Nitrogen determination by SEMEDS and elemental

Research article Received: 21 December 2012

Revised: 3 April 2013

Accepted: 9 April 2013

Published online in Wiley Online Library: 23 May 2013

(wileyonlinelibrary.com) DOI 10.1002/xrs.2490

Nitrogen determination by SEM-EDS and elemental analysis M.F. Gazulla,* M. Rodrigo, E. Blasco and M. Orduña This paper describes a methodology for the analysis of nitrogen by scanning electron microscope with an energy dispersive Xray spectrometer (SEM-EDS). The methodology was developed to have a rapid and accurate alternative method to the elemental analysis by combustion and thermoconductivity detection that does not imply the decomposition of the sample. Two methods by SEM-EDS were established: a quantitative method trying to construct a calibration curve with reference materials and another using the standardless method provided with the instrument software, and the results were compared with those obtained by elemental analysis using two instruments that work at different temperature. An important matrix effect was found when trying to construct a calibration curve for SEM-EDS for any kind of material, being corrected when using the standardless method because this method corrects the matrix effect. The quantification of nitrogen by SEM-EDS is a good alternative to elemental analysis by combustion and thermoconductivity detection in those cases where the sample has a very high decomposition temperature. Copyright © 2013 John Wiley & Sons, Ltd.

Introduction

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The determination of nitrogen in materials such as Si3N4, Si/C and Zr/C bilayers, and so on, has been carried out by nuclear techniques[1,2] but, apart from being complex techniques, no reference materials used in the measurement validation are available. In other type of materials such as plant materials, spectroscopic techniques were used.[3,4] There are studies that report the determination of nitrogen in other materials, such as silicates, soils and sediments, but with a huge disparity in the results obtained and methods used.[5] In a previous study, a compilation of reference materials containing nitrogen was carried out to enhance the knowledge about materials containing this element that can be used as reference materials for validation and calibration for the analysis of nitrogen.[6] The most widespread nitrogen determination method nowadays is the decomposition of the sample with temperature and the detection of the analyte by thermal conductivity.[5,7] The appearance of new materials with high technological properties and very high decomposition temperatures makes necessary the development of an alternative method, rapid and accurate to analyse materials that could not decompose by at the work temperature of the elemental analysis (combustion and thermoconductivity detection). Scanning electron microscopy with energy dispersive X-ray spectrometry (SEM-EDS) is an elemental microanalysis method widely applied across in physical and biological sciences, engineering, technology and forensic investigations,[8] which is able to identify and quantify all the elements in the periodic table except H, He and Li. One of the limitations of this technique is in the field of low Z elements detection due to the X-ray absorption phenomena, which occurs as consequence of the Be window of the detector. Some publications [9–11] show the Monte Carlo simulation technique for the quantification of the composition of microparticles, giving acceptable results for low Z elements.

X-Ray Spectrom. 2013, 42, 394–401

The aim of this paper is the development of an alternative method for the analysis of nitrogen, easy to conduct, using a technique where the decomposition of the sample is not required and capable to analyse different type of samples, such as nitrides, nitrates, oxynitrides, nitrogen doped materials, and so on. The alternative method was developed using an SEMEDS Si(Li) provided with super ultra thin window, which improves the sensitivity of low Z element signal. Results with elemental analysis are also shown in this paper using two different pieces of equipment, and a comparison of the results was carried out. The limits of detection and quantification were calculated for both methods.

Experimental Two methods were developed, one by SEM-EDS and another by elemental analysis, using two different elemental analysers, and the results were compared.

Selection of the materials Materials of different nature obtained from the industry were selected to be analysed by the different methods developed in this study: nitrides, nitrates, oxynitrides and nitrogendoped materials.

* Correspondence to: Maria Fernanda Gazulla, Instituto de Tecnología Cerámica, Asociación de Investigación de las Industrias Cerámicas, Universitat Jaume I. Castellón. España. E–mail: [email protected] Instituto de Tecnología Cerámica, Asociación de Investigación de las Industrias Cerámicas, Universitat Jaume I. Castellón, España

Copyright © 2013 John Wiley & Sons, Ltd.

