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 raw material and MnO. The sintered ceramic material can include about 0.01 wt % to about 10 wt % MnO, about 0.1 wt % to about 20 wt % Fe.sub.2O.sub.3, and about 0.01 wt % to about 10 wt % Mn.sub.2O.sub.3. The ceramic particle can have a size from about 8 mesh to about 170 mesh.

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 raw material and MnO. The sintered ceramic material can include about 0.01 wt % to about 10 wt % MnO, about 0.1 wt % to about 20 wt % Fe.sub.2O.sub.3, and about 0.01 wt % to about 10 wt % Mn.sub.2O.sub.3. The ceramic particle can have a size from about 8 mesh to about 170 mesh.

A porous material includes aggregate particles and a binding material. In the aggregate particles, oxide films containing cristobalite are provided on surfaces of particle bodies that are silicon carbide particles or silicon nitride particles. The binding material contains cordierite and binds the aggregate particles together in a state where pores are provided therein. The mass ratio of the cordierite to the whole of the porous material is in the range of 10 to 40 mass %. The oxide films that exist between the particle bodies and the binding material have a thickness less than or equal to 0.90 μm.

A porous material includes aggregate particles and a binding material. In the aggregate particles, oxide films containing cristobalite are provided on surfaces of particle bodies that are silicon carbide particles or silicon nitride particles. The binding material contains cordierite and binds the aggregate particles together in a state where pores are provided therein. The mass ratio of the cordierite to the whole of the porous material is in the range of 10 to 40 mass %. The oxide films that exist between the particle bodies and the binding material have a thickness less than or equal to 0.90 μm.

A ceramic containing, in mass %: Si.sub.3N.sub.4: 20.0 to 60.0%, ZrO.sub.2: 25.0 to 70.0%, at least one selected from SiC and AlN: 2.0 to 17.0%, where AlN is 10.0% or less, at least one selected from MgO, Y.sub.2O.sub.3, CeO.sub.2, CaO, HfO.sub.2, TiO.sub.2, Al.sub.2O.sub.3, SiO.sub.2, MoO.sub.3, CrO, CoO, ZnO, Ga.sub.2O.sub.3, Ta.sub.2O.sub.5, NiO and V.sub.2O.sub.5: 5.0 to 15.0%, wherein Fn calculated from the following equation (1) satisfies 0.02 to 0.40. This ceramic can be laser machined with high efficiency.

Fn=(SiC+3AlN)/(Si.sub.3N.sub.4+ZrO.sub.2)  (1)

A sintered body includes a crystal grain containing silicon nitride, and a grain boundary phase. If dielectric losses of the sintered body are measured while applying an alternating voltage to the sintered body and continuously changing a frequency of the alternating voltage from 50 Hz to 1 MHz, an average value ε.sub.A of dielectric losses of the sintered body in a frequency band from 800 kHz to 1 MHz and an average value ε.sub.B of dielectric losses of the sintered body in a frequency band from 100 Hz to 200 Hz satisfy an expression |ε.sub.A−ε.sub.B|≤0.1.

A sintered body includes a crystal grain containing silicon nitride, and a grain boundary phase. If dielectric losses of the sintered body are measured while applying an alternating voltage to the sintered body and continuously changing a frequency of the alternating voltage from 50 Hz to 1 MHz, an average value ε.sub.A of dielectric losses of the sintered body in a frequency band from 800 kHz to 1 MHz and an average value ε.sub.B of dielectric losses of the sintered body in a frequency band from 100 Hz to 200 Hz satisfy an expression |ε.sub.A−ε.sub.B|≤0.1.

The present invention provides a silicon nitride sintered substrate capable of reducing contamination caused by a boron nitride powder or the like used as a releasing agent and problems in bonding strength and dielectric strength at the time of laminating metal layers or the like, where the contamination is caused by a network structure provided by a silicon nitride crystal formed on the surface of the substrate in an unpolished state after sintering a silicon nitride powder. The silicon nitride substrate in an unpolished state after sintering is a silicon nitride sintered substrate where a cumulative volume of pores having a diameter in a range of 1 to 10 μm is not more than 7.0'10.sup.−5 mL/cm.sup.2 in a measurement by a mercury porosimetry. Preferably, Ra of the surface is not more than 0.6 μm and arithmetic mean peak curvature (Spc) of a peak is not more than 4.5 [l/mm].

Disclosed are a silicon nitride ceramic sintered body and a preparation method thereof. The silicon nitride ceramic sintered body has a content of a silicon nitride crystalline phase of not less than 98 wt %, a relative density of not less than 99%, a porosity of not larger than 1%, a grain boundary phase including Li, O, N, and Si elements, and a total content of C, F, Al, Mg, K, Ca, Na and rare-earth metals elements of less than 0.1 wt %.

According to an example, a green body may include from about 1 wt. % to about 20 wt. % of a metal nanoparticle binder and a build material powder, wherein the metal nanoparticle binder is selectively located within an area of the green body to impart a strength greater than about 3 MPa.