Patent ID: 8333833

Claim:
A method for preparing a low thermal expansion filler, comprising: dispersing a zirconium carbonate powder containing hafnium in an aqueous solution containing at least one ion selected from an alkali metal ion, an alkaline earth metal ion and an ammonium ion, and a phosphate ion; and aging the dispersion while wet-heating; and subsequently dry-heating the dispersion to give a low thermal expansion filler, wherein the low thermal expansion filler comprising a hexagonal zirconium phosphate powder represented by Formula (1) below, wherein a particle size of 0.8 μm to 50 μm is 95% or more on a volume basis with respect to the total particles, M a Zr b Hf c (PO 4 ) 3 .n H 2 O (1) wherein M represents at least one ion selected from the group consisting of an alkali metal ion, an alkaline earth metal ion, an ammonium ion and a hydrogen ion; a, b and c are values satisfying 1.75<b+c<2.25, and a+4(b+c)=9, in a case where M is monovalent, 2a+4(b+c)=9 in a case where M is divalent; a and b are each a positive number, c is a positive number, and n is zero or a positive number of 2 or less.