Nitrogen determination by SEM-EDS and elemental analysis Experimental procedure Determination of nitrogen by SEM-EDS

To approach the analysis of nitrogen by SEM-EDS, we set up two methods of quantification: one with the use of calibration standards to be able to construct a calibration curve that correlates the signal obtained with the concentration, and another using the standardless ZAF procedure, supplied with the instrument, where the calculation of the concentration is direct. I. Sample preparation Two types of samples were considered in the present study: powder samples and sintered samples. The homogeneity of the samples and the smoothness of the surfaces are very important to ensure good results in the EDS analysis. So, powder standards were milled using a HERZOG-tungsten carbide ring mill and were formed as pressed pellets with a CASMONhydraulic press. Sintered standards were mounted onto an epoxy resin and polished to a 1-mm finish using diamond paste. Figure 1 shows SEM micrographs of the two types of samples prepared. All the samples were mounted onto an aluminium holder, using a carbon double-sided adhesive tape and were coated with a 20–25-nm thick layer of carbon,[10,12,13] in an EMITECH K950 metalliser, using graphite bars and a current intensity of 25 A. Carbon is usually the material chosen for microanalysis, due to its excellent transparency and electrical conductivity.[12] The absorption of this layer of the X-rays emitted by the elements can influence in the results obtained by EDX. In this way, Ro et al.[9] and Limandri et al.[11] correct the effect of carbon coating absorption using Monte Carlo simulation, obtaining good results. However, Kato [10] verifies empirically that the carbon coating does not cause significant influence in the quantitative electron probe microanalyser of most geological samples. Laskin and Cowin[13] checked that a 15–25-nm thick layer of carbon gives exceptionally low background in the SEM-EDS analysis and allows satisfied automated analysis of particles down to 0.1-mm size, including detection of low Z elements. In the present study, quantification of nitrogen has been made using a spectrometer equipped with super ultra thin window and applying the standardless ZAF procedure for corrections. II. Calibration standards Reference materials and chemical reagents were used as calibration standards to prepare a calibration curve to be able to

quantify the nitrogen content by EDS. It was also necessary to prepare synthetic standards from the mixtures of different reference materials and chemical reagents to obtain calibration standards in the low concentration range. These synthetic standards were mixed in a tungsten carbide ring mill. The reference materials used in the calibration and validation are listed as follows: (1) Reference materials and chemical reagents for calibration: Fluka NaNO3, Fluka AgNO3, Fluka LiNO3, Fluka Pb(NO3)2, Alfa Aesar TiN, BCS-CRM No. 359 nitrogen bearing silicon carbide and Alfa Aesar Si3N4. (2) Reference materials and chemical reagents used for the preparation of synthetic standards for calibration: Fluka NaNO3, Merck CaF2, Merck Ca(OH)2, Merck Al2O3, Merck SiO2, Merck Na2B4O7 10H2O and GBW03102 Kaolin.

III. Calibration To approach the development of the quantification method, we measured the reference materials and chemical reagents. All the calibration standards available in the laboratory presented high nitrogen concentrations so, to obtain calibration standards with low nitrogen concentrations to construct a calibration curve usable for any type of sample, synthetic standards were prepared by mixing the reference materials and chemical standards available in the laboratory. Table 1 shows the composition of the synthetic standards analysed to construct a calibration curve. IV. Determination by SEM-EDS The determination of nitrogen by EDS was carried out using an FEI model FEG-ESEM Quanta 200 field-emission environmental scanning electron microscope equipped with an energy dispersive X-ray microanalysis [Si(Li) EDAX Genesis 7000 SUTW (super ultra thin window)]. Analyses were performed at 20 kV (beam voltage) in the high vacuum mode (~106 mbar). Acquisition time of the spectrums was 300 s, and the detector dead time was of about 30%. To ensure the representativeness of the sample, ten spectra of each specimen were acquired, on areas of 100 100 mm selected at random. Results were expressed as the average of the ten measurements obtained.

X-Ray Spectrom. 2013, 42, 394–401


wileyonlinelibrary.com/journal/xrs

395

Figure 1. Scanning electron microscopy micrographs of sample surfaces after preparation: (a) powder samples formed as pressed pellets and (b) sintered samples mounted onto an epoxy resin and polished.

