Thermal expansion

About: Thermal expansion is a research topic. Over the lifetime, 21040 publications have been published within this topic receiving 349407 citations. The topic is also known as: heat expansion.

Metals during rapid heating by dense currents

Abstract: Research on metals, particularly refractory metals, during pulsed heating by high density currents (up to ~ 107 A/cm2) is reported. In particular, studies have been made of the specific heat and thermal expansion of metals in the liquid state, the resistivity in the liquid state as a function of the amount of energy supplied during free and confined expansion generating pressures up to 4·104 atm, the visible emission, the heat effusion, the changes in certain properties upon melting, the dispersal of a metal during an electrical explosion, and the anomalies of the specific heat and electron emission of a metal in the solid state which stem from the high rate of Joule heating. The experimental data reported on liquid metals (Al, Cu, Mo, W) at high temperatures and high pressures (tens of kilobars) are of considerable interest for the theory of liquid metals.

Carbon nanotube-copper exhibiting metal-like thermal conductivity and silicon-like thermal expansion for efficient cooling of electronics

Abstract: Increasing functional complexity and dimensional compactness of electronic devices have led to progressively higher power dissipation, mainly in the form of heat. Overheating of semiconductor-based electronics has been the primary reason for their failure. Such failures originate at the interface of the heat sink (commonly Cu and Al) and the substrate (silicon) due to the large mismatch in thermal expansion coefficients (∼300%) of metals and silicon. Therefore, the effective cooling of such electronics demands a material with both high thermal conductivity and a similar coefficient of thermal expansion (CTE) to silicon. Addressing this demand, we have developed a carbon nanotube–copper (CNT–Cu) composite with high metallic thermal conductivity (395 W m−1 K−1) and a low, silicon-like CTE (5.0 ppm K−1). The thermal conductivity was identical to that of Cu (400 W m−1 K−1) and higher than those of most metals (Ti, Al, Au). Importantly, the CTE mismatch between CNT–Cu and silicon was only ∼10%, meaning an excellent compatibility. The seamless integration of CNTs and Cu was achieved through a unique two-stage electrodeposition approach to create an extensive and continuous interface between the Cu and CNTs. This allowed for thermal contributions from both Cu and CNTs, resulting in high thermal conductivity. Simultaneously, the high volume fraction of CNTs balanced the thermal expansion of Cu, accounting for the low CTE of the CNT–Cu composite. The experimental observations were in good quantitative concurrence with the theoretically described ‘matrix-bubble’ model. Further, we demonstrated identical in-situ thermal strain behaviour of the CNT–Cu composite to Si-based dielectrics, thereby generating the least interfacial thermal strain. This unique combination of properties places CNT–Cu as an isolated spot in an Ashby map of thermal conductivity and CTE. Finally, the CNT–Cu composite exhibited the greatest stability to temperature as indicated by its low thermal distortion parameter (TDP). Thus, this material presents a viable and efficient alternative to existing materials for thermal management in electronics.

Equations for the Calculation of the Thermo-physical Properties of Stainless Steel

Abstract: Equations have been derived to calculate values of the thermophysical properties of all stainless steels for temperatures between 300 and 1800 K (austenitic 3 series, ferritic-4 series and precipitation-hardened 6-series alloys). Values of the following properties are given in both figures and tables: density (ρ), thermal expansion coefficient (α), heat capacity (Cp), enthalpy (HT−H298), thermal conductivity (λ) and thermal diffusivity (a), electrical resistivity (R), viscosity (η) and surface tension (γ).

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.

Phase transformation and bond coat oxidation behavior of plasma-sprayed zirconia thermal barrier coating

Abstract: ZrO2–CeO2–Y2O3 and ZrO2–Y2O3 thermal barrier coatings were prepared using the air plasma spray process. Phase transformation in the ceramic top coating, bond coat oxidation and thermal barrier properties were investigated to compare ZrO2–CeO2–Y2O3 with ZrO2–Y2O3 at 1300°C under high temperature thermal cycles. In the as-sprayed condition, both coatings showed a 7∼11% porosity fraction and typical lamellar structures formed by continuous wetting by liquid droplets. In the ZrO2–CeO2–Y2O3 coating, the phase ratio of tetragonal to cubic phase was 75:25 and ZrO2–Y2O3 coating had a 100% non-transformable tetragonal phase. There was no monoclinic phase in either coating. However, the phase ratio of the coatings was changed after 1300°C thermal cycles. In the ZrO2–CeO2–Y2O3 coating, the ratio of tetragonal to cubic was changed to 88:12 and a monoclinic phase was still not detected, but a 10∼19% monoclinic phase had formed in the ZrO2–Y2O3 coating. The life of the coatings was found to be strongly dependent on the temperature which the bond coat experienced during exposure to a peak temperature of 1300°C. When the bond coat experienced a temperature higher than 1100°C, the useful life of both thermal barrier coatings was decreased drastically and this was related to the oxidation behavior of the bond coat. Al2O3 formed preferentially along the bond and top coat interface and the other oxides such as NiO and Ni(Cr,Al)2O4 spinel, which were believed to decrease the coating life by oxide growth stress, formed rapidly at the top coat side of the interface at temperature higher than 1100°C. The thermally shocked ZrO2–Y2O3 coating exhibited a non-linear thermal expansion curve which was probably due to a reversible tetragonal–monoclinic transformation. The ZrO2–CeO2–Y2O3 coating was found to have better thermal cycling behavior than the ZrO2–Y2O3 coating. The reason for this could be that there was no phase transformation from tetragonal to monoclinic phase, which induces volume expansion. The thermal expansion mismatch is smaller and the effect of oxide growth stress is relatively small because of better thermal insulation.