High thermal expansion KAlSiO4 ceramic

Orthorhombic kalsilite (KAlSiO₄) was prepared by solid-state reaction from K₂CO₃, Al₂O₃, and SiO₂. The axial thermal expansion coefficients of the orthorhombic kalsilite were 1.6×10^–5°C^–1 for the a-axis, 1.6×10^–5°C^–1 for the b-axis, 2.8×10^–5°C^–1 for the c-axis, and 2.0×10^–5°C^–1 for the average from room temperature to 1000°C. A high thermal expansion ceramic consisting of the orthorhombic kalsilite was prepared by sintering. The densification was promoted by adding Li₂CO₃. The KAlSiO₄ ceramic sintered at 1200°C for 2 h with 5 wt% Li₂CO₃ had a bending strength of 65 MPa and linear thermal expansion coefficient of 2.2×10^–5 °C^–1 from room temperature to 600°C.     

Thermal properties of MoSi2 and SiC whisker-reinforced MoSi2

The heat capacity, thermal conductivity and coefficient of thermal expansion of MoSi₂ and 18 vol % SiC whisker-reinforced MoSi₂ were investigated as a function of temperature. The materials were prepared by hot isostatic pressing between 1650 and 1700 °C, the hold time at temperature being 4 h. The heat capacity of MoSi₂ showed an increase from about 0.44 Wsg^–11K^–1 at room temperature to 0.53 at 700 °C. Whisker reinforcement increased heat capacity by about 10%. Thermal conductivity exhibited a decreasing trend from 0.63 Wcm^–1 K^–1 at room temperature to 0.28 Wem^–1 K^–1 at 1400°C. Whiskers reduced conductivity by about 10%. The thermal expansion coefficient increased from 7.42 °C^–1 between room temperature and 200 °C to 9.13 °C^–1 between room temperature and 1200 °C. There was a 10% decrease resulting from the whiskers. The measured data are compared with literature values. The trends in the data and their potential implications for high-temperature aerospace applications of MoSi₂ are discussed.     

Thermal and dielectric properties of zirconyl phosphate compact

Experimental results are presented on the measurements of thermal expansion (up to 1500°C), thermal conductivity (up to 1000°C), dielectric constant (up to 450 °C) and tan (up to 800 °C) of zirconyl phosphate compacts obtained by sintering at 1600°C. The thermal expansion coefficient of the samples at the temperature below 1100°C was less than 1.7 × 10^–6°C^–1. The samples showed a definite shrinkage at temperatures of 1110 and 1470°C due to the phase transformations. The expansion at 1500°C was less than that at 1100°C probably because of the phase transformation. The thermal conductivity at room temperature was a very small value (0.0046 to 0.0065 cal s^–1 cm°C^–1 cm^–2). The dielectric constant was close to 9. The value of tan° (–0.0001) measured is one of the lowest values for ceramic materials.     

Preparation and properties of ALCON (Al28C6O21N6) ceramics

The synthesis of Al₂₈C₆O₂₁N₆ powder (ALCON), starting from the binary compounds is described. The powder is resistant to oxidation in air up to 760°C. From the prepared powder, fully dense ceramics have successfully been prepared using hot pressing. The as-prepared ceramics had a thermal conductivity of 20 W m^–1 K^–1. Experiments showed that it is also possible to prepare ALCON ceramics by reactive hot-pressing, starting from Al₂O₃, AlN and Al₄C₃. Further optimization is expected to raise the thermal conductivity significantly. The strength, about 300 MPa, is similar to that of AlN. The thermal expansion coefficient of 4.8 × 10^–6K^–1 closely matches that of silicon, making application of ALCON ceramics as heat sinks an interesting possibility.     

