Technique for the analysis of insulating materials by glow discharge mass spectrometry

The present invention describes a technique for sample preparation and analysis of ceramics and oxides. The technique involves mixing the ceramic or oxide powder with a conducting powder such as gallium, indium or silver and adding a small amount of dopant. The dopant comprises approximately 5-30% by weight of the sample and is selected from the group comprising thoria, yttria or ytterbia. It is theorized that the addition of the dopant provides a source of electrons that stabilizes the plasma in the glow discharge mass spectrometer which allows for impurity analysis in the part-per-million range.

BACKGROUND OF THE INVENTION 
The present invention discloses a method for analyzing ceramic materials to 
determine their impurity content. More specifically the present invention 
describes a Glow Discharge Mass Spectrometry (GDMS) analysis technique for 
determining trace impurity content in ceramic and ceramic-type materials 
such as alumina and silica. 
Ceramic materials play an increasing role in meeting the needs of industry 
and society. Because of their high melting temperatures, thermal shock 
capabilities and resistance to harsh atmospheres, various ceramic products 
have found their way into such areas as automotive, lighting and space 
technologies. In order to minimize the chances of a part failing 
prematurely, the starting powders should be screened for contaminants, 
which may cause microscopic fracturing during forming operations or from 
mechanical stress during use. This quality control may be performed by 
various methods including spark source mass spectrometry, (SSMS), emission 
spectrography, inductively coupled plasma, spectrometry (ICP) or glow 
discharge mass spectrometry, (GDMS). 
While emission spectrography does offer adequate sensitivity, it lacks 
accuracy, is time consuming and the photoplate may be difficult to 
interpret. SSMS offers similar sensitivity and better accuracy, but it 
lacks the resolution necessary to separate interference peaks from the 
peaks of interest. For example, the identification of the silicon dimmers 
from the iron and nickel peaks are not readily determined using SSMS 
because of isotopic interferences. Also, matrix effects of this technique 
can be severe, making it more standard dependent than GDMS. ICP offers 
good sensitivity and excellent accuracy, but sample preparation is 
difficult and time consuming for many ceramic materials, and great care 
must be taken not to volatilize any elements such as boron or silicon 
during the dissolution process with hydrofluoric acid. 
In GDMS, the sample to be analyzed forms the cathode in a low pressure gas 
discharge. Argon is typically used as the gas. Positive gas ions are 
accelerated towards the cathode with energies of a few hundred electron 
volts thereby sputtering the sample. The sputtered neutral species diffuse 
through the discharge gas where some are ionized. The positive ions are 
extracted through a small slit and accelerated into a high resolution mass 
spectrometer for analysis. 
The glow discharge produces a stable ion beam with few multicharged species 
and is therefore suited to producing consistent data. In contrast, the 
traditional spark ion source has poor ion stability and produces complex 
mass spectra which requires long integration times to optimize the 
sensitivity, commonly uses photographic plates for detection, as well as 
the need for a skilled operator to interpret the mass spectra on the photo 
plates. 
While recently gaining in prominence, GDMS is an old analytical technique. 
Also, it is not the panacea for all elements. For example, potassium and 
calcium determinations at the low ppm range are not possible because of 
interferences from argon ions. Using a different discharge gas, such as 
xenon, reduces this problem, but at the expense of sensitivity. GDMS does 
offer excellent resolution (4000-10000 Daltons), straight forward sample 
preparation, and short analysis time. 
Samples that are analyzed by GDMS must be conducting since they serve as 
one of the electrodes of a small hollow cathode cell. Therefore 
nonconducting material, such as insulating and ceramic materials must be 
mixed with a high purity conducting powder such as In(indium), 
Ga(gallium), or Ag(silver). For most insulating materials this procedure 
is quite satisfactory. However, it has been shown that materials such as 
silica and alumina cannot be run using the standard approach because the 
discharge (voltage and current) in the hollow cathode cell is not constant 
enough to allow for stable cell operation. This instability can be reduced 
if the sample to binder (silver, indium or gallium) ratio is reduced to 1 
part Sample to 50 parts binder, or if the discharge parameters are 
extremely low, 0.2 mA. However, this stability is achieved at the expense 
of sensitivity, which now would be greater than 100 ppm for many elements. 
The present invention describes a technique wherein ceramic materials that 
were previously not possible to analyze, such as alumina and silica, can 
be analyzed for impurities in the part-per-million range using GDMS. 
