Showing papers on "Thermal expansion published in 1993"

Relaxation mechanism of thermal stresses in the heterostructure of GaN grown on sapphire by vapor phase epitaxy

Abstract: Thermal strains and stresses due to the thermal expansion coefficient difference in GaN(0001)/α-Al2O3(0001) layered structures are studied by varying the film thickness of GaN from 0.6 to 1200 µm. The strain in GaN is greater in films of less than a few microns thickness. It is decreased in films of thickness from several to about a hundred microns, and is almost completely relaxed in those thicker than 100 µm. The stresses and strains in the heterostructure are calculated using a model in which relaxation due to cracking in the sapphire is considered. Three relaxation mechanisms of the thermal strain are found for different film thicknesses as follows: (a) only lattice deformation ( 20 µm).

Application of dynamic mechanical analysis for the study of the interfacial region in carbon fiber/epoxy composite materials

Abstract: The application of dynamic mechanical analysis (DMA) for quantifying interfacial interactions in composites is briefly reviewed. Carbon fiber/epoxy composites with fiber volume fractions of 12, 17, 38 and 61 vol% were subjected to flexural deformation on a Dupont DMA 983 instrument. The dependencies of dynamic mechanical properties of the composites on experimental parameters such as oscillation mode, amplitude, frequency, and temperature were investigate. As opposed to the storage modulus, the loss modulus is found to be sensitive to all parameters. In a fixed multiple frequency mode, the loss modulus of the composites increases with oscillation amplitude and decreases with frequency and the number of tests. The information produced in the resonant mode is more reproducible. An additional damping at the interfaces, apart from those of the constituents, suggests a poor interface adhesion in these composites. A linear relationship between the excess damping at the interfaces and the fiber volume fraction shows a similar interface quality for these composites having different fiber volume fractions. The detection of interfacial properities was found to be more sensitive in the flexural deformation mode than in the torsional mode. At temperatures higher than the glass transition temperature of the matrix, the effective volume fraction of the matrix is reduced. Such a reduction can be interpreted from the mismatch of thermal expansion of the matrix and the fibers.

Principles of particle selection for dispersion-strengthened copper

Abstract: A new fundamental approach to the design of high strength, high thermal conductivity dispersion-strengthened copper alloys for applications in actively cooled structures is developed. This concept is based on a consideration of the basic principles of thermodynamics, kinetics and mechanical properties. The design requirements for these materials include a uniform distribution of fine particles for creep and fatigue resistance, a high thermal conductivity, thermodynamic and chemical stability at temperatures up to 1300 K, a small difference in the coefficients of thermal expansion between the particle and matrix, and low particle coarsening rates at the processing and service temperatures. The theory for creep of dispersion-strengthened metals developed by Rosler and Arzt is used to predict the optimum particle size for a given service temperature and to illustrate the need for a high interfacial energy. Resistance to coarsening leads to a requirement for low diffusivity and solubility of particle constituent elements in the matrix. Based on the needs for a low difference in the coefficients of thermal expansion to minimize thermal-mechanical fatigue damage and low diffusivity and solubility of the constituent elements, several candidate ceramic phases are compared using a weighted property index scheme. The results of this quantitative comparison suggest that CeO2, MgO, CaO and possibly Y2O3 may be good candidates for the dispersed phase in a copper matrix.

Ellipsometric study of the glass transition and thermal expansion coefficients of thin polymer films

Abstract: The glass transition temperature (T g ) of thin polystyrene films (ca. 3000 A) cast on silicon wafers was determined by a new technique. An ellipsometer was used to determine the refractive index and thickness of the polystyrene films. T g was determined by measuring the temperature dependence of the refractive index. The change in thickness with temperature was used to calculate the linear and bulk thermal expansion coefficients of the material. A significant shift in T g , possibly due to strains induced in the cooled films, was observed between heating and cooling for polystyrene films