Thermal expansion and conductivity of magnetite flakes taken from the Oconee-2 steam generator

Flakes consisting primarily of iron oxides (magnetite) have been discovered in the spaces between tubes and support plates in steam generators, increasing flow resistance and causing abnormal increases in water levels. To aid in the determination of the effects of tube scale on steam generators, Duke Power Company and MPR Associates, Inc. arranged for the author to measure the thermal expansion and thermal conductivity of tube scale specimens from the steam generator of the Oconee-2 pressurized water reactor. The study measured the thermal expansion of the flakes directly, using miniaturized specimens, rather than deriving these data from X-ray powder diffractometry as in past studies.The flakes are composed of multiple layers, each of which exhibits different thermal behaviour. Thermal expansion was higher than that for Fe₃O₄. The average thermal conductivity for two- and three-layered flakes is 0.026 J sec^–1 cm^–1 ° C^–1. The thermal conductivity for a singlelayered flake is 0.060 J sec^–1 cm^–1 ° C^–1.The work was performed during employment at Battelle.     

Properties of high reinforcement-content aluminum matrix composite for electronic packages

Aluminum matrix composites reinforced by a high volume fraction of ceramic particles provide a novel solution to electronic packaging technology, because of their high thermal conductivity, compatible and tailorable coefficient of thermal expansion (CTE) with chips or substrates, low weight, enhanced specific stiffness, and low cost. In this paper, SiC-particle-reinforced aluminum matrix composites are fabricated by the cost-effective squeeze-casting technology, and their microstructure characteristics, thermo-physical, and mechanical properties are investigated. The reinforcement volume fraction is as high as 70% and composites with linear CTE of 6.9–9.7×10^–6 °C^–1 and thermal conductivity of 120–170 W m^–1 °C^–1 are produced. The composites can be electric-discharge machined, ground, and electric-spark drilled. An electroless nickel layer is plated on the composite by the conventional procedures. Finally, their potential applications in electronic packaging and thermal management are illustrated via prototype examples.     

Synthesis and characterization of (Sr1−x′K2x)Zr4(PO4)6 ceramics

Single phase (Sr_1–x K_2x )Zr₄(PO₄)₆, where x lies between 0.0 and 1.0, ceramic powder with a submicron scale particle size has been synthesized successfully at calcination temperatures as low as 650–750°C by a sol-gel technique. The formation of the powder strongly depends on calcination temperature, but is independent of solution pH in the studied range. Dilatometric measurement shows an ultra-low linear coefficient of thermal expansion of 0.1×10^–6°C^–1 when x=0.5 at temperature intervals of 25–1000°C. Thermal conductivity and flexural strength of the materials were determined at ambient temperature to be 1.0 Wm^–1K^–1 and as high as 280 MPa, respectively, indicating that this material can be an excellent candidate in many applications, especially those subjected directly to severe environments.     

Phase Relations along the Y2O3–CuWO4Join in the Y2O3–WO3–CuO System

The phase relations in the Y₂O₃–WO₃–CuO system were studied by x-ray diffraction and thermal analysis. The results were used to construct the 800°C section of the phase diagram. Based on the new and earlier data on the liquidus relations, the section through the Y₂O₃–WO₃–CuO phase diagram along the Y₂O₃–CuWO₄join was mapped out.     

Preparation, thermal expansion, high pressure and high temperature behavior of Al2(WO4)3

The titled compound Al₂(WO₄)₃ was synthesized by a conventional solid state reaction and characterized by powder XRD. It crystallizes in an orthorhombic (Pbcn, No. 60) lattice, with unit cell parameters as 12.582(2), 9.051(1), 9.128(2) Å, and V = 1039.5(3) (Å)³. The compound was found to show negative thermal expansion (NTE) behavior in the temperature range of 25 to 850°C. The average linear NTE coefficient (₁), in this temperature range, was –1.5 × 10^–6 K^–1. The effect of pressure at ambient temperature, was studied by a Bridgman Anvil (BA) apparatus, to reveal that there is no irreversible phase transition up to 8 GPa. The effect of high pressure and high temperature on this compound was studied by a Toroid Anvil (TA) apparatus. This compound has a limited stability under high pressure and temperature, as it undergoes a decomposition to AlWO₄ and WO_3–x with a partial oxygen loss. As an off-shoot of this work, certain new modifications of WO_3–x under pressure and temperature were observed, viz., monoclinic, tetragonal and an orthorhombic modifications at 5 GPa/1400°C, 3 GPa/900°C and 1.8 GPa/1030°C, respectively. The detailed XRD studies of the products are presented here.     

