A method of fabricating a polycrystalline CVD
synthetic diamond material having an average
thermal conductivity at
room temperature through a thickness of the polycrystalline CVD
synthetic diamond material of at least 2000 Wm−1K−1, the method comprising: loading a
refractory metal substrate into a CVD reactor; locating a
refractory metal guard ring around a
peripheral region of the
refractory metal substrate, the refractory metal
guard ring defining a gap between an edge of the refractory
metal substrate and the refractory metal
guard ring having a width 1.5 mm to 5.0 mm; introducing microwaves into the CVD reactor at a power such that the
power density in terms of power per unit area of the refractory
metal substrate is in a range 2.5 to 4.5 W mm−2; introducing process gas into the CVD reactor wherein the process gas within the CVD reactor comprises a
nitrogen concentration in a range 600 ppb to 1500 ppb calculated as
molecular nitrogen N2, a carbon containing
gas concentration in a range 0.5% to 3.0% by volume, and a
hydrogen concentration in a range 92% to 98.5% by volume; controlling an average temperature of the refractory
metal substrate to lie in a range 750° C. to 950° C. and to maintain a
temperature difference between an edge and a centre point on the refractory metal substrate of no more than 80° C. growing polycrystalline CVD
synthetic diamond material to a thickness of at least 1.3 mm on the refractory metal substrate; and cooling the polycrystalline CVD synthetic
diamond material to yield a polycrystalline CVD synthetic
diamond material having a thickness of at least 1.3 mm, an average
thermal conductivity at
room temperature through the thickness of the polycrystalline CVD synthetic
diamond material of at least 2000 Wm−1K−1 over at least a central area of the polycrystalline CVD synthetic diamond material, wherein the central area is at least 70% of a total area of the polycrystalline CVD synthetic diamond material, a single substitutional
nitrogen concentration no more than 0.80 ppm over at least the central area of the polycrystalline CVD synthetic diamond material, and wherein the polycrystalline CVD synthetic diamond material is substantially
crack free over at least the central area thereof such that the central area has no cracks which intersect both external major faces of the polycrystalline CVD synthetic diamond material and extend greater than 2 mm in length.