Thermal expansivity, bulk modulus, and melting curve of H2O–ice VII to 20 GPa

Abstract: Equation of state properties of ice VII and fluid H2O at high pressures and temperatures have been studied experimentally from 6 to 20 GPa and 300-700 K. The techniques involve direct measurements of the unit-cell volume of the solid using synchrotron X-ray diffraction with an externally heated diamond-anvil cell. The pressure dependencies of the volume and bulk modulus of ice VII at room temperature are in good agreement with previous synchrotron X-ray studies. The thermal expansivity was determined as a function of pressure and the results fit to a newly proposed phenomenological relation and to a Mie-Gruneisen equation of state formalism. The onset of melting of ice VII was determined directly by X-ray diffraction at a series of pressures and found to be in accord with previous volumetric determinations. Thermodynamic calculations based on the new data are performed to evaluate the range of validity of previously proposed equations of state for fluid water derived from static and shock-wave compression experiments and from simulations.

Au-In bonding below the eutectic temperature

Abstract: A bonding method using Au-In alloy which requires a low process temperature of 200 degrees C to produce high temperature (454 degrees C) bonds is reported. Multiple layers of Au and In are deposited on semiconductor wafers in one vacuum cycle to reduce In oxidation. The semiconductor dice are then bonded to substrates coated with Au. Above 157 degrees C, the indium layer melts and dissolves the Au layers to form a mixture of liquid and solid. The solid-liquid interdiffusion process continues until the mixture solidifies to form the Au-In bond. A scanning acoustic microscope (SAM) was used to determine the excellent bonding quality before and after thermal shock tests while an energy dispersive X-ray (EDX) was employed to determine the composition of the resulting bonds. The resulting bond has an unbonding temperature greater than 545 degrees C. Due to the low process temperature, the stress on the bonded structure caused by thermal expansion mismatch is reduced. This type of bonding is useful when bonding at a low temperature is followed by a subsequent higher temperature process. >

Effects of crosslinking on physical properties of phenol-formaldehyde novolac cured epoxy resins

Abstract: To clarify the relationship between crosslinking density and physical properties of phenol–formaldehyde novolac cured epxy resin and factors governing their physical properties, we studied various properties of cured resins having different crosslinking densities. The resins were prepared with various curing accelerators and raw epoxy resins having different molecular weights. We found that as the crosslinking density of a cured resin increases, glass transition temperature (Tg) rises and the relaxation time becomes longer. Furthermore, in the rubbery region, the coefficient of linear thermal expansion drops and the elastic modulus become larger, while, in the glassy region, the coefficient of linear thermal expansion, specific volume, water absorption, diffusion coefficient, and permeability all increase but the elastic modulus becomes smaller. The WLF analysis on the relaxation behaviors of typical cured resin showed that cured resin with a higher crosslinking density decreases in the fractional free volume. This behavior is completely opposite from the relationship predicted from the temperature dependency of specific volume. While the coefficient of thermal expansion of free volume decreases as the crosslinking density increases for the cured resin, it coincides well with the tendency predicted from the difference in coefficient of cubic thermal expansion in the rubbery and glassy regions of each cured resin. That the free volume obtained from WLF analysis shows a relationship opposite to the predicted free volume as based on the temperature dependency of specific volume is explained as follows: Namely, the free volume obtained from the WLF analysis is a hole free volume Vh which contributes to fluidity and Vh decreases with the crosslinking density. On the other hand, the free volume predicted from the specific volume is a sum of the interstitial free volume Vi and Vh. Vi increases with the crosslinking density and this Vi increase exceeds the decrease of Vh. Therefore, the free volume predicted from the specific volume increases with the crosslinking density. Consequently, the influence of free volume on the relationship between the crosslinking density and physical properties of cured resin can be interpreted as follows. As the crosslinking density increases on cured resins, Tg rises, the relaxation time is lengthened, and the coefficient of linear thermal expansion becomes smaller in the rubbery region because, as the crosslinking density increases, Vh decreases. Since crosslinking density increases on cured resins, the coefficient of linear thermal expansion, water absorption, diffusion coefficient, and permeability become larger, and the elastic modulus becomes smaller in the glassy region because, as the crosslinking density increases, Vi increases and, accordingly, molecular chain packing becomes looser; i.e., the specific volume increases. © 1993 John Wiley & Sons, Inc.