Thermal stabilization of aluminium titanate and properties of aluminium titanate solid solutions

Aluminium titanate has a near zero thermal expansion coefficient (=0.8×10^–6 °C^–1) in the range 20 to 1000 °C, nevertheless it decomposes below 1200 °C.The thermal stabilization of Al₂TiO₅ without altering its thermal expansion has been considered by partial substitution in the structure compound of Al³⁺ ions by Fe³⁺ ions.The solid solutions prepared by solid state reaction are in agreement with the general formula Al(1–x)₂Fe_2x TiO₅(0<x<0.2)The iron ions present in the crystal structure of Al₂TiO₅ act on its lattice parameters and bring about a catalytic effect in the formation of materials.Solid solutions show a strong thermal stability and a thermal expansion coefficient specially for the solid solution (x=0.1) which is not far from the Al₂TiO₅ value even after annealing for 300 h at 1000 °C.The mechanical properties of such materials corresponding to that solid solution present strength values lower than Al₂TiO₅ ones. After annealing, however, these are improved later due to a microcrystallization.     

Effects of moisture on the thermal expansion of poly(methylmethacrylate)

The effects of moisture on the thermal expansion of compression-moulded poly(methyl methacrylate) (PM MA, Plexiglas V811) were investigated by pushrod dilatometry. Moisture conditioning at high relative humidity, immersion in water, or vacuum drying were used to prepare PMMA specimens at different moisture contents. Weight and length change measurements were used to indicate water uptake. A substantial, reversible effect of moisture content on the coefficient of thermal expansion of PMMA was demonstrated. At 50°C, the expansion coefficient was observed to increase 32% (from 70.4 to 92.8 X 10^–6 K^–1 ) for a 2% gain in weight. A significant correlation was observed betweenT _g and moisture content, of approximately –1°C per 0.1% weight increase. Increases in weight and length of the PMMA with time were suggestive of the dual mode kinetic sorption process advanced by others, and indicated that a significant fraction of water was accommodated in microvoids.     

Effect of lithium salts addition on sintering temperature, cationic ordering and dielectric properties of Ba(Mg1/3Ta2/3)O3 ceramic

The effect of different lithium salts (LiF, Li₂CO₃, LiNO₃) addition on sintering temperature, cationic ordering and dielectric properties of Ba(Mg_1/3Ta_2/3)O₃ ceramic has been investigated. This perovskite was synthesised by solid state reaction from BaCO₃, MgO and Ta₂O₅. It was shown that, by addition of LiNO₃, the sintering temperature was decreased to 1300°C (versus 1550°C–1600°C for the pure Ba(Mg_1/3Ta_2/3)O₃ ceramic) without altering dielectric properties at 1 MHz. A cationic ordering was also observed. Sintering with co-fired nickel electrodes in Ar/H₂ atmosphere was also successfully performed for Multi Layer Ceramic Capacitors applications.     

Carbon fibre-reinforced silicon nitride composite

The processing of silicon nitride reinforced with carbon fibre was studied. The problems of physical and chemical incompatibility between carbon fibre and the silicon nitride matrix were solved by addition of a small amount of zirconia to the matrix and by low-temperature hot-pressing. The composite material possesses a much higher toughness than hot-pressed silicon nitride. Its work of fracture increased from 19.3 J m^–2 for unreinforced Si₃N₄, to 4770 J m^–2; its fracture toughness,K _lc , increased from 3.7 MN m^–3/2 for unreinforced material, to 15.6 MN m^–3/2. The strength remains about the same as unreinforced Si₃N₄ and the thermal expansion coefficient is only 2.51×10^–6 ° C^–1 (RT to 1000° C). It is anticipated that this composite may be promising because of its mechanical and good thermal shock-resistance properties.     

