of the flakes the shape can be observed by a scanning electron microscope or particularly in refractory aggregate powders of al 2 o 3 sio 2 zro 2 etc and graphites a reflection microscope the surface of a layer as used in the specification is defined by using a standard sieve that is when graphites are sieved using a standard sieve having a mesh of 0 105 mm flaky graphites remaining on the sieve are expressed as d u2267 0 105 mm the size d can be readily confirmed by using a reflection microscope the thickness t of the flaky graphites as used in the present invention refers to a numerical value obtained by the following measurement method twenty 20 g of a graphite specimen is mixed with 80 g of fine alumina powders having a size of not greater than 44 u03bcm using a thermosetting resin using an amsler test molding machine the mixture is molded under 500 kg cm 2 to provide a briquette of u03c6 40 mm u00d7 15 mm this briquette is subjected to a hardening treatment at 150 u00b0 c for 24 hours and then provided as a specimen the surface perpendicular to the molding direction is ground and then observed with a reflection microscope twenty relatively long graphites are selected from the visual field the thickness is measured around the center in the longitudinal direction the average thickness of the intermediate 10 pieces n 10 eliminating the 5 largest and the 5 smallest is determined to be the thickness of the flaky graphites referring to the content of the thin flaky graphites improved performance is noted.