A method can be used for producing a powdery precursor material of the following general composition I or II or III or IV: I: (Ca.sub.ySr.sub.1y) AlSiN.sub.3:X1 II:(Ca.sub.bSr.sub.aLi.sub.1ab) AISi (N.sub.1cF.sub.c)3:X2 III: Z.sub.5Al.sub.42Si.sub.8+2N.sub.18: X3 IV: (Z.sub.idLi.sub.d).sub.5Al.sub.42Si.sub.8+2(N.sub.1XF.sub.X).sub.18: X4. The method includes A) producing a powdery mixture of starting materials, wherein the starting materials comprise ions of the aforementioned compositions I and/or II and/or III and/or IV, B) annealing the mixture under a protective gas atmosphere, subsequent milling. In method step A), at least one silicon nitride having a specific area of greater than or equal to 5 m.sup.2/g and smaller than or equal to 100 m.sup.2/g is selected as starting material. The annealing in method step B) is carried out at a temperature of less than or equal to 1550 C.

A silicon nitride sintered body includes a silicon nitride crystal grains and grain boundary phases. Further, when D stands for width of the silicon nitride sintered body before being subjected to surface processing, relations between an average grain diameter dA and an average aspect ratio rA of the silicon nitride crystal grain in a first region from an outermost surface to a depth of 0 to 0.01D and an average grain diameter dB and an average aspect ratio rB of the silicon nitride crystal grain in a second region inside the first region satisfy the inequalities:
0.8dA/dB1.2; and
0.8rA/rB1.2.

A silicon nitride sintered body includes a silicon nitride crystal grains and grain boundary phases. Further, when D stands for width of the silicon nitride sintered body before being subjected to surface processing, relations between an average grain diameter dA and an average aspect ratio rA of the silicon nitride crystal grain in a first region from an outermost surface to a depth of 0 to 0.01D and an average grain diameter dB and an average aspect ratio rB of the silicon nitride crystal grain in a second region inside the first region satisfy the inequalities:
0.8dA/dB1.2; and
0.8rA/rB1.2.

Ceramic particles for use in a solar power tower and methods for making and using the ceramic particles are disclosed. The ceramic particle can include a sintered ceramic material formed from a mixture of a ceramic raw material and a darkening component comprising MnO as Mn.sup.2+. The ceramic particle can have a size from about 8 mesh to about 170 mesh and a density of less than 4 g/cc.

Ceramic particles for use in a solar power tower and methods for making and using the ceramic particles are disclosed. The ceramic particle can include a sintered ceramic material formed from a mixture of a ceramic raw material and a darkening component comprising MnO as Mn.sup.2+. The ceramic particle can have a size from about 8 mesh to about 170 mesh and a density of less than 4 g/cc.

A silicon nitride sintered body includes a silicon nitride crystal grains and grain boundary phases. Further, when D stands for width of the silicon nitride sintered body before being subjected to surface processing, relations between an average grain diameter dA and an average aspect ratio rA of the silicon nitride crystal grain in a first region from an outermost surface to a depth of 0 to 0.01 D and an average grain diameter dB and an average aspect ratio rB of the silicon nitride crystal grain in a second region inside the first region satisfy the inequalities:

0.8dA/dB1.2; and

0.8rA/rB1.2.

A silicon nitride sintered body includes a silicon nitride crystal grains and grain boundary phases. Further, when D stands for width of the silicon nitride sintered body before being subjected to surface processing, relations between an average grain diameter dA and an average aspect ratio rA of the silicon nitride crystal grain in a first region from an outermost surface to a depth of 0 to 0.01 D and an average grain diameter dB and an average aspect ratio rB of the silicon nitride crystal grain in a second region inside the first region satisfy the inequalities:

0.8dA/dB1.2; and

0.8rA/rB1.2.

It is provided a sintered body on the basis of -sialon and 15R-sialon, which as a cutting material has a high cutting performance as compared to workpieces made of nickel-based alloy or Heat Resistant Super Alloys. For this purpose, a ceramic sintered body is shown, which includes a sialon phase and an amorphous or semi-crystalline grain boundary phase. The sialon phase includes a proportion of 20-80 wt-% of 15R-sialon polytypoid. The amorphous or semi-crystalline grain boundary phase possibly includes an YbAl garnet and constitutes up to 15 wt-% of the entire sintered body. The sintered body is manufactured from an inorganic raw material mixture which includes 40 to 57 wt-% of Si.sub.3N.sub.4; 40 to 55 wt-% of a mixture of AlN and Al.sub.2O.sub.3, wherein the ratio of Al.sub.2O.sub.3 to AlN lies in the range of 1-1.5:1, and 3 to 5 wt-% of Yb.sub.2O.sub.3 as sintering aid.

It is provided a sintered body on the basis of -sialon and 15R-sialon, which as a cutting material has a high cutting performance as compared to workpieces made of nickel-based alloy or Heat Resistant Super Alloys. For this purpose, a ceramic sintered body is shown, which includes a sialon phase and an amorphous or semi-crystalline grain boundary phase. The sialon phase includes a proportion of 20-80 wt-% of 15R-sialon polytypoid. The amorphous or semi-crystalline grain boundary phase possibly includes an YbAl garnet and constitutes up to 15 wt-% of the entire sintered body. The sintered body is manufactured from an inorganic raw material mixture which includes 40 to 57 wt-% of Si.sub.3N.sub.4; 40 to 55 wt-% of a mixture of AlN and Al.sub.2O.sub.3, wherein the ratio of Al.sub.2O.sub.3 to AlN lies in the range of 1-1.5:1, and 3 to 5 wt-% of Yb.sub.2O.sub.3 as sintering aid.

Provided is a highly heat-conductive silicon nitride sintered compact capable of achieving the improvement in both of a heat conductivity and a relative permittivity. The highly thermally-conductive silicon nitride sintered compact according to an embodiment includes silicon nitride crystal grains and a grain boundary phase. The thermal conductivity of the silicon nitride sintered compact is not less than 100 W/m.Math.K. A grain boundary phase present in a 5 m5 m measurement area in any cross section includes Mg and a rare-earth element (RE). The atom ratio of Mg to rare-earth element is within the range of not less than 0.01 and not more than 1.5. A relative dielectric constant .sub.10M-25 at 10 MHz and room temperature is not more than 9.0.