A new group of microwave dielectric ceramics in the RE(Ti0.5W0.5)O4 [RE=Pr, Nd, Sm, Gd, Tb, Dy, and Y] system

RE(Ti_0.5W_0.5)O₄ [RE=Pr, Nd, Sm, Gd, Tb, Dy, and Y] ceramics have been prepared as single-phase materials by a conventional solid-state ceramic route. The ceramics have been characterized by X-ray diffraction and microwave methods. The RE(Ti_0.5W_0.5)O₄ ceramics showed an increase in cell volume and a corresponding decrease in density with ionic radius of RE³⁺ ions. The dielectric constant varies from 21 to 25, Q_u×f from 6000 to 11 000 GHz and _f from –5 to –22 ppm °C^–1. The microwave dielectric properties indicate that these materials are possible candidates for dielectric resonator as well as substrate applications in microwave integrated circuits.     

Ultralow expansion ceramics in the HfO2-TiO2 system synthesized by an hydrolysis and polycondensation technique

Ultra-refractory ceramics from the HfO₂-TiO₂ system in the range 30–40 mol% TiO₂, with a near-zero thermal expansion, have been synthesized by hydrolysis and polycondensation of titanium alkoxide and hafnium dichloride alcoholic solutions and sintered at moderate temperature. Thermal stability, crystallization, density and microstructure of these materials have been examined. The as-prepared powder, amorphous at room temperature, crystallized quickly when heated at 500 ° C. Entire crystallization occurred after treatment at 1000 °C. Sintering at 1500 °C on cold-pressed samples led to ceramics with weak porosity (7%), low expansion coefficient <1×10^–6 °C^– with a minimum for 30 mol% TiO₂ content. SEM examination on sintered materials at 1500 °C reveals a grain size from 2–6 m, increasing with TiO₂ content.     

Thermal expansion of TiAl + TiB2 alloys and model calculations of stresses and expansion of continuous fiber composites

Thermal expansion values for three TiAl alloys with different additions of TiB₂ can be represented using a third-order equation at temperatures between 20 and 800°C. Expansion values were obtained on both heating and cooling temperature cycles. The total expansion at 800°C is between 0.917 and 0.931% for three different samples. The expansivity increases from about 10×10^–6°C^–1 at 80°C to 14×10^–6°C^–1 at 750°C. A five-coaxial cylinder elastic model for multizone-coated continuous fiber composites is developed for predicting stresses and thermal expansion of composites. Either isotropic or transversely isotropic material properties can be assigned to the various cylinder zones.Paper presented at the Tenth International Thermal Expansion Symposium, June 6–7, 1989, Boulder, Colorado, U.S.A.     

The high-temperature thermal expansion of BiPbSrCaCuO superconductor and the oxide components (Bi2O3, PbO,CaO, CuO)

The thermal expansion of superconducting Bi_1.6Pb_0.4Sr₂Ca₂Cu₃Ox (BiPbSrCaCuO) and its oxide components Bi₂O₃, PbO, CaO and CuO have been studied by high-temperature dilatometric measurements (30–800°C). The thermal expansion coefficient for the BiPbSrCaCuO superconductor in the range 150–830°C is =6.4×10^–6K^–1. The temperature dependences of L/L of pressed Bi₂O₃ reveals sharp changes of length on heating (T ₁=712°C), and on cooling (T ₂=637°C and T ₃=577°C), caused by the phase transition monoclinic-cubic (T ₁) and by reverse transitions via a metastable phase (T ₂ and T ₃). By thermal expansion measurements of melted Bi₂O₃ it is shown that hysteresis in the forward and the reverse phase transitions may be partly caused by grain boundary effect in pressed Bi₂O₃. The thermal expansion of red PbO reveals a sharp decrease in L/L, on heating (T ₁=490°C), related with the phase transition of tetragonal (red, a=0.3962 nm, c=0.5025 nm)-orthorhombic (yellow, a=0.5489 nm, b=0.4756 nm, c=0.5895 nm). The possible causes of irreversibility of the phase transition in PbO are discussed. In the range 50–740°C the coefficient of thermal expansion of pressed Bi₂O₃ (_m=3.6 × 10^–6 and _c=16.6×10^–6K^–1 for monoclinic and cubic Bi₂O₃ respectively), the melted Bi₂O₃ (_m=7.6×10^–6 and _c=11.5×10^–6K^–1), PbO (_t=9.4×10⁶ and _or=3.3×10^–6K^–1 for tetragonal and orthorhombic PbO respectively), CaO (=6.1×10^–6K^–1) and CuO (=4.3×10^–6K^–1) are presented.     

