Analyrica C&z&a Acta, 276 (1993) 161-166 Elsevier Science Publishers B.V., Amsterdam

161

major and minor elements in sintered fine

’

Hiroshi Uchida

Industriul Research Institute of Kanagawa Prefecture, 3173, Showa-machi, Kanazawa-ku, Yokohama 236 (Japan)

(Received 10th August 1992; revised manuscript received 17th November 1992)

Abstract

A sputtering technique was applied to the decomposition of sintered fine ceramics for contamination-free analysis. Alumina-, zirconia- and lanthanum-doped lead zirconate titanates (PLZT) prepared as sputtering target were converted into thin films on a quartz plate by r.f. sputtering. The sample films were dissolved in hydrochloric or sulphuric acid as easily as the powdered samples. Major, minor and trace elements were determined by inductively coupled plasma atomic emission spectrometry (ICP-AES). The analytical results for alumina and zirconia agreed well with the original target composition under wide sputtering conditions, but it was necessary to optimize the r.f. power used in the sputtering process and the sputtering gas pressure for PLZT analysis. The relative standard deviations were approximately 1% for major, l-3% for minor and 3-8% for trace components.

Keywords: Atomic emission spectrometry; Ceramics; Plasma

Sintered fine ceramics have been used in a variety of fields because of their excellent charac- teristics such as high solidity and high resistance to chemical substances. There are many kinds of constitution in sintered ceramics; some of them are complex oxides and some are non-oxides. The exact determination of major, minor and trace elements is necessary in order to maintain the excellent characteristics, but the high resistance to chemicals of these materials sometimes ham- pers their chemical analysis. There are significant matrix effects in the analysis of solids, and the detection power without chemical separation is not always sufficient for ultra-trace determina-

Correspondence to: H. Uchida, Industrial Research Institute of Kanagawa Prefecture, 3173, Showa-machi, Kanazawa-ku, Yokohama 236 (Japan). ’ Part of this work was presented the International Congress

on Analytical Sciences (Chiba, Japan, 1991) [ll.

tions. Dissolution of samples is appropriate for the simple and exact determination of major and minor elements, and chemical separation is nec- essary for the determination of trace elements. Acid dissolution in a PTFE-lined bomb is simple to apply and suitable for the subsequent atomic absorption (AAS) and inductively coupled plasma atomic emission spectrometric (ICP-AES) deter- mination of major and minor elements in silicates [2,3]. The decomposition of powdered alumina 141, zirconia 151 and silicon nitride [6] has been investigated by using such sealed bombs. Pow- dered alumina [71, zirconia [81 and silicon carbide [9] are decomposed by fusion with alkali metal carbonate, but many kinds of sintered fine ceram- ics cannot be easily decomposed by acid dissolu- tion or alkali fusion. Further, crushing of solid ceramics causes contamination from the tools used.

In this work, a radiofrequency (r.f.) magnetron

0003-2670/93/$06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved

162 H. Uchida /AMI. Chim. Acta 276 (1993) 161-W

sputtering technique was applied to the decom- position of sintered fine ceramics without con- tamination. The sputtered sample was collected on the quartz plate, followed by dissolution in acids. Major and minor elements were deter- mined by ICP-AES, and the results obtained were compared with reported values and values ob- tained by other decomposition methods. Opti- mization of the sputtering conditions is discussed.

where the plate voltage was 2.0-4.5 kV and the current was 50-200 mA. Pure argon and argon mixed with 30% of oxygen were used as sputter- ing gases at pressures of 0.4-11.0 Pa. The sput- tered sample was collected on a quartz plate (3 99.99%) (25 x 25 x 2 mm). The sputtering time was 0.5-3.0 h to collect several mg of the thin-film sample on the plate. Details of the sputtering conditions are discussed later.

l%in-film dissolution and ICP analysis EXPERIMENTAL

Fine ceramic and other chemical samples Sintered samples of pure alumina (> 99%),

mixed alumina containing about 3% of calcium and magnesium oxides, stabilized zirconia con- taining about 6% of yttrium oxide (Kojundo Chemical Laboratory) and lanthanum-doped lead zirconate titanate (PLZT) (Kyodo International) were prepared as sputtering targets (10 cm diam- eter, 5 mm thick). A Siemens D500 x-ray diffrac- tometer was used for studying the crystallization.

