Chemistry Reference
In-Depth Information
or yttrium-alumina-garnets for lasers, and used as heat-resistant combustion
chambers in high-pressure sodium lamps.
The second group of technical ceramics consists of a great variety of
nonoxides, such as carbides (Be
2
C, Al
4
C
3
,B
4
C, WC, and SiC also called
carborundum), nitrides (e.g., BN, TiN, Si
3
N
4
), silicides (e.g., MoSi
2
, NbSi
2
,
WSi
2
), and borides (e.g., SiB
3
, LaB
6
). Some of them are applied as thin metal
coatings.
The third group of ceramics includes composite materials, including combi-
nations of oxides and nonoxides possibly reinforced by particulates or fibers.
Most of these products are available as high purity powders with impurities of
less than 0.1%.
The first application of TXRF on the analysis of high-purity ceramic
powders was reported by Graule
etal
. in 1989 [129]. Several powders of
Al
2
O
3
, AlN, Si
3
N
4
, and SiC were analyzed by ICP-OES with slurry atomization
using a special nebulizer. TXRF was proposed and introduced as a competitive
method and was shown to have comparable capabilities for routine analysis and
production control. More than 15 years later, TXRF was used for the determi-
nation of nine elements in Al
2
O
3
ceramic powders [130]. Slurries containing
10 mg of powder per ml of pure water were prepared with cobalt as an internal
standard (200
μ
g/g). Aliquots of 5
μ
l were deposited on quartz carriers, evapo-
rated, and excited by synchrotron radiation. Systematic errors are reduced for
fine powders with a grain size of less than 10
μ
m and reliable results were
obtained. The detection limits for Ca, V, Cr, Mn, Fe, Ni, Cu, Zn, and Ga were
found to be in the range of 0.3-7
μ
g/g (ppm).
In another paper, boron nitride powders were studied by TXRF [131].
Slurries of BN with a good stability were obtained by the addition of PEI
(polyethylenimine) at a concentration of 1 mg/g and a pH value of 4. This time,
5 mg ceramic powder was suspended in 1 ml pure water and gallium was added
as an internal standard. Six elements (Ca, Ti, Cr, Mn, Fe, and Cu) could be
determined in four different ceramic powders. Detection limits for the elements
calcium and copper determined by TXRF were found to be 0.06-1.6
μ
g/g, and
herewith are higher than those obtained for GF-AAS with slurry sampling and
for ICP-OES with electrothermal evaporation. However, detection limits of
TXRF are lower than those of GF-AAS and ICP-OES after a previous
chemical digestion of the boron nitride powders.
In addition to the mentioned ceramic powders, TXRF was applied to the
analysis of the certified reference materials SiC and Al
2
O
3
. Three elements Ca,
Ti, and Fe, were determined by calibration straight lines in a concentration
range of 2.5
μ
g/g to 1.5 mg/g and correlation coefficients greater than 0.991 were
achieved. The advantage of a simple calibration by an internal standard valid
for all matrices was emphasized.
A similar study was carried out for boron carbide powders after stabiliza-
tion [132]. Eight elements (Ca, Ti, Cr, Mn, Fe, Co, Ni, and Cu) could be
determined in seven different ceramic powders. Again, gallium served as the
internal standard. Detection limits ranging from 0.1 to 1.2
μ
g/g (ppm) were
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