Abstract: A sintered aluminum nitride substrate having a thermal conductivity of about 60 W/m-K to about 150 W/m-K, a flexural strength of about 200 MPa to about 325 MPa, a volume resistivity of greater than 1010 Ohm cm, a density of at least about 95% of theoretical, optionally at least 97%, and a reflectance factor of at least about 60% substantially over the wavelength range of 360 nm to 820 nm. A low temperature process for sintering aluminum nitride includes providing an AlN sintering formulation of AlN powder and a sintering aid of yttria, calcia, and optionally added alumina, forming the AlN sintering formulation into a green body, and sintering the green body at a temperature of about 1675° C. to 1750° C.