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SHORT COMMUNICATION - Determination of Trace Impurities In High Purity Aluminium Oxide Using INAA.

Byline: Yusuf Islam, Shahida Waheed, Naila Siddique and M. Mansha Chaudhry

Summary: Instrumental neutron activation analysis (INAA) technique was used to investigate 24 trace impurities in high purity alumina. Na was found to be a major element with concentration 159 ug/g while Fe, Cr, Sr, K and Ba were in lower amounts having concentrations from 9 to 27 ug/g. The low amounts of impurities suggest that deleterious effects upon the properties of high purity Al2O3 should not occur. Remaining impurity elements were in trace amounts; i.e. concentrations from 0.003 to 1.50 ug/g.

Keywords: High purity; Aluminum oxide; Neutron activation analysis (INAA); Trace impurities.

Introduction

Increasing awareness of the deleterious effects of impurities at trace level on specific properties of high purity aluminum oxide used in various fields of modern technology has led to an urgent need for highly sensitive and accurate instrumental analytical techniques with low limits of detection (LODs). Moreover extensive use of alumina for different high technology applications such as electrical systems components, petroleum cracking catalyst, and thermoluminescence (TL) dosimetry crystal requires their precise characterization. High purity alumina is used in microelectronics as a substrate and package material in integrated circuits, as digital microelectronic circuitry and as insulators in electronic devices [1], in medicine as material for bi-ceramic endoprothesis, in the oil and in Petrochemical industries as effective catalyst for fuel cracking and in thermoluminescence (TL) dosimetry [2-6].

Chemical analysis of aluminum oxide powders can be performed by direct or solution-based methods. There are numerous problems in the direct analysis of high purity aluminum oxide, such as volatilization of aluminum oxide, non-availability of solid reference material for calibration, grain size limitations and high limits of detection (LODs). Electrothermal vaporization-inductively coupled plasma-atomic emission spectrometry (ETV-ICP-AES), electrothermal vaporization-inductively coupled plasma-mass spectrometry (ETV-ICP-MS), electrothermal vaporization-inductively coupled plasma-optical emission spectrometry (ETV-ICP-OES) and electrothermal atomic absorption spectrometry (ETAAS) are widely used for the direct analysis of aluminum oxide powders. Aluminum oxide is thermally stable and often requires reagents to improve volatilization [1, 7-11]. Optimum sample pretreatment and vaporization temperatures and the most suitable thermo-chemical reagents are often analyte dependent.

Moreover in case of multi-element determinations, compromising conditions have to be used.

Several approaches may be used for calibration in direct solids sampling analysis: i.e. calibration 1) with solid standard reference materials, 2) with analyte solutions or 3) by standard addition using aqueous multi-element solutions. In the direct analysis of solid samples calibration is usually a problem as the behaviour of the same element may be dependent on matrix composition. Generally calibration via standard addition is feasible but requires further sample preparation, especially in measurement of multiple elements [4-7].

In solution-based methods such as ICP-AES-MS and atomic emission spectrometry (AES) difficulties in Al2O3 digestion, pre-concentration and/or matrix separation steps are the major problems that result in higher detection limits and lower precision due to the dilution and contamination of the sample. Additionally due to the high melting-point, hardness and chemical inertness of aluminum oxide, dissolution is tedious and time-consuming [6-12].

As the concentrations of most of trace elements in high purity aluminum oxide are at parts per billion (ppb) or parts per million (ppm) levels, multi-element trace analysis by conventional chemical analysis is very difficult and problematic. Instrumental neutron activation analysis (INAA) followed by high resolution gamma-ray spectrometry is a well-established, sensitive, non-destructive, accurate, precise, specific, and universally applicable analytical technique, which can provide multi-elemental characterization of aluminum oxide with low limits of detection (LODs) and high accuracy [13-15]. The accuracy and the limit of detection of trace impurities using INAA is strongly dependent on the type of material analyzed and on the presence of any interfering elements, which may result in a high background masking the I3-peaks of trace impurities. The relative form of the NAA technique has been utilized extensively in our laboratory for the analyses of diverse matrices.

We report the analysis of high purity aluminum oxide using INAA technique for the measurement of trace impurities.

