Density measurements of silicon crystals by hydrostatic weighing

Through research for a determination of the Avogadro constant by the X-ray crystal density method, a primary density standard based on silicon crystals was established at the National Research Laboratory Metrology (NRLM) with a relative combined standard uncertainty of 1.0 x 10(-7). To investigate the cause of the e;existing discrepancy in the molar volume of silicon in single crystals, a new hydrostatic weighing system has been developed for evaluating the consistency in the absolute density values of the silicon crystals used for the determination of the Avogadro constant. This system measures the density of a solid material under study with respect to that of a primary standard with a relative combined standard uncertainty of 1.3 x 10(-7). Some measurement results on silicon spheres fabricated from different silicon ingots are described.