SUMMARY OF THE INVENTION 
The present invention describes a method for analyzing an oxide or ceramic 
sample for trace impurities. The method involves preparing the sample by 
mixing the oxide or ceramic material with a conducting powder such as 
gallium, indium or silver and a dopant such as yttria (Y.sub.2 O.sub.3), 
thoria (ThO.sub.2), or ytterbia (Yb.sub.2 O.sub.3) to stabilize the 
discharge. The sample is then formed into a small rod suitable for 
analysis by glow discharge mass spectrometry. If the additive or dopant is 
not included in the mixture, the resulting cathode rod does not give a 
stable discharge and accurate analysis of impurities at the 
part-per-million level is not achievable.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The present invention describes a method of preparing nonconducting samples 
for GDMS. Nonconducting samples such as alumina and silica must be mixed 
with a conducting powder and a dopant additive to form a conducting 
electrode in order to produce a stable discharge. The method involves 
mixing powders of alumina or silica which are less than 37 microns with 
approximately 5-30% by weight of thoria (ThO.sub.2), yttria (Y.sub.2 
O.sub.3) or ytterbia (Yb.sub.2 O.sub.3) along with a conducting species 
such as indium, silver or gallium. The results of the analyzed samples 
indicate that improved plasma stability is obtained compared to mixing 
alumina or silica with only the conducting powders. 
A VG9000 Glow Discharge Mass Spectrometer, equipped with a standard 
discharge cell was used for all of the following examples. The mass 
spectrometer is a double focusing, dual detector system with a maximum 
resolution of 10000 Daltons and a peak intensity dynamic range of 
5.times.10.sup.7 at 4000 Daltons, the typical operating resolution. This 
allows base line resolution of the major iron and nickel isotopes in a 
quartz matrix from Si.sub.2.sup.+ peak, a difference of 19 milli-mass 
units (FIG. 1). The standard cell is liquid nitrogen cooled to 
-190.degree. C. to minimize the contribution of water and CO in the 
discharge, which would have an adverse affect on its efficiency, and the 
stability of the discharge. The discharge gas is argon, which was obtained 
from liquid argon boil off and purified by passing it through a gettering 
furnace, Centorr model 2G-100-SS, prior to its introduction into the mass 
spectrometer. 
EXAMPLE 1 
Sample preparation for alumina, silica, silicon carbide and silicon nitride 
was as follows. Approximately 50 milligrams of sample powder were placed 
in a plastic vial with 300 mg of 37 micron 5N pure indium powder, which 
was obtained from Cerac Inc., Milwaukee Wisc., as well as two teflon balls 
3/32" diameter. This was shaken for 5 minutes on a Spex 5100 Mixer/Mill to 
obtain a uniform blend. The sample-indium mix was loaded into a 
polypropylene plug that has a 2 mm.times.18 mm hole drilled perpendicular 
in its cyclindrical axis, and pressed at 20000 pound for one minute to 
form the sample electrode. The sample was mounted into the mass 
spectrometer and a discharge was struck. The plasma was allowed to 
stabilize for thirty minutes before data collection commenced. 
Samples of alumina and silica prepared in this manner would not discharge 
in a stable manner. The glow discharge power supply fluctuated over 
several hundred volts, and the accelerating voltage showed variations of 2 
kV or more. Because of the nature of the sample (an insulator), it was 
theorized that a source of more freely liberated electrons was needed to 
sustain the discharge. It was found that the addition, several percent of 
yttria, silica, thoria or ytterbia to alumina; or yttria or thoria to 
silica would satisfy this need. Table I summarizes the composition of the 
optimum mixtures. It is also critical that the particle size of the 
alumina and silica powders be on the order of less than 50 microns. 
Therefore, these samples were passed through a 37 micron nylon sieve prior 
to the addition of indium, mixing and compacting. 
TABLE I 
______________________________________ 
Ceramic-Dopant Mixes 
Matrix mg Matrix mg Dopant mg Indium 
______________________________________ 
Alumina 65 20 Y.sub.2 O.sub.3 
330 
Alumina 50 10 Yb.sub.2 O.sub.3 
325 
Alumina 35 5 SiO.sub.2 
330 
Alumina 50 15 ThO.sub.2 
325 
Silica 52 16 Y.sub.2 O.sub.3 
300 
______________________________________ 
Because the concentrations of silicon and yttrium were of interest in the 
alumina samples, the majority of the analyses involved the use of thoria 
as the additive to the electrode mixture to sustain the discharge. 
The operating parameters of the discharge were also sample dependent, 
therefore, the optimum signal to noise ratio had to be determined for each 
sample type. Most oxide-nitride/indium mixtures could be analyzed with a 
discharge current of 1.5 mA and a discharge voltage of 800-900 volts 
without any significant breakdown. Alumina and silica on the other hand 
could only withstand a discharge voltage of 500-600 volts at a current of 
1.5 mA. These parameters yielded a matrix signal of 2.times.10.sup.-12 to 
1.times.10.sup.-11 amps for the alumina or silica samples, and greater 
than or equal to 1.times.10.sup.-11 amps for most other oxides-nitrides 
while maintaining a background signal of less than 1.times.10.sup.-17 
amps. To minimize the chance that a stray arc would distort a peak beyond 
what is software correctable, the Daly counting time for the alumina and 
silica experiments was 200 milliseconds with each peak scanned once. For 
all other samples, the Daly counting time was set to 200 milliseconds, and 
two or three scans were taken of each peak. For all experiments, the 
Faraday counting time was 160 milliseconds, with 2 scans recorded for the 
major peaks. 