Experimental

Sample Preparation

Sample of high purity aluminum oxide was purchased from Johnson Matthey, Materials Technology U.K, which contains low amounts of impurities. Sample contamination was avoided by taking care during sampling. Therefore all sample preparation procedures, up to the final sealing of the samples, were carried out at a clean bench facility. About 50-100 mg of the high purity aluminum oxide sample, in triplicate, were taken in 3 pre-cleaned labeled polyethylene capsules and heat-sealed. IAEA-SL-1 [16], lake sediment and IAEA-S7 [17], soil were employed as reference materials (RMs) for Quality Assurance (QA) purposes. The capsules containing samples and standards were finally packed and thermally sealed into polyethylene rabbits. The prepared targets along with the matrix appropriate standards were subjected to different irradiation protocols as discussed later.

Standard Preparation

Reference standards for the quantification of inorganic elements by INAA were prepared by taking ultrapure spectrographically standardized materials from Johnson, Matthey and Co., Limited, London. About 1 mg cm-3 of the materials was used to prepare stock solutions of the required elements. The solutions were diluted from 5 ug/g to a few ng/g to offer a wide range of standards for the elements to be determined. The solutions were dried on ash-less filter papers and sealed in polyethylene capsules for irradiation. Any undesired contribution of the filter paper was eliminated by simultaneous irradiation and counting of the blank filter paper and subtracting their contributions.

Neutron Irradiations

The samples and standards were irradiated for optimized protocols consisting of durations of 1 to 5 hours. Irradiations of samples were carried out at the Pakistan Atomic Research Reactor (PARR-2) pneumatic irradiation facility. PARR-2 is a 27 kW, Miniature Neutron Source Research Reactor (MNSR) with a thermal neutron flux of 1.0 x 1012 n/cm2 sec. The optimized procedures for the determination of trace impurities in high purity aluminum oxide were developed using the nuclear data [15, 18] listed in Table-1a and 1b. To monitor the variations in the thermal neutron flux Au and Al foil flux monitors were placed between the samples. The thermal neutron flux was found to be the same for all samples with negligible fluctuations. Prior to counting the irradiated samples and standards were transferred to clean pre-weighed polyethylene vials. The capsules were re-weighed to assess the exact weight of the irradiated sample.

Gamma-ray Spectrometry

A high purity germanium detector (Canberra Model AL-30) was used to acquire I3-ray spectra. Gamma spectrometric system used comprised of a PC-based Intertechnique Multichannel Analyzer (MCA) (Intertechnique) and a sensitive spectroscopy amplifier (ORTEC model 2010) connected to the detector. The system has a resolution of 1.9 keV for the 1332.5 keV peak of 60Co and a peak to Compton ratio of 40: 1. For acquiring data GammaVision, Version 6.01 (Advanced Measurement Technology, Inc.)" software was used. The detector is energy calibrated daily using 152Eu and 60Co sources. As relative NAA is used the irradiation, cooling and counting times are identical for all samples and standards for each irradiation protocol. Moreover all samples/ standards are counted at the same distance from the detector. Results were obtained using validated in-house computer programs.

All sources of errors were estimated to obtain the overall combined uncertainty in the calculations and background subtractions were also made for each irradiation [19].

Table-1: (a) Nuclear Data for Intermediate Irradiation Scheme [18].

###Isotope used###Half life###I3-ray used (keV)###Irradiation time (h)###Cooling time (d)###Counting time (sec)

###24 Na###14.96 h###1368.6###1###2###900

###42 K###12.36 h###1524.7###1###2###900

###140 La###1.7 d###1591.5###1###2###900

###122 Sb###2.7 d###564.1###1###2###900

###82 Br###35.4 h###554.3###1###2###900

###153 Sm###1.9 d###103.2###1###2###900

###175 Yb###4.2 d###396.3###1###2###900

Table-1: (b) Nuclear Data for Long Irradiation Scheme [18].