Diamond/Al metal matrix composites formed by the pressureless metal infiltration process

Abstract: Diamond particles are unique fillers for metal matrix composites because of their extremely high modulus, high thermal conductivity, and low coefficient of thermal expansion. Diamond reinforced aluminum metal matrix composites were prepared using a pressureless metal infiltration process. The diamond particulates are coated with SiC prior to infiltration to prevent the formation of Al4C3, which is a product of the reaction between aluminum and diamond. The measured thermal conductivity of these initial diamond/Al metal matrix composites is as high as 259 W/m-K. The effects of coating thickness on the physical properties of the diamond/Al metal matrix composite, including Young's modulus, 4-point bend strength, coefficient of thermal expansion, and thermal conductivity, are presented.

Viscosity and elastic constants of amorphous Si and Ge

Abstract: The biaxial modulus and coefficient of thermal expansion of ion‐beam‐sputtered amorphous Si and Ge thin films were determined from curvature changes induced by differential thermal expansion. Viscous flow was measured by stress relaxation and was found to be Newtonian. The viscosity increased linearly with time as a result of structural relaxation, and its isoconfigurational activation enthalpy was 1.8±0.3 and 2.6±1.3 eV for amorphous Si and Ge, respectively. An atomistic model, based on a chain reaction of broken bond rearrangements, is proposed to describe the observation.

Microwave Characteristics of TiO2-Bi2O3 Dielectric Resonator

Abstract: The microwave characteristics of the binary-system TiO2-Bi2O3 ceramics, which is widely known to exhibit a comparatively high dielectric constant in the lower-frequency band, are investigated from a new standpoint It is shown in this article that the composition of 0919TiO2-0081Bi2O3 has a high dielectric constant (er=80), high Q (1800 at 5 GHz) and low temperature coefficient of resonant frequency (τf=+21 ppm/°C) From the X-ray diffraction and X-ray microanalysis, it is indicated that the crystal structure of this material is composed of two phases, TiO2 and Bi2Ti4O11, which possess a high er with a negative temperature coefficient and a high er with a positive temperature coefficient, respectively The measurements of linear thermal expansion and temperature coefficient of capacitance are also carried out in order to study the behavior of temperature compensation in this system

Polyethylene fibers‐polyethylene matrix composites: Preparation and physical properties

Abstract: Drawing on the difference in melting points of UHMPE fiber (150°C) and HDPE matrix (130°C), single-polymer composites were fabricated under various processing conditions. Because of the chemical similarity of the composite components, good bonding at the fiber-matrix interface could be expected. The matrix, the fiber, and unidirectional composite laminae were studied using TMA and DSC analyses, a hot-stage crystallization unit attached to a polarizing microscope, and an universal tensile testing machine. The TMA showed negative thermal expansion of the fiber over the complete temperature range of the experiment. Three regimes of contraction according to the values of the thermal expansion coefficient were detected. DSC analyses of either the fiber or the composite specimens did not show any appreciable changes after various thermal treatments. They also showed no evidence of fiber relaxation during manufacture, probably because of the pressure-related transverse constraint. The tensile strength and modulus values of the composite appeared to be fairly high and close to those reported for other composites reinforced with polyethylene (PE) fibers. An apparent maximum on the temperature dependencies of tensile properties was observed. A study of the matrix microstructure did not give any proof of transcrystalline growth at the fiber-matrix interface even for chemical or plasma surface-treated fibers. © 1993 John Wiley & Sons, Inc.