RESULTS 
For silica, the data were corrected for sensitivity variations by entering 
the standard VG sensitivity factors. These are based on the fact that the 
ion yield of all elements are within a factor of three of iron as 
discussed in the article by Guidobini et al., Journal of Crystal Growth 
89, 1988. The results were accurate to approximately 50% for most samples, 
and no worse than a factor of two for any certified impurity when 
corrected using relative sensitivity factors (RSF's) based on NBS SRM 102 
(a silica powder) and illustrated in Table II. Results are listed in ppm 
by weight. 
TABLE II 
______________________________________ 
SRM 102 Silica Using Y.sub.2 O.sub.3 Additive 
Element Cert. PPM Calc. PPM 
______________________________________ 
Al 10400 20200 
Fe 4620 4000 
Ti 960 680 
Zr 150 240 
P 110 83 
Mn 38 30 
Ca 16400 15000 
Mg 1260 1400 
K 2410 2500 
______________________________________ 
Detection limits of 1 ppm are attainable in high purity quartz as shown in 
Table III. 
TABLE III 
______________________________________ 
Quartz Powder Using ThO.sub.2 Additive 
Element 
PPM 
______________________________________ 
Na 4.2 
Al 18 
Ti 0.93 
Fe 5.5 
Mg 1.1 
P 0.91 
Cr 0.54 
Ni 2.0 
______________________________________ 
Base line resolution is easily achieved between .sup.56 Fe.sup.+ and 
.sup.28 Si.sub.2.sup.+, as demonstrated in FIG. 1. 
Because of a great interest in developing a rapid and accurate method of 
alumina assaying at this laboratory, a set of sensitivity factors based on 
SRM 699 and BCS 394 (a British standard designation) alumina standards 
were developed. These improved the accuracy of the results from a factor 
of two, to less than 50%, (Table V). A list of these factors is outlined 
in Table IV. 
TABLE IV 
______________________________________ 
Alumina Relative Sensitivity Factors (RFS) 
Element 
Factor 
______________________________________ 
Al 1.00 
Na 0.51 
Mg 0.62 
Si 0.99 
P 1.94 
Ca 0.33 
Ti 0.26 
Fe 0.67 
Zn 2.80 
______________________________________ 
TABLE V 
______________________________________ 
Standard vs Calculated RSF's SRM 699 
Without PPM 
Element 
RSF'S Standard RSF 
Calculated 
Cert. 
______________________________________ 
Na 8600 12000 4400 4377 
Mg 5.7 6.5 3.5 3.6 
Si 120 180 120 65 
Ca 530 250 180 257 
Cr 2.6 3.5 2.6 1.4 
Mn 3.1 3.0 3.1 3.9 
Fe 69 47 46 91 
Zn 21 76 60 100 
______________________________________ 
As with silica, the detection limits of alumina samples were in the ppm 
range, with an aluminum signal of 1.times.10.sup.-11 amps achieved, (Table 
VI). 
TABLE VI 
______________________________________ 
Polycrystaline Alumina Powder 
Element PPM Element PPM 
______________________________________ 
Na 19 Mg 200 
Si 480 P 1.8 
Cl 65 Ti 9.1 
Cr 5.2 Mn 1.8 
Fe 23 Ni 3.0 
______________________________________ 
From the experiments and results outlined above, it is apparent that glow 
discharge mass spectrometry can be used to analyze the impurity content of 
oxides and ceramics. Detection limits approaching 1 ppm are readily 
obtained. Sample preparation and analysis time are two hours per sample 
for up to 15 impurities. This is achieved with accuracy, sensitivity and 
precision that i lacing in other techniques such as SSMS or emission 
spectrography. 
As shown in the above samples, thoria can be used to replace yttria as an 
additive to analyze ceramic materials under GDMS. The above examples show 
that less thoria than yttria is required to facilitate the running of 
silica and alumina. Typically 5-30% of weight of thoria is sufficient to 
produce a stable discharge. Polycrystalline alumina arc tubes often 
contain a minor amount of yttria as a sintering aid. Therefore the use of 
thoria allows the determination of the concentration of yttria in the 
alumina. This would not be possible if yttria was used as an additive to 
create a stable discharge. Another advantage of using thoria is that 
thorium has a higher atomic mass and therefore results in fewer 
interferences in the mass spectrum which allows for lower detection limits 
and easier data interpretation. 
While the present invention has been shown and described what is at present 
considered the preferred embodiment of the invention, various changes and 
modifications will be obvious to those skilled in the art. All such 
modifications are intended to fall within the scope of the invention as 
defined by the appended claims.