Provided is a large-sized silicon nitride sintered substrate and a method for producing the same. The silicon nitride sintered substrate has a main surface 101a of a shape larger than a square having a side of a length of 120 mm. A ratio dc/de of the density dc of the central area and the density de of the end area of the main surface 101a is 0.98 or higher. The void fraction vc of the central area of the main surface 101a is 1.80% or lower, and the void fraction ve of the end area is 1.00% or lower. It is preferred that the density dc of the central area is 3.120 g/cm.sup.3 or higher, the density de of the end area is 3.160 g/cm.sup.3 or higher, and a ratio ve/vc of the void fraction vc of the central area and the void fraction ve of the end area is 0.50 or higher.

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 .sub.A of dielectric losses of the sintered body in a frequency band from 800 kHz to 1 MHz and an average value .sub.B of dielectric losses of the sintered body in a frequency band from 100 Hz to 200 Hz satisfy an expression |.sub.A.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 .sub.A of dielectric losses of the sintered body in a frequency band from 800 kHz to 1 MHz and an average value .sub.B of dielectric losses of the sintered body in a frequency band from 100 Hz to 200 Hz satisfy an expression |.sub.A.sub.B|0.1.

A probe card board in the present disclosure includes a plurality of through holes designed to receive a probe brought into contact with a measurement object. The probe card board is composed of silicon nitride based ceramics. The probe card board includes a first surface opposed to the measurement object and a second surface located opposite to the first surface. The probe card board contains a plurality of crystal phases of metal silicide. Metal constituting the metal silicide is at least one kind selected from among molybdenum, chrome, iron, nickel, manganese, vanadium, niobium, tantalum, cobalt and tungsten.

A probe card board in the present disclosure includes a plurality of through holes designed to receive a probe brought into contact with a measurement object. The probe card board is composed of silicon nitride based ceramics. The probe card board includes a first surface opposed to the measurement object and a second surface located opposite to the first surface. The probe card board contains a plurality of crystal phases of metal silicide. Metal constituting the metal silicide is at least one kind selected from among molybdenum, chrome, iron, nickel, manganese, vanadium, niobium, tantalum, cobalt and tungsten.

A flow path member may include silicon nitride ceramics. The flow path member may have an inlet port, an outlet port, and a flow path connected to the inlet port and the outlet port inside the flow path member. A plurality of needle-shaped crystals may be arranged on a surface of the flow path where the needle-shaped crystals intersect each other.

A flow path member may include silicon nitride ceramics. The flow path member may have an inlet port, an outlet port, and a flow path connected to the inlet port and the outlet port inside the flow path member. A plurality of needle-shaped crystals may be arranged on a surface of the flow path where the needle-shaped crystals intersect each other.

A sintered material includes a first material and a second material, the first material being partially stabilized ZrO.sub.2 having a crystal grain boundary or crystal grain in which 5 to 90 volume % of Al.sub.2O.sub.3 is dispersed with respect to a whole of the first material, the second material including at least one of SiAlON, silicon nitride and titanium nitride, the sintered material including 1 to 50 volume % of the first material.

A sintered material includes a first material and a second material, the first material being partially stabilized ZrO.sub.2 having a crystal grain boundary or crystal grain in which 5 to 90 volume % of Al.sub.2O.sub.3 is dispersed with respect to a whole of the first material, the second material including at least one of SiAlON, silicon nitride and titanium nitride, the sintered material including 1 to 50 volume % of the first material.

Disclosed herein are a ceramic particle comprising a ceramic core substrate and a conformal coating of a sintering aid film on a surface of the core substrate, wherein the conformal coating includes a plurality of distributed islands of the sintering aid film across the surface of the core substrate; methods for producing the ceramic particle by ALD or MLD; and methods of using the coated ceramic particles in additive manufacturing or in solid oxide fuel cells. In one example, the film may have a thickness of less than three nanometers. The disclosed ceramic particle may be non-reactive with water.