Thermal expansion of NASICON materials

Our and others’ results on the thermal expansion of compounds and the crystal-chemical information about their structure have been used to properly select compositions of new NASICON materials with small thermal expansion and low thermal expansion anisotropy: M _x M_1?x ′Ti₂(PO₄)₃ (M and M′ are alkali metals, 0 ≤ x ≤ 1), M″FeTi(PO₄)₃ (M″ is an alkaline-earth metal), and Ca_{0.5(1 + x)}Fe _x Ti_{2 ? x} (PO₄)₃ (0 ≤ x ≤ 1). Such materials have been synthesized through salt coprecipitation from aqueous solutions and their thermal expansion has been assessed by high-temperature X-ray diffraction. We have determined their unit-cell parameters as functions of temperature and correlated their thermal expansion parameters and composition. The K_0.5Rb_0.5Ti₂(PO₄)₃ and Rb_0.8Cs_0.2Ti₂(PO₄)₃ materials obtained combine small volumetric thermal expansion and near-zero thermal expansion anisotropy.     

Thermal expansion of silicon carbide materials

The influence of porosity and sintering additives on the thermal expansion of silicon carbide ceramics with additions of Al₂O₃, B(B₄C), and BeO is investigated in the temperature range 50–700°C.Notation thermal expansion coefficient - b constant in the exponent - n porosity - T temperature Daghestan State University, Makhach-Kala. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 66, No. 6, pp. 739–741, June, 1994.     

Thermal expansion studies on ferroelectric materials

Thermal expansion data is reported over a wide temperature range (80–800 K) for BaTiO₃ (BT) and Pb(Fe_1/2 Nb_1/2)O₃ (PFN), belonging to different classes of the ferroelectric materials. The sharp structural phase transitions of BT result in lowering of the thermal expansion coefficient (α) at the transitions which is proportional to the change in polarization at the transitions. In PFN, a relaxor ferroelectric, lowering of α is spread over a wider temperature range across the dielectric maxima.     

Thermal expansion uniformity of materials for large telescope mirrors

Uniformity of thermal expansion has been measured for fused quartz (Heraeus-Amersil TO8E) and borosilicate glass (Schott Duran and Ohara E6). The variation of expansion coefficient for three melts of TO8E was 5 x 10(-9)/K over a temperature range of 300 to 100 K and was found to vary linearly with position in the melt. This spatial gradient averaged 3.5 x 10(-11)/K cm. The room-temperature thermal expansivity variation of Duran (Tempax) glass was approximately 27 x 10(-9)/K, while that of E6 glass was approximately 52 x 10(-9)/K.     

Thermal,hygroscopic and irradiation expansion of composite materials

Journal of Materials Science Letters -     

Photoluminescence of semiconductor materials

Photoluminescence has been developed as a sensitive technique for semiconductor analysis. In this paper we review some of the basic physics necessary for interpreting photoluminescence spectra. The necessary equipment is discussed. Recent applications of photoluminescence to analysis of silicon and GaAs are covered with emphasis on impurity identification and possible analytical methods.This article is intended to be an introduction to photoluminescence with a brief review of recent literature.     

Polishing of semiconductor materials

Different abrasive processes such as grinding and lapping are necessary to produce semiconductor wafers. However grinding and lapping leads to deterioration of the surface integrity of monocrystalline wafers. Therefore polishing and planarization is of utmost importance to produce microelectronic components. In this lecture the basics of polishing technology as well as different process models are presented. Additionally the properties of different semiconductor substrate materials Si, GaAs are discussed.     

Thermal expansion of CaZnSO

The temperature stability range of the zinc calcium oxysulfide CaZnSO has been determined in an inert atmosphere using high-temperature x-ray diffraction, thermogravimetry, and x-ray microanalysis. The lattice parameters and thermal expansion coefficients of CaZnSO have been measured in the range 298–1170 K and have been represented by best fit equations.     

Thermal expansion of composites

A theory is developed for the overall thermal expansion of a composite consisting of either spherical or long cylindrical inclusions of one material in a matrix of another. The strain field of a single inclusion consists of a uniform expansion and a short-range strain field. These two components are related by minimizing the elastic strain energy. To account for a dense array of inclusions, average properties of the mixture are used for the long-range field, but those of the matrix alone for the short-range field. The net dilatation is thus found for inclusions of mismatching volume; hence one finds a differential expression for the thermal expansion in terms of the volume fraction of inclusions, the individual thermal expansivities, the bulk moduli of inclusion and matrix, the shear modulus of the matrix, and, in the case of cylinders, the shear modulus of the inclusions. This expression is integrated over temperature; one accounts for plasticity by letting the shear modulus depend on the temperature and on the accumulated shear strain. A representative numerical example is given.Invited paper presented at the Ninth Symposium on Thermophysical Properties, June 24–27, 1985, Boulder, Colorado, U.S.A.     