The mixed alumina and PLZT samples con- verted into thin-films were dissolved in hydro- chloric acid (1 + 1) in a PTFE beaker on a hot- plate. The pure alumina and stabilized zirconia were decomposed with sulphuric acid (1 + 3) in a PTFE-lined bomb (San-ai Kagaku) at 230°C for 16 h [4,5].

Ultra-pure hydrochloric acid (36%) (Kant0 Kagaku Kogyo) and sulphuric acid (96%) (Kant0 Kagaku Kogyo) were used to dissolve the sample, and 1000 mg 1-r standard solutions for atomic absorption spectrometry (Kant0 Kagaku Kogyo) were used to prepare the solutions for calibra- tion.

The ICP spectrometer used and the operating conditions including analytical lines are given in Table 1. For the zirconia and PLZT analyses, the Al I 394.40 mn and Ca I 422.67 nm lines were employed to avoid overlap of emission caused by co-existing zirconium, titanium and yttrium. Ana- lytical results were calculated as a typical oxide (indicated as M,O, below).

RESULTS AND DISCUSSION

Sputtering method An Anelva SPF-210 sputtering device was used

for r.f. sputtering. The r.f. power was 200-800 W,

Thin-flm decomposition and limit of detection The sintered mixed alumina sample after

crushing with an agate mortar and pestle was decomposed with sulphuric acid (1 + 3) in a sealed

TABLE 1

ICP spectrometer and operating conditions

ICP spectrometer Mounting Grating Slit width R.f. power Argon flow-rate Observation height Signal integration Analytical lines (nm)

Seiko SPS12OOVR Czerny-Turner (1.0 m) 3600 lines mm-’ 20-40 pm 1.3 kW 16.0, 0.8,0.6 1 min-’ 15 mm above the induction coil 3 times 3 s Al I 3%.15 (I 394.40 “1, Ca II 393.37 (I 422.67 “1, Mg II 279.53, Si I 251.61, Fe II 259.94, Na I 590.54,

Pb II 217.00, La II 408.67, Zr II 343.82, Ti II 334.90, Y II 371.03, Hf II 264.14

’ Used for determinations in zirconia and PLZT.

H. Uchida /Anal. Chim. Acta 276 (1993) 161-166 163

TABLE 2

Limits of detection’ (%) for the sputtering method using ICP-AES

Analyte HCI dissolution H2SOd decomposition

~203 CaO MgO SO,

Fe@3 Na,O PbO

ho3

=lQ,

TiO,

w3

HfO2

0.02 0.01 0.005 0.02 b 0.01 0.01 0.008 0.002 0.001 0.006

- 0.03 0.005 0.002 -

0.01 0.02

0.002 0.001 0.0003 0.004

a Limit of detection was calculated as the concentration which gave a net signal equal to three times the standard deviation of five blank determinations, and converted to the content in 8 mg of collected sample. b The blank value of 0.093% was subtracted in each determination.

bomb, but could not dissolved in hydrochloric acid Cl+ 1) in a beaker on the hot plate. How- ever, the converted mixed alumina on the quartz plate was easily dissolved in hydrochloric acid (1 + 1). This might be due to a change in the chemical structure or an increase in the specific surface area of the sintered mixed alumina in the sputtering process. The crystal structures of (Y- Al,O,, MgAl,O, and CaAl,O, were observed in the original target by x-ray diffractometry, but no crystallization was found in the converted thin film. The converted PLZT was also dissolved in hydrochloric acid Cl+ 11, but converted pure alu- mina and stabilized zirconia samples were only decomposed in sulphuric acid Cl+ 3) in a sealed bomb.