Thermal expansion of mantle and core materials at very high pressures

Abstract: The thermal expansivities (α) of MgO and high-pressure phases of CaO, CaMgSi2O6, and Fe at ultrahigh pressure are obtained by comparing existing shock compression and temperature measurements to 300 K compression curves constructed from ultrasonic elasticity and static compression data. For MgO, α can be represented by: α = ρoγoCV(ρo/ρ)0.5±0.5/KT where γ is the Gruneisen parameter, CV is the constant volume specific heat, KT is the isothermal bulk modulus, and ρ is the density. Using this expression, the thermal expansivity of MgO is 28-32×10−6K−1 at the pressure of the top of the lower mantle and 10-16×10−6K−1 at its base (at 2000 K). New data for α of e-Fe, together with an inner core temperature of 6750 K, constrain the density of the inner core to be 5±2% less than the density of e-Fe, implying the inner core contains a light element.

Ceramic-metal composite structure and process of producing same

Abstract: A composite substrate 30 is constituted by an alumina substrate 16, a metallic layer 34, and a copper sheet 26 bonded to the alumina substrate 16 via the metallic layer 34. The metallic layer 34 is constituted by a tungsten sub-layer 34a having a low coefficient of thermal expansion, a tungsten/silver-copper alloy mixture sub-layer 34b, and a silver-copper alloy sub-layer 34c having a high coefficient of thermal expansion. In the mixture sub-layer 34b, the ratio of the percentage content of the silver-copper alloy to that of the tungsten increases with distances from the alumina substrate 16.

Thermal-stress relaxation behavior in thin-films under transient laser-pulse heating

Abstract: Transient laser pulse has been used to generate nonsynchronous change of temperature rise and thermal expansion in thin films. The transient process of thermal expansion is recorded by a photoelectric high‐speed acquisition system. By comparison of the calculated temperature rise and thermal expansion, thermal stress relaxation processes in Al films of 20–50 μm thickness is obtained. This result shows that, for transient heating, thermal stress exists even in the case of uniform temperature distribution and free expansion. And also there is a transient high stress in thin films under high‐speed heating.

Solids: Thermal expansion and contraction

Abstract: For most close-packed crystals, the thermal expansion coefficient varies with temperature in much the same manner as the heat capacity although the magnitude is largely controlled by the elastic moduli. The macroscopic picture is fairly clear even when the microscopic description in terms of interatomic forces is rather intractable. More varied effects arise in the ‘open’ structures of diamond-like solids, vitreous silica, anisotropic crystals, many magnetic materials, heavy-electron metals and some superconductors.

Residual stresses and their effects on deformation

Abstract: Residual stresses induced by thermal expansion mismatch in metal-matrix composites are studied by three-dimensional (3-D) elastic-plastic finite element analyses. Typically, the stress-free state is 150 to 300 K above room temperature. The coefficient of thermal expansion of the matrix is 3 to 5 times larger than that of the ceramic inclusion, resulting in compressive stresses of order 200 MPa in the inclusions. Both compressive and tensile stresses can be found in the matrix. Since the stress may exceed the matrix yield strength near the particles, plastic flow occurs. The authors find a significant influence of this flow on the elastic and plastic properties of the composite. The calculated residual strains in TiC particles due to thermal expansion mismatch and external loads compare well with recent neutron diffraction experiments (Bourkeet al.) The present work is the first reported three-dimensional analysis of spherical inclusions in different arrays (simple cubic (sc) and face-centered cubic (fcc)) that permit a study of particle interactions.

Origin and development of residual stresses in the NiNiO system: in-situ studies at high temperature by X-ray diffraction