Thermal expansion of berlinite

Journal of Materials Science Letters -     

Thermal expansion of composites

Coefficient of thermal expansion (CTE) has been determined for selected composite materials using differential thermal analysis. Variables evaluated were: type of material, with particular emphasis on filler content; annealing; thermal history, with particular attention being payed to the effects of multiple heating and cooling cycles; ageing in wet or dry conditions. Filler content was a major factor involved in controlling CTE, although clearly other factors such as the type of filler, resin and degree of conversion are important. For an inlay material, annealing at 120°C significantly reduced the value of coefficient of thermal expansion and this is most likely due to an increase in conversion of methacrylate groups. The findings of this study confirm those of previous studies regarding the reduction in CTE following an initial heating. This is most likely due to the relief of internal stress. New information reported here relates to the fact that stress release can occur slowly without heating and that rapid stress release can be achieved through water storage at mouth temperature. These results suggest that, clinically, internal stresses induced by polymerization will be dissipated rapidly. A further finding was that long-term water storage causes an increase in CTE, which may reflect changes at the resin-filler interface.     

Thermal instabilities in semiconductor amplifiers

We introduce here a model which includes the thermal dynamics in the time evolution of a semiconductor multiple quantum well microresonator, driven by a coherent holding beam. The active layer is electrically pumped, in order to obtain population inversion, but it is maintained below the lasing threshold. We show that the inclusion of thermal effects introduces a Hopf instability which may dominate the dynamical behaviour of the system in some operational regimes. In those cases our numerical simulations show that both spatial patterns and cavity solitons perform a drift motion in the transverse direction. This motion develops over the slow time scale which characterizes thermal effects.     

Photoelectrochemical reactions on semiconductor materials

The cathodic reduction of Fe³⁺ ions and I₂ on ann-GaAs electrode was studied. The variation in the current density with the concentration of the oxidant has been interpreted as being the result of a reduction mechanism involving interface or surface states, an interpretation that is amply substantiated by experimental data. The effect of the surface modification with SiW₁₂O₄₀ ^4– on the reduction process was studied. Prior to this electrode activation, the rate constant for electrons being transferred from the conduction-band to the interface or surface states,k _S, was observed to be independent of electrode potential, whereas in the case of the modifiedn-GaAs,k _S depends on band-bending. On the other hand, the rate constant for electrons being transferred from the interface or surface states to oxidant species,k _ox, does depend on electrode potential in the case of the unmodifiedn-GaAs, and is independent of band-bending in the case of the modifiedn-GaAs. This change may be attributed to the filling of the active surface or interface states or their redistribution after the electrode surface activation.     

Negative thermal expansion materials

Materials exhibiting negative thermal expansion through room temperature have a variety of applications, mainly in controlling the overall thermal expansion of various composites. Several materials showing negative thermal expansion have recently been identified. The most dramatic of such behavior is exhibited by ZrW₂O₈, which shows strong isotropic thermal expansion from 20 to 425K.     

Thermal expansion measurements of high-temperature carbon/carbon materials from room temperature to 3000 K

The thermal expansion of carbon/carbon material has been measured to 3000 K using a variety of methods. Coupon samples have been measured in a directview dilatometer and an automatic recording dilatometer. Cylindrical or hoop samples, 5 to 50 cm in diameter, have been measured by a fiber-wrap technique. The direct-view or, as it is sometimes referred to, the twintelemicroscope method is generally used when good accuracy is required for measurements to elevated temperatures. The change in length is observed with the use of telemicroscopes aligned on fiducial marks machined on opposite ends of the specimen. An Apple IIe computer controls the automatic recording dilatometer and also provides an output to an X-Y recorder, giving a continuous curve of thermal expansion percentage versus temperature through the range from RT to 300 K. Circumferential thermal expansion is defined as the change in circumference per unit of initial circumference. The change in the circumference of the hoop is measured by wrapping the hoop with a graphite yarn and measuring the change in length with a linear variable differential transformer.Paper presented at the Ninth International Thermal Expansion Symposium, December 8–10, 1986, Pittsburgh, Pennsylvania, U.S.A.