The limit of detection was obtained from five blank samples using only acid without ceramic sample and treated in the same decomposition process. Results calculated as the typical oxide assuming that 8 mg of sample were collected on the plate (5-10 mg collected under the investi- gated sputtering conditions) are given in Table 2. The values obtained with hydrochloric acid disso- lution were almost identical with those for sealed bomb decomposition with sulphuric acid, except for SiO,. A trace amount of silicate in the quartz

200 400 8W 8m

RF pOW8r (WI

Fig. 1. Effect of r.f. power on the composition of mixed alumina. Mixed argon-oxygen gas pressure, 0.8 Pa (solid line); pure argon gas pressure, 0.8 Pa (dashed line).

plate is dissolved in hydrochloric acid, which might make the SiO, detection limit poorer then those for other elements. Further, the detection limit of SiO, could not be obtained in the sealed vessel decomposition with sulphuric acid, because approximately 1 mg of silicate was dissolved from the quartz plate. The limits of detection in Table 2 indicate that major and minor components ex- cept silicate are determined when several mg of the sputtered sample are collected on the quartz plate.

Effect of sputtering r.f power and gas The amount of sample collected on the plate

increased with increase in the r.f. power. The

70

J 0.. ---__ -_,__Pbo__o-------” o-o-o-.

8

5’ 200 400 800

RF power tW1 Fig. 2. Effect of r.f. power on the composition of PLZT. Mixed argon-oxygen gas pressure, 0.8 Pa (solid line); 11.0 Pa (dashed line).

164 H. Uchida /Anal Chim. Acta 276 (1993) 161-166

.-.-

Ar + 02 pressure (Pa) Fig. 3. Effect of mired argon-oxygen gas composition of PLZT. R.f. power, 300 W.

pressure on the

amount with pure argon sputtering was greater than that with mixed argon-oxygen, which means that pure argon is better suited for the determi-

TABLE 4

Analytical results (%)

TABLE 3

Optimized sputtering conditions

Parameter Alumina Zirconia PLZT

Incident r.f. 0 power 670 580 510 Gas composition Ar-0, ‘k-02 Ar-0, Gas (Pa) pressure 0.8 2.0 3.0 Sputtering time (h) 3.0 1.0 0.5 Amount collected (mg) 8.46 b; 8.75 ’ 6.78 7.14 R.S.D. (%) a 1.4 b; 5.3 c 2.5 0.9

a Obtained from five samples. b Mixed Ar-0,. c Pure Ar.

nation of minor elements. However, the content of MgO significantly decreased and that of CaO slightly increased on increasing the r.f. power in pure argon sputtering, as shown in Fig. 1. On the other hand, the contents of CaO and MgO were constant for r.f. powers varying from 200 to 800 W when mixed argon-oxygen sputtering was ap-

Oxide

‘+GZ CaO MgG SiO, Fe203 Na,O

Mired ahnnina

Proposed R.S.D. ’ method

94.3 0.9 2.95 1.5 2.67 1.3 0.08 18 0.02 4.7 0.03 5.6

Stabilized zirconia

Proposed R.S.D. a method

Crushed tablet b

93.0 2.92 2.71 0.98 0.03 0.03

Powder a

Report ’

3.1 3.0

Report ’

Pure alumina

Proposed R.S.D. a Report c method

99.8 1.0 0.03 5.2 0.01 4.3

0.14 0.03 6.3 0.10 0.08 8.4 0.14

PL

Proposed R.S.D. a Scraping e Report c method

PbO

La2o3

ZrO, Ti02

Hfo2

w3

A12o3

CaO

MgG SiO,

Fe@3 Na,O

64.7 0.2 64.2 63.0 3.99 0.8 3.90 3.91

21.2 1.3 22.4 24.4 9.16 0.3 8.66 8.64 0.40 2.0 0.49

92.8 0.3 0.16 2.5 0.61 1.1 6.21 1.7 0.021 21 0.24 3.6 0.046 9.1

0.021 0.064

-

22 4.8

93.0 0.022 0.65 6.05 0.023 0.18 0.040 0.091 0.012 0.038

0.02 1.2 5.5 0.01 0.1 0.03

0.01

0.16 4.0 0.13 0.058 9.6 0.074 0.013 7.5 0.013 0.15 20 0.089 0.011 24 0.013 0.047 7.2 0.034

