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 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.     

Characteristics of aerosol-synthesized AlN particles

Particulate characteristics were investigated for AlN synthesized by a thermochemical reaction of aluminium aerosol with ammonia gas. The product powders had a decreasing specific surface area between 15.7 and 36.7 m²g^–1, with increasing reaction temperature from 1100–1500 °C, and the powders with large surface area were strongly hygroscopic. Although the powders were severely aggregated and had a small amount of unreacted aluminium, light milling and post heat treatment made them ultrafine and completely converted. When sintered with 0.5% yttrium at 1900 °C, full densification and high thermal conductivity of about 130 Wm^–1 K^–1 were obtained.     

Thermal expansion of the cubic (3C) polytype of SiC

Thermal expansion of the cubic beta or (3C) polytype of SiC was measured from 20 to 1000° C by the X-ray diffraction technique. Over that temperature range, the coefficient of thermal expansion can be expressed as the second order polynominal: ₁₁=3.19×10^–6+ 3.60×10^–9 T–1.68×10^–12 T ² (1/° C). It increases continuously from about 3.2×10^–6/° C at room temperature to 5.1×10^–6/° C at 1000° C, with an average value of 4.45 × 10^–6/° C between room temperature and 1000° C. This trend is compared with other published results and is discussed in terms of structural contributions to the thermal expansion.     

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.     

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).     

Low temperature sintering and crystallisation behaviour of low loss anorthite-based glass-ceramics

Anorthite-based glass-ceramics including TiO₂ as nucleating agent were melted and quenched in this study. The effect of particle size on the sintering behaviour of glass powders was investigated in order to obtain low-temperature sintered glass-ceramics. Anorthite glass-ceramic starts to densify at the transition temperature of glass (T _g = 770°C) and is fully sintered before the crystallisation occurrence (880°C). Therefore, a dense and low-loss glass-ceramic with predominant crystal phase of anorthite is achieved by using fine glass powders (D ₅₀ = 0.45 m) fired at 900°C. The as-sintered density approaches 99% theoretical density and the apparent porosity is as low as 0.05 Vol%. The dense and crystallized anorthite-based glass-ceramic exhibits a fairly low dielectric loss of 4 × 10^–4 at 1 MHz and a thermal expansion coefficient of 4.5 × 10^–6°C^–1. Furthermore, the microwave characteristics were measured at 10 GHz with the results of K = 9.8, Q _f = 2250, and temperature coefficient of resonant frequency _f = –30 ppm/°C.     

Dielectric spectra of fresh cement paste below freezing point using an insulated electrode

The spectra of complex dielectric constant were measured on a fresh cement paste with a water/cement ratio of 0.4 sandwiched between insulated electrodes in the frequency range 10 kHz–1 MHz and temperature range between 0 °C and — 30 °C. The bulk dielectric constant, 30–20, and conductivity, 6.14×10^–5–0.65×10^–5, in the temperature range –10 to –28 °C were much lower than those at room temperature, owing to the great decrease of ionic mobility caused by freezing the cement paste. The activation energy of 0.31 eV for the ionic conduction in fresh cement paste was obtained from an Arrhenius plot of conductivity at subzero temperature.     

Electrical properties of ZrF4-BaF2-GdF3-AlF3-glass

The electrical and dielectric behaviour of a gadolinium-based zirconium fluoride glass have been studied over a wide temperature (23–275 °C) and frequency (100 Hz–1 MHz) range. The conductivity at 200 °C was of the order of 10^–6 (ohm-cm)^–1 and the activation energy was 0.75 eV. The dielectric constant measured at 1 kHz was 14 and was independent of frequency at low temperatures. The observed dielectric dispersion has been attributed to the conduction mechanism.     

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.     