Abstract: In order to characterize the respective importance of the growth stresses, thermal stresses and stress relaxation developed in oxide scales, two high temperature chambers for X-ray diffraction were designed, allowing us to determine the stresses in both the oxide and the substrate with the sin2 ϕ technique, at high temperatures or room temperature and during heating-cooling sequences. It was applied to NiNiO. At room temperature after oxidation, NiO is subjected to compressive stresses whose level depends on the substrate thickness and on the oxidation time and temperature. In the substrate, compressive stresses are mainly due to internal oxidation. During oxidation at 900 °C, the oxide scale is subjected to slight tensile stresses which can be due partially to anionic diffusion, internal oxidation or the heating process. During heating-cooling sequences, the stresses in the scale decrease with increasing temperature and become negligible when the oxidation temperature is reached. The reversibility of the stress-temperature curve indicates that no stress relaxation occurs. The stresses found at room temperature are due only to thermal stresses and fit well the theoretical calculation of thermal stresses in NiO scale based on the newly determined thermal expansion coefficients of Ni and NiO. All these results show that the stresses found at room temperature are mainly generated during cooling and that the effect of the Pilling-Bedworth ratio or of factors playing a role during isothermal growth is negligible.

Residual Stresses in Fiber‐Reinforced Ceramics due to Thermal Expansion Mismatch

Abstract: Principles for the calculation of residual stresses due to thermal expansion mismatch and previous solutions are reviewed. A general model for this subject is proposed. Considering the effective thermoelastic properties of a unidirectional composite in axial and transverse directions allows a more comprehensive treatment of the residual stress situation in fiber-reinforced composites. Parameter variations show the important influence of the thermomechanical properties of an interfacial layer, which is disregarded in most of the conventional calculations.

Damping behavior of 6061Al/Gr metal matrix composites

Abstract: The damping behavior of graphite particulate-reinforced 6061A1 alloy metal matrix composites (MMCs) processed by spray atomization and codeposition is studied. Four spray deposition experiments are made, yielding materials with graphite volume fractions of 0, 0.05, 0.07, and 0.10. A dynamic mechanical thermal analyzer is used to measure the damping capacity and elastic modulus at 0.1, 1, and 10 Hz over the temperature range of 30 °C to 250 °C. The damping capacity of the materials is shown to increase with increasing volume fraction of graphite. Hot extrusion of the spray-deposited MMCs is shown to further increase the damping capacity. The elastic moduli of the spray-deposited MMCs are reduced with the addition of graphite but are improved by hot extrusion. At low temperatures (below 150 °C), the high damping capacity of the MMCs is attributed primarily to thermal expansion mismatch-induced dislocations and the high intrinsic damping of graphite. At high temperatures (above approximately 200 °C), the damping capacity is attributed to Al/graphite interface viscosity, preferred orientation of the graphite, and the presence of dislocations.

On the Determination of Residual Stresses in Fiber-Reinforced Thermoset Composites

Abstract: Residual processing stresses in fiber-reinforced composites arise primarily from a mismatch in the coefficients of thermal expansion of fiber and matrix, and from the anisotropy in the thermal expansion of the individual plies. These stresses significantly in fluence the mechanical properties of the laminate. In this study an attempt was made to vary the processing-induced strains by curing the same graphite/epoxy (AS4/3501-6) com posite system at different temperatures. Residual stress-free temperatures were determined from the deflection of unsymmetric laminate strips as a function of temperature and from residual strains in symmetric laminates measured at room temperature using a peel-ply technique. The first method gave stress-free temperatures above the maximum cure tem perature, and the difference between the two is attributed to the onset of shrinkage stresses at the cure temperature. Stress-free temperatures determined by the latter technique were, for the most part, in the vicinity of the maximum...

Experimental verification of a predicted negative thermal expansivity of crystalline zeolites

Abstract: Computational techniques predict that several microporous zeolitic structures have unusual negative coefficients of thermal expansion. High-resolution powder diffraction techniques confirm the qualitative aspects of these simulations.

Thermodynamic properties of CaCO3 calcite and aragonite: A quasi-harmonic calculation

Abstract: A quasi-harmonic model has been used to simulate the thermodynamic behaviour of the CaCO3 polymorphs, by equilibrating their crystal structures as a function of temperature so as to balance the sum of inner static and thermal pressures against the applied external pressure. The vibrational frequencies and elastic properties needed have been computed using interatomic potentials based on two-body Born-type functions, with O-C-O angular terms to account for covalency inside the CO3 molecular ion. A good agreement with experimental data is generally shown by simulated heat capacity and entropy, while the thermal expansion coefficient seems to be more difficult to reproduce. The results obtained for aragonite are less satisfactory than those of calcite, but they are improved by using a potential specifically optimized on properties of that phase itself.

