An alumina sintered body according to the present invention has a degree of c-plane orientation of 90% or more as determined by Lotgering's method from an X-ray diffraction profile obtained by irradiating a plate surface with X-rays in a range of 2=20 to 70. The alumina sintered body has no pores when a cross-sectional surface formed in a direction perpendicular to the plate surface is polished using an Ar.sup.+ ion beam and a mask and is examined under a scanning electron microscope at a magnification of 5,000 times. The alumina sintered body has a total mass fraction of impurity elements other than Mg and C of 100 ppm or less. This alumina sintered body has a high degree of orientation, high density, and high purity and thus has a higher optical translucency than those known in the art.

A method for producing a transparent alumina sintered body includes (a) the step of preparing an alumina raw material powder containing a plate-like alumina powder having an aspect ratio of 3 or more so that the mass ratio R1 of F to Al in the alumina raw material powder is 5 ppm or more, and forming a compaction raw material containing the alumina raw material powder into a compact, and (b) the step of pressure-sintering the compact at a temperature at which F evaporate to yield a transparent alumina sintered body.

A voltage nonlinear resistive element includes a resistor containing a joined body in which a zinc oxide ceramic layer composed mainly of zinc oxide and having a volume resistivity of 1.010.sup.1 cm or less is joined to a bismuth oxide layer composed mainly of bismuth oxide, and a pair of electrodes disposed on the resistor such that an electrically conductive path passes through a joint surface between the zinc oxide ceramic layer and the bismuth oxide layer. In this element, the zinc oxide ceramic layer of the joined body has a lower volume resistivity than before. This can result in a lower clamping voltage in a high-current region than before.

The voltage nonlinear resistive element includes a resistor containing a joined body in which a zinc oxide ceramic layer composed mainly of zinc oxide and having a volume resistivity of less than 1.010.sup.2 cm is joined to a rare-earth metal oxide layer composed mainly of a rare-earth metal oxide, and a pair of electrodes disposed on the resistor such that an electrically conductive path passes through a junction between the zinc oxide ceramic layer and the rare-earth metal oxide layer. In this element, the zinc oxide ceramic layer of the joined body has a lower volume resistivity than before. This can result in a lower clamping voltage in a high electric current region than before.

The present invention concerns spheroidal alumina particles, catalysts comprising such particles as a support and a process for the production of spheroidal alumina particles, comprising the following steps: a) preparing a suspension comprising water, an acid and at least one boehmite powder for which the ratio of the crystallite dimensions in the [020] and [120] directions obtained using the Scherrer X-ray diffraction formula is in the range 0.7 to 1; b) adding a pore-forming agent, a surfactant and optionally water, or an emulsion comprising at least one pore-forming agent, a surfactant and water to the suspension of step a); c) mixing the suspension obtained in step b); d) shaping the spheroidal particles by the oil-drop method using the suspension obtained in step c); e) drying the particles obtained in step d); f) calcining the particles obtained in step e).

There is provided a platy zinc oxide sintered compact containing 0.80 wt % or less at least one first dopant element selected from the group consisting of Al, Ga and In, the balance consisting essentially of ZnO and optionally at least one second dopant element selected from the group consisting of Br, Cl, F, Sn, Y, Pr, Ge, B, Sc, Si, Ti, Zr, Hf, Mn, Ta, W, Cu, Ni, Cr, La, Gd, Bi, Ce, Sr and Ba, the second dopant element being optional component, wherein the (002)-plane orientation in the plate surface is 60% or more. The zinc oxide sintered compact of the present invention has excellent properties such as high orientation in addition to transparency and conductivity.

Abrasive particles include: 0.50 to 5.00 weight percent of seed particles for alpha-alumina formation selected from the group consisting of alpha-Fe.sub.2O.sub.3 seed particles, alpha-Al.sub.2O.sub.3 seed particles, and combinations thereof; 0.50 to 5.00 weight percent of MgO; 0.02 to 0.40 weight percent of Li.sub.2O or a molar equivalent amount of at least one alkali metal oxide selected from the group consisting of: Na.sub.2O; K.sub.2O; combinations of Na.sub.2O and Li.sub.2O; combinations of K.sub.2O and Li.sub.2O; combinations of Na.sub.2O and K.sub.2O; and combinations of Li.sub.2O, Na.sub.2O, and K.sub.2O; and 91.08 to 99.48 weight percent of Al.sub.2O.sub.3. Methods of making the abrasive particles and abrasive articles incorporating them are also disclosed.

The present invention relates to a composite ceramic which comprises a conversion phosphor and a further material, characterized in that the further material has a negative coefficient of thermal expansion, and to a process for the preparation thereof. Furthermore, the present invention also relates to the use of the composite ceramic according to the invention as emission-converting material, preferably in a white light source, and to a light source, a lighting unit and a display device.

A porous ceramic composition having improved thermal stability is comprised of ceramic grains bonded together by grain boundary phase comprised of silica, yttrium and oxygen wherein the amount of alkali metals, alkaline earth metals and other transition metals not including rare earth metals is at most 2% by weight of the grain boundary phase.

An extruded, solid honeycomb body comprises a copper-promoted, small pore, crystalline molecular sieve catalyst for converting oxides of nitrogen in the presence of a reducing agent, wherein the crystalline molecular sieve contains a maximum ring size of eight tetrahedral atoms, which extruded, solid honeycomb body comprising: 20-50% by weight matrix component comprising diatomaceous earth, wherein 2-20 weight % of the extruded, solid honeycomb body is diatomaceous earth; 80-50% by weight of the small pore, crystalline molecular sieve ion-exchanged with copper; and 0-10% by weight of inorganic fibres.