Abstract: Provided is a polycrystalline ceramic substrate to be bonded to a compound semiconductor substrate with a bonding layer interposed therebetween, wherein at least one of relational expression (1) 0.7<?1/?2<0.9 and relational expression (2) 0.7<?3/?4<0.9 holds, where ?1 represents a linear expansion coefficient of the polycrystalline ceramic substrate at 30° C. to 300° C. and ?2 represents a linear expansion coefficient of the compound semiconductor substrate at 30° C. to 300° C., and ?3 represents a linear expansion coefficient of the polycrystalline ceramic substrate at 30° C. to 1000° C. and ?4 represents a linear expansion coefficient of the compound semiconductor substrate at 30° C. to 1000° C.

Abstract: Provided is a joined body including a piezoelectric substrate and a polycrystalline spinel substrate provided on one main surface of the piezoelectric substrate, wherein the polycrystalline spinel substrate has a porosity of 0.005% or more and 0.6% or less.

Abstract: A ceramic substrate is formed of a polycrystalline ceramic and has a supporting main surface. The supporting main surface has a roughness of 0.01 nm or more and 3.0 nm or less in terms of Sa. The number of projections and depressions with a height of 1 nm or more in a square region with 50 ?m sides on the supporting main surface is less than 5 on average, and the number of projections and depressions with a height of 2 nm or more in the square region is less than 1 on average.

Abstract: Provided is a polycrystalline ceramic substrate to be bonded to a compound semiconductor substrate with a bonding layer interposed therebetween, wherein at least one of relational expression (1) 0.7<?1/?2<0.9 and relational expression (2) 0.7<?3/?4<0.9 holds, where ?1 represents a linear expansion coefficient of the polycrystalline ceramic substrate at 30° C. to 300° C. and ?2 represents a linear expansion coefficient of the compound semiconductor substrate at 30° C. to 300° C., and ?3 represents a linear expansion coefficient of the polycrystalline ceramic substrate at 30° C. to 1000° C. and ?4 represents a linear expansion coefficient of the compound semiconductor substrate at 30° C. to 1000° C.

Abstract: Provided are a liquid-crystal-display protection plate that has a high strength, is produced at a reduced cost, and has a shape including a curved surface; and a method for producing the liquid-crystal-display protection plate. The liquid-crystal-display protection plate is formed of a spinel sintered body. The spinel sintered body has an average grain size of 10 ?m or more and 100 ?m or less. The liquid-crystal-display protection plate has a shape including a curved surface.

Abstract: A ceramic substrate is formed of a polycrystalline ceramic and has a supporting main surface. The supporting main surface has a roughness of 0.01 nm or more and 3.0 nm or less in terms of Sa. The number of projections and depressions with a height of 1 nm or more in a square region with 50 ?m sides on the supporting main surface is less than 5 on average, and the number of projections and depressions with a height of 2 nm or more in the square region is less than 1 on average.