Tungsten-oxide thin films as novel materials with high sensitivity and selectivity to NO2, O3, and H2S. Part II: Application as gas sensors

The electrical response of tungsten-oxide thin films as-deposited by electron-beam deposition and annealed (at 350–800 °C for 1–3 h in O₂) to NO₂, O₃ and H₂S was studied both experimentally and theoretically. In order to interpret the kinetic characteristics of tungsten-oxide thin films on exposure to different gases, a model based on surface adsorption/desorption processes coupled with bulk diffusion was used. A link between the geometrical and chemical heterogeneities of the tungsten-oxide film surfaces and their performance characteristics as gas sensors was established. It was shown that the nature and amount of surface-adsorption sites in the different nonstoichiometric phases (W _n O_3n–2 or W _n O_3n–1) and WO₃ as well as their conduction mechanisms are defined from the phase composition of the film, the crystallographic and electronic structures of the phases, the orientation of the crystallites within the film and the geometrical shape and dimensions of the crystallites. All tungsten-oxide thin films investigated in this work are suitable for detection of very low concentrations of NO₂ (0.05–0.5 ppm in N₂ and synthetic air), ozone (25–90 ppb) and H₂S (3–15 ppm in N₂ and synthetic air) at very low working temperatures (80–160 °C). The films annealed at 400 °C for 1–2 h are very selective to ozone at 120–160 °C; the films annealed at 400 °C for 1–3 h and at 800 °C for 1 h are very sensitive to NO₂ (in N₂).     

Microwave dielectric properties of doped Ba6−x(Sm1−y′Ndy)8+2x/3Ti18O54 oxides

Dielectric ceramic compositions for microwave applications belonging to the (BaO)-(Nd₂O₃)-(Sm₂O₃)-(TiO₂) phase diagram were studied. Two compositions were selected for study, the doping effect of MnO₂, WO₃, CaO on dilatometry, microstructure and microwave properties. The effect of the nature and the amount of dopants on microstructure and microwave properties were clearly demonstrated. The effect of the addition of 1 or 2wt% WO₃, MnO₂ and CaO to Ba₄ (Sm_0.6, Nd_0.4)_9.33Ti₁₈O₅₄ and Ba_3.75(Sm_0.5, Nd_0.5)_9.5Ti₁₈O₅₄ was studied. It was found that these two compositions lead to dense ceramics exhibiting excellent microwave properties, for instance =74–81, Q × f up to 9000 Ghz at 3 Ghz and _f around +9 p.p.m.°C^–1.     

Fabrication and properties of alumino-borosilicate glass-ceramic systems

In current microelectronics packaging applications, low-temperature fired substrates with low dielectric constant are required. Formulations of SiO₂, B₂O₃, Al₂O₃, and CaO have been used as substrate materials which can be sintered as low as 1000C in air. The electrical behaviour, thermal expansion coefficient, and mechanical property of the fabricated substrate materials are evaluated. The as-sintered substrates possess the following characteristics: low dielectric constant of 4–5 at 1 MHz; a loss factor smaller than 0.2% at 1 MHz; and a thermal expansion of 3.57 × 10^–6C^–1 which is very close to that of silicon (3.5 × 10^–6C^–1).