Iwao Yamai

Low thermal expansion and low thermal expansion anisotropy ceramic of Sr0.5Zr2(PO4)3 system

A low thermal expansion ceramic with a very low thermal expansion anisotropy was synthesized from the Sr0.5Zr2(PO4)3 system. The sintering was promoted by addition of MgO, and the solgel technique also improved the sinterability. The thermal expansion of the crystal was lowered by substituting Nb5+ for Zr4+ and 1/2Sr2+ pairs, becoming near-zero for Sr0.25Nb0.5Zr1.5(PO4)3. All dense ceramics in this system had a strength of about 80 MPa, and did not suffer microcracking even in the coarse-grained polycrystalline ceramics, owing to the very low thermal expansion anisotropy of the crystals.

Vapor phase growth of alumina whiskers by hydrolysis of aluminum fluoride

Abstract The vapor phase growth of alumina whiskers by the hydrolysis of aluminium flouride has been studied at an elevated temperature. The product was obtained in various forms: wool-like whiskers, needle-like whiskers, jointed needle-like crystals which consisted of linked small single crystals, tightly coagulated powder or as thin film covering the substrate. Aluminium oxi-fluoride was suggested as the intermediate gaseous species in the formation of alumina. The relation between the growth mechanism and crystal morphologies was studied.

Synthesis of potassium hexatitanate fibers by the hydrothermal dehydration method

Abstract K 2 Ti 6 O 13 fibers were synthesized from the system TiO 2 · n H 2 O-KOH-H 2 O at 390°C for 20 h to investigate: (1) the application of dehydration to the ordinary hydrothermal reaction, (2) the influence of the molar ratio of K 2 O/TiO 2 and the concentration of the KOH solution and (3) the influence of other types of starting materials on crystallization. The so-called hydrothermal dehydration method appeared to be an advantageous technique by which the fibers of some millimeter length were easily formed. Favorable conditions were found to be: 2 N KOH as concentration of solution; about 20% of fill; Zn metal as a dehydration reagent.

Flux growth of perovskite-type La2/3TiO3-x crystals

Crystals of perovskite-type lanthanum titanate La2/3TiO3-x were grown from a flux of the system KF-Na2B4O7. Cubic crystals were obtained above 1000°C and tetragonal crystals were obtained below 950°C. Oxygen deficiency seemed to be not so much a problem as has been reported for powdered sample. Growth conditions, X-ray diffraction analysis, crystal morphology and dielectric properties are described.

Decomposition behaviours of dopant-free and doped solid solutions in the TiO2-SnO2 system

Decomposition behaviours, including phase separation by the spinodal mechanism, were investigated for selected compositions ofx = 0.7, 0.5 and 0.3 in TixSn1−xO2 alloy. Inside the coherent spinodal, an equimolar alloy was found to decompose most rapidly at 1100° C and the decomposition rate decreased as the annealing temperature increased. The difference in the rate of decomposition outside the spinodal betweenx = 0.7 andx = 0.3 alloys suggested that the ionic mobility of diffusing tin was much slower than that of titanium in the alloy system. The effect of some dopants on the decomposition rate was also examined. Doping with tungsten or antimony atoms strongly suppressed the phase separation both inside and outside the spinodal, especially resulting in a prolonged stabilization ofx = 0.7 alloy. The role of the dopants in affecting the decomposition rate is discussed in relation to selective substitution of the doping atoms in the tin or titanium sublattice.

Thermal expansion of ZrP2O7 and related solid solutions

The effect of solute ions on the thermal expansion of ZrP2O7 solid solutions was studied. The abrupt thermal expansion at the high-low inversion can be interpreted as due to the rotation of the polyhedra from the low-temperature form in the partially collapsed state to the high-temperature form in the fully expanded state. The replacement of zirconium ions by larger ions and/or the stuffing of cations into the cavities of the framework stabilized the expanded structure, and then depressed the abruptness of the expansion. Consequently the thermal expansion was reduced in CexZr1-xP2O7 or (Li, Y)xZr1-xP2O7 solid solutions.

Vapor phase growth of titania whiskers by hydrolysis of titanium flouride

Abstract The chemical vapor deposition of TiO 2 crystals from the system TiF 4 −H 2 O was studied at elevated temperatures. When Na 2 TiF 6 was used as a TiF 4 source, rutile-type TiO 2 needles grew to 30 mm in length in a comparatively short time over the melt in a platinum crucible in air. The needles were mostly skeletal or twinned intergrowths of acicular crystals. Rutile gradually changed to Na 2 Ti 6 O 13 by the reaction with NaF and moisture in the air. The formation of Na 2 Ti 6 O 13 was inhibited by addition of TiO 2 to the raw material. The rutile whiskers were grown under controlled conditions by means of a flow method from the system Na 2 TiF 6 −TiO 2 −H 2 O. The needles or whiskers were found to grow preferentially in the [001] direction.

Flux growth of rare-earth stabilized zirconia and hafnia crystals

Abstract Crystals of zirconia and hafnia stabilized with rare-earth oxides (Nd 2 O 3 , Sm 2 O 3 , Gd 2 O 3 , Dy 2 O 3 , Er 2 O 3 , Yb 2 O 3 ) were grown from a flux of the system KF - Na 2 B 4 O 7 . Flourite-type or pyrochlore-type crystals were obtained according to the kind of rare-earth oxide doped. The rare-earth oxide content in the grown crystal was estimated by precision lattice parameter measurement. Crystal morphology and growth process are described.

Low-thermal expansion polycrystalline tantalum tungstate ceramics

The synthesis of thermal-shock-resistant materials from the system Ta2O5WO3 was investigated. Ta2WO8 had a very low unit-cell thermal expansion coefficient (+0.5 X 10−6° C−1). Ta30W2O81 also had a relatively low coefficient (+4.0 X 10−6 ° C−1) and a thermal durability over 1600° C. The thermal expansion curves of these polycrystalline ceramics were lowered because of microcracks caused by the large thermal expansion anisotropy of the crystal axes and were accompanied by hysteresis loops. The densification of Ta2WO8 ceramic was promoted by the addition of some metal oxides, and the strong ceramic of Ta30W2O81 was obtained by controlling grain growth.

Preparation of potassium hexatitanate long fibres by the flux evaporation method

Potassium hexatitanate long fibres were successfully prepared by the flux evaporation method. The addition of a small amount of Li2O to K2MoO4 flux was found to be very advantageous for growth of the long fibres. The crystal length was increased with the increase of the amount evaporated. The results of the experiment on the effects of flux to raw material ratio, filled amount and evaporation area on crystal growth are described.