Electrical and electromechanical properties of hot-pressed Pb(Fe1/2Nb1/2)O3 ferroelectric ceramics

Abstract: Dense ceramics of Pb(Fe1/2Nb1/2)O3 with high resistivity were fabricated below 1000°C by making use of the hot-pressing technique For these hot-pressed specimens, measurements of the spontaneous polarization, dielectric constants, electromechanical coupling factors, piezoelectric constants and elastic compliance constants were carried out as a function of temperatures between -30°C and 200°C Temperature variations of the resistivity and linear thermal expansion were also studied At room temperature, the following values were obtained: e33T/e0=2640, Ps=113 µC/cm2, k33=0266, d33=145×10-12 C/N and s33E=123×10-12 m2/N These values were briefly compared with those of BaTiO3 ceramics

Full width array read or write bars having low induced thermal stress

Abstract: A full width read and/or write assemblies, such as a full width thermal ink jet printbar, is disclosed, having materials with both a high thermal coefficient of expansion and a low thermal coefficient of expansion. A suitable adhesive which provides lateral give while firmly holding the respective components together provides dimensional stability to the printbar element having a low thermal coefficient of expansion when components having high thermal coefficient of expansion are assembled thereto. The flexible or floating mounting enabled by lateral give of the adhesive allows for the application of cost effective materials with a high thermal coefficient of expansion to be used for support functions such as, for example, circuit boards and ink manifolds. The flexible or floating mounting relieves shear stress cased by a differential in the expansion or contraction of materials having a different thermal coefficient of expansion. Since the thermal expansion of the various components expand and contract from a center location thereof, this center location is bonded by an adhesive which does not provide lateral give, so that alignment between parts are maintained while the remainder of the respective components float relative to each other and prevent thermally induced stresses which tend to cause warpage.

Thermal recording head

Abstract: A thin-film thermal recording head, used in a facsimile machine or a thermal printer, including heat resistors formed from only a Cr--Si--SiO or Ta--Si--SiO alloy thin-film resistor layer and a chromium, molybdenum, nickel, or tungsten thin-film conductor layer. The heat resistor is formed on a substrate having a linear thermal expansion coefficient from room temperature to 300° C. of 5×10 -8 /°C. or less. The heat resistor is also described having a thin anti- abrasion layer with thickness of 0.5 μm or less. The thin-film thermal recording head is also described in monolithic form with a portion of the heat resistor formed to directly contact an output electrode of a drive LSI circuit. In this case, a double-layer thermal-insulation layer can be formed between the substrate and the portion of the heat resistor that contacts and heats heat-sensitive recording paper. The two layers of the thermal-insulation layer are formed from a heat-resistant resin and an inorganic insulator.

R.f. magnetron sputtered iridium coatings on carbon structural materials

Abstract: R.f. magnetron sputtering was used to produced iridium coatings on carbon-carbon composite and isotropic graphites (coefficient of thermal expansion (CTE) approximately (0.92–7.6) × 10−6 K−1) at room temperature and 1073 K. Sputtered iridium coatings exhibit a homogenous surface irrespective of the deposition temperature. Extremely fine grained columnar structures of iridium were observed when deposited at room temperature. Compared with room temperature deposition, substrate firing at 1073 K produced denser coatings with less strains and larger grain size. Cracking was observed on the coating deposited at 1073 K in the case of c-c composite and isotropic graphite with a thermal expansion coefficient of approximately (0.92–4.8) × 10−6 K−1. For iridium coatings deposited onto isotropic graphite (CTE approximately 7.6 × 10−6 K−1) at 1073 K, after heat treatment at high temperatures in N2, there was no cracking, no outward diffusion of carbon, no dislocations and pores were reduced or eliminated.