/mK 1 , a compatible CTE with silicon chips and a low dielectric constant. Commercial AlN fillers are angular in shape, because they are prepared via grinding coarse AlN powders synthesized by direct nitridation of aluminum metal and classification. The angular AlN are not expected to have high fluidity when mixed with polymers and hence low packing density. Recently, we successfully obtained single-crystalline spherical AlN fillers. Furthermore, polymer composites filled with the spherical AlN showed excellent thermal conductivity (>8 W/mK) as encapsulants for dissipating the heat generated in electronic devices.

Zirconium tungstate (ZrW2O8)/polyimide nanocomposites exhibiting reduced coefficient of thermal expansion

Abstract: ZrW2O8 is obtained as microparticulate powder following traditional methods or as nanoparticulate powder using an inverse micelle sol−gel synthesis strategy. As-prepared ZrW2O8 powders, surface-derivatized with (3-aminopropyl)siloxy linker molecules, disperse well in BTDA-ODA polyamic acid resins to give thermally cured ZrW2O8/polyimide hybrid films showing good wetting of embedded ceramic particles. Measured CTE values of ZrW2O8/BTDA-ODA hybrid films containing 0−50 wt % (0−22 vol %) ceramic loading show a controlled reduction in thermal expansion with increasing ceramic content. Reduction in CTE is modeled best assuming mixed-law behavior with incorporation of an interfacial phase region. A 22 vol % ceramic loading gives a 30% (10 ppm/K) reduction of CTE. ZrW2O8/BTDA-ODA films containing 0.8 vol % (3 wt %) ceramic loading exhibit unusually low thermal expansion, although the significance of this observation remains to be confirmed.

The thermal expansion of wood cellulose crystals

Abstract: We have investigated tension wood cellulose obtained from Populus maximowiczii using X-ray diffraction at temperatures from room temperature to 250 °C. Three equatorial and one meridional d-spacings showed a gradual linear increase with increasing temperature. For temperatures above 180 °C, however, the equatorial d-spacing increased dramatically. Thus, the linear and volume thermal expansion coefficients (TECs) below 180 °C were determined from the d-spacings. The linear TECs of the a-, b-, and c-axes were: α a = 13.6 × 10−5 °C−1, α b = −3.0× 10−5 °C−1, and α c =0.6× 10−5 °C−1, respectively, and the volume TEC was β = 11.1× 10−5 °C−1. The anisotropic thermal expansion in the three coordinate directions was closely related to the crystal structure of the wood cellulose, and it governed the macroscopic thermal behavior of solid wood.

Silicon Melt Infiltrated Ceramic Composites (HiPerComp

Abstract: Silicon melt infiltrated, SiC-based ceramic matrix composites (MI-CMCs) have been developed for use in gas turbine engines. These materials are particularly suited to use in gas turbines due to their low porosity, high thermal conductivity, low thermal expansion, high toughness and high matrix cracking stress. Several variations of the overall fabrication process for these materials are possible, but this paper will focus on “prepreg” and “slurry cast” MI-CMCs with particular reference to applications in power generation gas turbines. These composites have recently been commercialized under the name of HiPerComp™.

Structural anomalies associated with the electronic and spin transitions in LnCoO3

Abstract: A powder X-ray diffraction study, combined with the magnetic susceptibility and electric transport measurements, was performed on a series of LnCoO3 perovskites (Ln = Y, Dy, Gd, Sm, Nd, Pr and La) over a temperature range 100 - 1000 K. A non-standard temperature dependence of the observed thermal expansion was modelled as a sum of three contributions: (1) Weighted sum of lattice expansions of the cobaltite in the diamagnetic low spin state and in the intermediate (IS) or high (HS) spin state. (2) An anomalous expansion due to the increasing population of excited (IS or HS) states of Co3+ ions at the course of the diamagnetic-paramagnetic transition. (3) An anomalous expansion due to excitations of Co3+ ions to another paramagnetic state accompanied by an insulator-metal transition. The anomalous expansion is governed by parameters that are found to vary linearly with the Ln ionic radius. In the case of the first magnetic transition it is the energy splitting E between the ground low spin state and the excited state, presumably the intermediate spin state. The energy splitting E, determined by a fit of magnetic susceptibility, decreases with temperature. The values of E determined for LaCoO3 and YCoO3 at T = 0 K as 164 K and 2875 K, respectively, fall to zero at T = 230 K for LaCoO3 and 860 K for YCoO3. The second anomalous expansion connected with a simultaneous magnetic and insulator-metal transition is characterized by its center at T = 535 K for LaCoO3 and 800 K for YCoO3. The change of the unit cell volume during each transition is independent on the Ln cation and is about 1% in both cases.

Stiffness and thermal expansion of ZrB 2 : an ab initio study

Abstract: The stiffness and thermal expansion coefficient of ZrB2 are calculated within the density functional theory formalism The stiffness tensor obtained here using the static finite strain technique is in good agreement with the results of resonant ultrasonic measurements and points to a possible misinterpretation of the experimentally obtained compression data The methodology of evaluating thermal expansion coefficients from molecular dynamics simulations for small unit cells is validated for a number of systems: metals, semiconductors and insulators

System and method for providing thermal compensation for an interferometric modulator display

Abstract: Various embodiments of the invention relate to methods and systems for thermal compensation of a MEMS device. In certain embodiments, an interferometric modulator includes a first electrode and a flexible second electrode situated on a substrate. The flexible second electrode is a movable layer that can comprise aluminum or an aluminum-containing material, while the substrate can comprise glass. When the interferometric modulator undergoes a temperature change, the difference in thermal expansion rates results in a decrease in the tensile strain on the movable layer. Embodiments of the present invention provide a film configured to compensate for the thermal expansion. The film has a thermal expansion coefficient less than the substrate so as to compensate for expansion of the movable layer with respect to the substrate when the MEMS is exposed to thermal energy. The film compensates for mismatch in thermal expansion between the materials of the substrate and movable layer so as to inhibit undesirable optical characteristics.