Thermal expansion of epoxy-fiberglass composite specimens

The thermal expansion behavior of three epoxy-fiberglass composite specimens was measured from 20 to 120°C (70 to 250°F) using a fused quartz push-rod dilatometer. Billets produced by vacuum-impregnating layers of two types of fiberglass cloth with an epoxy were core-drilled to produce cylindrical specimens. These were used to study expansion perpendicular and parallel to the fiberglass layers. This type of composite is used to separate the copper conductors that form a helical field coil in the Advanced Toroidal Facility, a plasma physics experiment operated by the Fusion Energy Division at Oak Ridge National Laboratory. The coil is operated in a pulsed mode and expansion data were needed to assess cracking and joint stresses due to expansion of the copper-composite system. The dilatometer is held at a preselected temperature until steady state is indicated by stable length and temperature data. Before testing the composite specimens, a reliability check of the dilatometer was performed using a copper secondary standard. This indicated thermal expansion coefficient () values within ±2% of expected values from 20 to 200°C. The percentage expansion of the composite specimen perpendicular to the fiberglass layers exceeded 0.8% at 120°C, whereas that parallel to the fiberglass layers was about 0.16%. The expansion in the perpendicular direction was linear to about 70°C, with an value of over 55×10^–6 °C^–1. Anomalous expansion behavior was noted above 70°C. The expansion in the direction parallel to the fiberglass layers corresponds to an value of about 15×10^–6 °C^–1. The lower values in the parallel direction are consistent with the restraining action of the fiberglass layers. The values decreased with the specimen density and this is consistent with literature data on composite contraction from 20 to –195°C.Nomenclature Thermal expansion coefficient, °C^–1 - L L(T ₂)–L(T ₁), cm - T T ₂–T ₁, °C - L ₀ Length at room temperature, cm - L(T _i ) Length at temperature T _i , cm - T _i Temperature, °C - T ₀ Room temperature, °C Paper presented at the Ninth International Thermal Expansion Symposium, December 8–10, 1986, Pittsburgh, Pennsylvania, U.S.A.     

The synthesis,characterization and sintering of sol-gel derived cordierite ceramics for electronic applications

Cordierite ceramics were synthesized by sol-gel processing using alkoxides and acetate with an aim to use the material as substrate and packaging material. Preparation conditions were optimized by varying the amount and pH of water added and the amount of acetic acid as chelating agent. The powders were characterized by different analytical techniques such as thermogravimetric analysis, differential thermal analysis, surface area by BET, X-ray diffraction, transmission and scanning electron microscopies and infrared spectroscopy. The best product was obtained using 19.6 mol water and 0.34 mol acetic acid with respect to silicon ethoxide. The pH of the water added did not make any significant difference. Sintered materials were characterized by measuring different physical properties such as density, electrical and dielectric properties, thermal expansion, microstructure and composition. Well-sintered bodies could be achieved at 1000 °C in air with a soaking time of 2 h having a density of 99% theoretical, electrical resistivity of 10¹⁴ cm, dielectric constant of 5, dielectric loss 0.008 and thermal expansion coefficient of 28.5 × 10^–7 °C^–1, 25–200 °C. X-ray diffraction studies show the phase evolution in these materials is predominantly -cordierite (hexagonal high cordierite) and some -quartz. SEM reveals a uniformly dense microstructure with crystals of granular habit. X-ray photoelectron spectroscopy indicates that the surface composition of the sintered material is slightly enriched with aluminium and deficient in silicon.     

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.     

Fabrication, characterization and sintering of glass-ceramics for low-temperature co-fired ceramic substrates

Cordierite-based glass-ceramics with non-stoichiometric composition doped with rare earth oxide (CeO₂) and heavy metal oxide (Bi₂O₃) respectively were fabricated from glass powders. After sintering and crystallization heat treatment, various physical properties, including compact density and apparent porosity, were examined to evaluate the sintering behavior of cordierite-based glass-ceramics. Results showed both that the additives heavy metal oxide and rare earth oxide promoted the sintering and lowered the phase temperature from - to -cordierite as well as affecting the dielectric properties of sintered glass-ceramics. The complete-densification temperature for samples was as low as 900 °C. This material has a low dielectric constant (5.3), a low dielectric loss (0.2%) and a low thermal expansion coefficient (2.8–3.52×10^–6 K^–1), and can be co-fired with high conductivity metals such as Au, Ag, Cu, Ag/Pd paste at low temperature (below 950 °C), which makes it a promising material for low-temperature co-fired ceramic substrates.     