M. F. Gazulla et al. Table 1. Composition of the synthetic standards used for calibration of the SEM-EDS method Reference material/ chemical reagent Merck Na2B4O7 10H2O Fluka NaNO3 GBW03102 Kaolin Merck CaF2 Merck Ca(OH)2 Merck Al2O3 Merck SiO2

Composition (wt%) C1

C2

C3

C4

C5

C6

C7

C8

10.8 51.7 37.5 — — — —

29.8 5.7 64.5 — — — —

20.0 10.0 70.0 — — — —

10.0 70.0 20.0 — — — —

— 12.0 — 10.0 20.0 25.0 33.0

— 31.7 — 10.0 15.0 20.0 23.3

— 48.5 — 4.4 10.4 14.6 22.1

— 61.8 — 5.5 6.2 8.3 20.2

Determination of nitrogen by elemental analysis (combustion and thermoconductivity detection)

TruSpec, it is also necessary to carry out measurements without sample or tin foil to cause the flow of helium through the cell.

The analysis of nitrogen by elemental analysis was conducted using two different elemental analysers: a LECO model TN-400, suitable for inorganic samples and a LECO model TruSpec CHNSO (TruSpec hereinafter), more appropriate for organic materials. For the measurement by elemental analysis using TN-400 analyser, the sample was weighed in tin foils and introduced into a nickel basket to ease the fusion process. Then, the sample was introduced in single-use graphite crucibles. For the measurement by elemental analysis using TruSpec CHNSO, the only material necessary to carry out the assay is a tin foil.

II. Calibration and validation standards

I. Considerations prior to the analysis Depending on the type of sample to be analysed, the appropriate amount of sample to carry out the assay is different. It depends on the amount of nitrogen present in the sample and the design of the instrument. Table 2 shows the sample mass used in this study depending on the type of sample in each analyser. Both pieces of equipment must be stabilised before starting any analysis, so they have to be switched on 2 h before. In the case of

Table 2. Range of sample masses used in each analyser Type of sample

Range of sample mass

Samples with high nitrogen concentration (15–40 wt% N): nitrates, nitrides Samples with low nitrogen concentration (4–15 wt% N): nitrogen bearing materials, oxynitrides

TN-400

TruSpec

0.0200–0.0300

0.0500–0.0700

0.0400–0.0500

0.0700–0.1000

The calibration of the elemental analysers was performed using certified reference materials, chemical reagents and synthetic standards to obtain compositions with the suitable concentration of nitrogen. The synthetic standards were prepared mixing reference materials and/or chemical reagents with known concentration of nitrogen with Al2O3 from Merck to obtain the desirable nitrogen content. The following reference materials were used in the calibration and validation: • •

Reference materials and chemical reagents for calibration: Fluka NaNO3, Fluka KNO3, PART Nº 502–092 EDTA (traceable to NIST SRM 17e sucrose reagent grade nicotinic acid). Reference materials and chemical reagents for validation: Merck KNO3, Merck NaNO3, BCS-CRM No. 359 nitrogen bearing silicon carbide, BCS-CRM No. 360 sialon-bonded silicon carbide and Alfa Aesar Si3N4.

III. Calibration Two different calibration curves were prepared to analyse the materials studied in this paper: one for the analysis samples with high nitrogen concentration (nitrates, nitrides, etc.) and another for the analysis of samples with low nitrogen concentration. Each calibration curve was prepared with the calibration standards shown in Table 3. Table 4 shows the composition of the calibration standards (CS) specified in Table 3, prepared from the mixture of different reference materials. Each calibration curve was constructed with a minimum of ten points by different weighings of the calibration standards.

Table 3. Calibration curves used to analyse nitrogen by elemental analysis Calibration curve Calibration 1 Calibration 2

TN-400

TruSpec

Reference materials/calibration standards

N (wt%)

Reference materials/calibration standards

N (wt%)

Fluka NaNO3 Fluka KNO3 CS1(*) CS2(*)

16.47 13.85 5.01 10.00

Fluka NaNO3 Fluka KNO3 PART Nº 502–092 EDTA CS1

16.47 13.85 9.57 5.01

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* CS1 and CS2 are synthetic standards prepared from the mixture of chemical reagents, which composition is shown in Table 4.