###Isotope used###Half life###I3-ray used (keV)###Irradiation time (h)###Cooling time (w)###Counting time (h)

###177 Lu###6.710 d###208.4###5###2###2

###147 Nd###10.98 d###91.1, 531.0###5###2###2

###131 Ba###11.80 d###469.3###5###2###2

###117m Sn###13.61 d###158.5###5###2###2

###86 Rb###18.60 d###1078.8###5###2###2

###233 Th###27.00 d###311.9###5###2###2

###51 Cr###27.80 d###320.1###5###2###2

###141 Ce###32.38 d###145.4###5###2###2

###203 Hg###41.60 d###279###5###2###2

###181 Hf###42.50 d###482.0###5###2###2

###59 Fe###44.60 d###1099.3, 1291.6###5###2###2

###46 Sc###83.90 d###889.3###5###2###2

###85 Sr###64.84 d###514.0###5###2###2

###75 Se###120.0 d###264.5###5###2###2

###65 Zn###244.0 d###1115.5###5###2###2

###134 Cs###2.060 y###795.8###5###2###2

###60 Co###5.270 y###1173.2, 1332.5###5###2###2

Limit of Detection

The lowest concentration, which can be determined by any analytical technique, is its limit of detection. For the undetected elements, limit of detection (LOD) were calculated from the spectra using the following equation [19]:

LOD = 3I / 100 * X (1)

where, X is the mean value of concentration and I is the standard deviation.

Like most techniques limit of detection for NAA depends on analyte, matrix, standard used for analysis and experimental conditions or system used for counting.

Results and Discussion

Two irradiation protocols were used for the quantification of elements using PARR-2 irradiation facilities. The devised optimum conditions employing 1 hour irradiation and 1-2 days cooling time were employed to quantify intermediate indicator radionuclides, i.e. 24Na, 140La, 153Sm, 42K, 122Sb, 82Br and 175Yb, whereas 5 hours irradiation along with 2-3 weeks cooling time were used to determine isotopes with longer half-lives i.e. 177Lu, 147Nd, 86Rb, 233U, 51Cr, 141Ce, 181Hf, 79Hg, 60Co, 59Fe, 117mSn, 46Sc, 75Se, 134Cs, 85Sr, 131Ba, 134Cs and 65Zn. Interferences by the matrix elements (Al and O) are eliminated using the above irradiation, cooling and counting protocols as aluminum is short-lived and decays during the cooling time whereas oxygen is not activated. Corrections for self-absorption due to penetration of gamma rays are not needed due to the small amount of samples (50-100 mg) used for analysis.

The thermal to fast flux provided at the pneumatic irradiation facility of PARR-2 has a ratio of 5.2. This reduces the production of 24Na via the 27Al(n, [alpha]) reaction and also prevents the possibility of spectral interferences for most of the measured elements. Selection of I3-peak energies, their abundance and yield ratios provide interference free analysis of some trace elements. The interference free photo-peaks 1596.5 keV, 145.5 keV, 91.11 keV, 103 keV, 208.4 keV, 264.66 keV, 889.3 keV, 482.2 keV and 554.35 keV of 140La, 141Ce, 147Nd, 153Sm, 177Lu, 75Se, 46Sc, 181Hf and 82Br respectively, were used for the measurement of these elements. The multiple I3-peaks of 59Fe (1099.2 and 1291.6 keV), 60Co (1173.2 and 1332.5 keV) and 134Cs (604.7 and 795.8 keV), were used for the determining these elements. These multiple I3-peaks of radioactive nuclei allowed a cross-check of the accuracy.

La and Yb are measured via the intermediate lived interference free I3-peaks of 1596.5 keV for 140La and 396.3 keV for 175Yb, respectively. 153Sm and 124Sb can be determined using both intermediate as well as long irradiation and counting schemes. For these elements the results corresponding to intermediate irradiation and counting schemes were taken because of the best quality assurance data. The observed interferences for 134Cs in the presence of 124Sb were overcome as mentioned earlier [16-20]. The I3-peaks of all the radionuclides were well resolved. Therefore the full energy peak area of 1115.5 keV of 65Zn was well separated from the 1120.5 keV peak of 46Sc [20-23].

Table-2: Analysis of IAEA Reference Materials (Concentration in ug/g) using INAA.