Fabrication process and thermal properties of SiCp/Al metal matrix composites for electronic packaging applications

The fabrication process and thermal properties of 50–71 vol% SiC_p/Al metal matrix composites (MMCs) for electronic packaging applications have been investigated. The preforms consisted with 50–71 vol% SiC particles were fabricated by the ball milling and pressing method. The SiC particles were mixed with SiO₂ as an inorganic binder, and cationic starch as a organic binder in distilled water. The mixtures were consolidated in a mold by pressing and dried in two step process, followed by calcination at 1100 °C. The SiC_p/Al composites were fabricated by the infiltration of Al melt into SiC preforms using squeeze casting process. The thermal conductivity ranged 120–177 W/mK and coefficient of thermal expansion ranged 6–10 × 10^–6/K were obtained in 50–71 vol% SiC_p/Al MMCs. The thermal conductivity of SiC_p/Al composite decreased with increasing volume fraction of SiC_p and with increasing the amount of inorganic binder. The coefficient of thermal expansion of SiC_p/Al composite decreased with increasing volume fraction of SiC_p, while thermal conductivity was insensitive to the amount of inorganic binder. The experimental values of the coefficient of thermal expansion and thermal conductivity were in good agreement with the calculated coefficient of thermal expansion based on Turner's model and the calculated thermal conductivity based on Maxwell's model.     

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.     

Silicon carbide whisker copper-matrix composites fabricated by hot pressing copper coated whiskers

Copper-matrix SiC whisker composites containing 33–54 vol % SiC whiskers and with < 5 vol % porosity were fabricated by hot pressing SiC whiskers that had been coated with copper by electroless plating followed by electroplating. The highest Brinell hardness of 260 was attained at 50 vol % SiC whiskers. The lowest coefficient of thermal expansion (CTE) of 9.6 × 10^–6°C^–1 (at 25–150°C) was attained at 54 vol % SiC whiskers. The composites exhibited lower porosity, higher hardness, higher compressive yield strength, lower CTE, lower electrical resistivity and higher thermal conductivity than the corresponding composites made by hot pressing mixtures of copper powder and bare SiC whiskers.     

Formation of oxide phases in the system Fe2O3-NiO

Mixed metal oxides in the system Fe₂O₃-NiO were prepared by coprecipitation of Fe(OH)₃/Ni(OH)₂ and the thermal treatment of hydroxide coprecipitates up to 800 or 1100°C. X-ray diffraction showed the presence of -Fe₂O₃, NiO and NiFe₂O₄ in samples prepared at 800°C. The oxide phases -Fe₂O₃, NiO, NiFe₂O₄ and a phase with structure similar to NiFe₂O₄ were found in samples prepared at 1100°C. Fourier transform-infrared spectra of oxide phases formed in the system Fe₂O₃-NiO are discussed. Two very strong infrared bands at 553 and 475 cm^–1, a weak intensity infrared band at 383 cm^–1 and two shoulders at 626 and 441 cm^–1 were observed for -Fe₂O₃ prepared at 1100°C. NiFe₂O₄, prepared at the same temperature, showed two broad and very strong infrared bands at 602 and 411 cm^–1, while NiO showed a broad infrared band at 466 cm^–1. Fourier transform infrared spectroscopic results were in agreement with X-ray diffraction.     

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.     

Fabrication and properties of novel composites in the system Al–Zr–C

Composite bodies in the system Al–Zr–C, with about 95% relative density, were obtained by heating the compact body of powder mixture consisting of Al and ZrC (5 : 1 mol %) in Ar at 1100–1500°C for various lengths of time. Components of the material heated at more than 1200°C were Al, Al3Zr, ZrC and AlZrC2. The Al3Zr exhibited plate-like aggregation, and its size increased with increasing temperature. In the material heated at 1500°C for 1 h, the largest plate-like Al3Zr aggregation was 2000 m long and 133 m thick. Then the AlZrC2 was present as well-proportioned hexagonal platelet particles with a 8–9 m diameter and a 1–2 m thickness in the interior of the plate-like Al3Zr aggregation and Al matrix phase. The average three-point bending strength of the bodies was 140–190 MPa, and the maximum strength was 203 MPa in the body heated at 1300°C for 1 h. The body heated at 1500°C for 1 h showed high oxidation resistivity to air up to 1000°C.