Table 1
Impurity concentrations (ppm) before (SPw) and after thermal annealing
	SPw (ref.)	700°C	800°C	900°C
Fe	5,100	20.77	10.54	0.41
Al	2,200	100.15	20.17	2.13
Ti	421	23.9	2.93	2.83
As	2	0.7	<0.05	<0.05
P	16	11	10	10
B	0.7	<0.05	<0.05	<0.05
Ni	5.6	3.1	1.4	0.17
Cu	5.5	1.8	1.5	0.14
Ca	98.1	10.21	9.85	9.1
Na	38	15.09	8.84	1.44
Mn	793	297	139	13.9
Mg	55	83	71	7
K	20	3.7	2.4	2.3
Cr	230	107	13	1.7
Co	7	6	6	6
Total (%)	0.9	0.07	0.03	0.01
Purity (%)	99.10081	99.93166	99.97034	99.99489

Annealing temperatures are 700°C, 800°C, and 900°C of silicon powder with a thin porous layer.

Khalifa et al. Nanoscale Research Letters 2012 7:444   doi:10.1186/1556-276X-7-444