’ Obtained from five independent determinations. b Analytical value for tablet with composition and structure the same as those of the target, crushed with an agate mortar and pestle. c Value reported by the target manufacturer. d Analytical value for the powder sample before sintering. e Analytical value for a scraping taken from the target.

H. Uchida /Anal. Chim. Acta 276 (1993) 161-166 165

plied. This result indicates that the species in the target are selectively sputtered under different sputtering conditions. Mixed argon-oxygen was subsequently used as the sputtering gas to avoid selective sputtering. In this work, selective sput- tering between metals and oxygen is not consid- ered. Excess and lack of oxygen were observed in collected samples obtained using mixed argon- oxygen and pure argon, respectively. The amount of the sample collected was defined as the total of the obtained typical oxides.

Variation of the r.f. power did not affect the contents of ZrO, and Y203; they were constant, as with alumina sputtering with mixed argon.

In the case of PLZT sputtering at 0.8 Pa, the contents of the three major oxides were slightly shifted when the r.f. power increased, but they fluctuated at 11.0 Pa, as shown in Fig. 2. Further, the measured content of PbO increased by more than 2% and that of ZrO, decreased oppositely when the pressure of mixed argon-oxygen was increased from 0.8 to 11.0 Pa. The r.f. power and the sputtering gas pressure should be optimized to make a thin film with a composition the same as that of the original target.

Hardly any effect of mixed argon-oxygen pres- sure on the composition of mixed alumina and stabilized zirconia was observed. The contents of major oxides were almost constant in the range 1.0-11.0 Pa.

However, in the PLZT sputtering shown in Fig. 3, the measured contents of three major oxides were changes significantly below 2.0 Pa. The content of TiO, was constant above 2.0 Pa, but that of PbO decreased and that of ZrO, increased. The pressure of mixed argon-oxygen as a sputtering gas should also be optimized in the case of PLZT.

Analytical results and reproducibility The final sputtering conditions used and the

sample amounts collected are summarized in Table 3. In the PLZT optimization, the r.f. power for sputtering was higher and the gas pressure was lower than in the previous work [l]. A sput- tering time was chosen such that about 6-8 mg of the sample were collected. The actual sputtering time for alumina, zirconia and PLZT was 3.0, 1.0

and 0.5 h, respectively. The amount of alumina collected per hour was less than those for the other two ceramics. The relative standard devia- tions (R5D.s) from five samples were in the range l-5%.

Analytical results obtained by the proposed method are summarized in Table 4, with R.S.D. values obtained from five independent determi- nations. The analytical value for SiO, in crushed mixed alumina was higher than by the proposed method because of contamination from the agate mortar and pestle used. The R.S.D.s of the deter- mination of SiO, with hydrochloric acid dissolu- tion were ca. 20%, which was caused by the trace dissolution of SiO, from the quartz plate. Fur- ther, SiO, could not be determined by sealed bomb decomposition with sulphuric acid, where the blank level was ca. 100 times larger than the amount to be determined. The final results agreed well with values reported by the target manufac- turer and values obtained by other methods. The R.S.D.s were approximately 1% for major, l-3% for minor and 3-8% for trace components.

Conclusion The proposed sputtering technique is effective

for the determination of major and minor ele- ments in sintered fine ceramics with low contami- nation. The collected samples are dissolved in acids just as easily as the powdered samples. Thin films with the same content are prepared from the original target under widely varying sputter- ing conditions in the case of alumina and zirco- nia. Optimization on the r.f. power and the gas pressure are required for PLZT to avoid selective sputtering.

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