###Element###IAEA-S7 (Soil)17###IAEA-SL-1 (Lake Sediment)16

###Observed values###Certified values###Observed values###Certified values

###Ba###164.0 +- 28.8###(159)###658.0 +- 46.7###639.0 +- 53.0

###Br###7.23 +- 1.80###7.00 +- 3.50###6.24 +- 1.23###6.82+-1.73

###Ce###63.7 +- 6.6###61.0 +- 6.5###105.0 +- 13.8###117.0 +- 17.0

###Co###8.89 +- 0.98###8.90 +- 0.85###19.6 +- 0.4###19.8+-1.5

###Cr###64.5 +- 9.2###60.0 +- 12.5###101.0 +- 11.7###104.0 +- 9.0

###Cs###5.25 +- 0.63###5.40 +- 0.75###6.96 +- 0.78###7.01 +- 0.88

###Fe###25860 +- 600###(25700)###67420 +- 1400###67400 +- 1700

###Hf###4.94 +- 0.32###5.10 +- 0.35###4.30 +- 0.50###4.16 +- 0.58

###Hg###0.047 +- 0.050###(0.04)###0.16 +- 0.03###(0.13)

###K###12340 +- 925###(12100)###14860 +- 865###(15000)

###La###29.1 +- 2.4###28.0 +- 1.0###48.6 +- 4.5###52.6 +- 3.1

###Lu###0.32 +- 0.15###(0.30)###0.52 +- 0.08###(0.54)

###Na###2370 +- 86###(2400)###1769 +- 153###1720 +- 120

###Nd###27.30 +- 3.18###30.00 +- 4.50###45.00 +- 4.02###43.80 +- 2.80

###Rb###52.6 +- 4.4###51.0 +- 4.5###102.0 +- 9.4###113.0 +- 11.0

###Sb###1.59 +- 0.19###1.70 +- 0.20###1.28 +- 0.11###1.31 +- 0.12

###Sc###8.54 +- 0.94###8.30 +- 1.05###17.10 +- 1.28###17.30 +- 1.10

###Se###0.37 +- 0.45###(0.40)###2.95 +- 0.45###(2.9)

###Sm###5.05 +- 0.64###5.10 +- 0.45###9.30 +- 0.83###9.25 +- 0.51

###Sr###102.0 +- 7.2###108.0 +- 5.5###85.70 +- 1.72###(80.0)

###Th###8.14 +- 0.96###8.20 +- 1.10###14.30 +- 1.29###14.0 +- 1.0

###Yb###2.24 +- 0.28###2.40 +- 0.35###3.30 +- 0.37###3.42 +- 0.64

###Zn###108.0 +- 9.4###104.0 +- 10.5###218.0 +- 18.5###223.0 +- 10.0

Method reliability was checked by analyzing the reference materials; IAEA-S7 (Soil) and IAEA-SL-1(Lake Sediment) employing the above mentioned optimized conditions and procedures. Table-2, in which the results for these standards are presented along with their certified data, shows that our values are in fairly good agreement with the certified/information values. Accuracy of the results maybe explained through plots of % difference between measured values and the certified values for each reference material as shown in Fig. 1a and 1b. In both these figures the % difference between our values and the certified values is >10 % for only Hg with most elements having this parameter 20%, indicating that the data for each element falls in a narrow range. Moreover the concentrations for some elements are near or below the detection limits (Ba, Br, Fe, Hf, Rb, Se and Sm and Ce, K, La, Lu and Yb respectively). Therefore, such low amounts should not impart any deleterious effects upon the properties of high purity Al2O3.

Conclusions

The INAA methodology presented in this work was effectively applied for the measurement of trace impurities in high purity Al2O3. Most of the interferences, particularly Compton background, were noticeably reduced using relative method and proper selection of I3-peak energies, their abundance and yield ratios. Generally detection limits were low for most of the measured elements. It can be inferred from the analysis that impurities levels of some elements were high (Ba, Cr, Fe, K, Na and Sr) and their adverse effects may be notable depending upon its applications.

Acknowledgement

The authors are grateful to the staff of the NAA/MNSR Lab and the Reactor Operations Group (ROG), Nuclear Engineering Division (NED) at PINSTECH for providing the irradiation facilities at PARR-II

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