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Nitrogen determination by SEM-EDS and elemental analysis u2method ¼ u2VR þ u2VL þ u2REPRO

Table 4. Composition of the calibration standards used for elemental analysis measurement Reference material

where uVR is the uncertainty of the certified value of the reference material, uVL is the uncertainty of the measurement of the reference material and uREPRO is the uncertainty of the measurement of the sample. uVL and uREPRO were calculated from the expression psffiffin, where s

Composition (wt%)

Merck Al2O3 Fluka NaNO3 Fluka KNO3

CS1

CS2

69.6 30.4 —

27.8 — 72.2

IV. Validation of the analytical methods The validation of the developed methods was performed by analysing reference materials and comparing the results obtained in this study with the known values. V. Determination by TN-400 analyser

VI. Determination by TruSpec analyser The LECO model TruSpec consists of an electric furnace working at a temperature of 950 C where the sample is introduced and combusted. The detection system of nitrogen is a thermal conductivity cell, with the same particularities that TN-400 presents. Calculation of the detection limit and quantification limit

The detection limit (LD) was calculated from the measurement of a sample with a concentration 0.5 times the concentration of the lowest standard in the calibration curve for each analyte. The sample was measured ten times under reproducibility conditions. The LD was obtained in accordance with the International Union of Pure and Applied Chemistry guidelines from the following expression: (1)

where s = value of the standard deviation of the measurements. The quantification limit (LQ), which expresses the quantifiability of an analyte, was calculated according to the International Union of Pure and Applied Chemistry guidelines as ten times the standard deviation of the measurement, for a number of measurements equal to ten[14,15]: LQ ¼ 10s

(2)

Calculation of the measurement uncertainty

Determination by elemental analysis Validation The results obtained in the analysis of the validation standards, together with their uncertainty (U), calculated from expression (3), are presented in Table 5. To compare the results obtained with the known values of the validation standards and verify the goodness of the method, the difference between both was compared, together with the related uncertainty, that is, the combined uncertainty of the known and measured values, as specified in the literature.[15] The absolute value of the difference between the measured and the known value is calculated as follows: Δm ¼ jcm cknown j

(4)

where Δm = absolute value of the difference between the measured and the known value cm = measured value cknown = known value

Table 5. Results of validation standards in the analysis by elemental analysis Validation Standards Merck NaNO3 Merck KNO3 Merck LiNO3 Alfa Aesar Si3N4 BCS-CRM No. 359 nitrogen bearing silicon carbide BCS-CRM No. 360 sialon-bonded silicon carbide

Known value (wt%)

N (wt%) TN-400

TruSpec

16.3 0.1 13.7 0.1 20.15 0.05 38.78 0.05 7.84 0.11

16.6 0.3 13.9 0.4 20.1 0.6 35.5 2.9 7.49 0.55

15.6 0.5 13.7 0.5 21.0 1.4 * *

4.77 0.10

4.62 0.58

*

(*) No results are presented for these materials with this instrument because recuperation less than 50% was obtained in all cases, due to its high decomposition temperature.



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The measurement uncertainty [16,17] was calculated as U = k umethod, where umethod is the combined uncertainty calculated from the expression:

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is the standard deviation of the reference material measurement or the standard deviation of the sample measurement under reproducibility conditions, depending on the term calculated and n is the number of measurements under reproducibility conditions. The coverage factor k is determined from the student’s tdistribution corresponding to the appropriate degrees of freedom and 95% confidence.

Results

The LECO model TN-400 comprises an electrode furnace that reaches a temperature around 2400 C, where the decomposition of the sample takes place, in the presence of helium, which acts as a carrier. The N2 released is detected by a thermal conductivity cell detector. The thermal conductivity detector is not specific for nitrogen, measuring any gas that arrives to the detector different from helium. To avoid this, the analyzer is provided with various traps that retain all the combustion products different from nitrogen. So, it is very important to change these reagents when they are saturated because if not, other gases apart from nitrogen can be analysed and the results obtained be wrong.

LD ¼ 3:29s

(3)

M. F. Gazulla et al. The uncertainty of Δm is calculated from the uncertainty of the known value and the uncertainty of the measured value from the following formula: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi uΔm ¼ u2m þ u2known (5) where uΔm = combined uncertainty of the result and of the known value um = uncertainty of the measured value uknown = uncertainty of the known value

Table 7. Results of nitrogen determination in the samples analysed by elemental analysis Sample

S1 S2 S3 S4 S5 S6 S7

N (wt%) TN-400

TruSpec

4.3 0.3 8.3 0.4 3.7 0.3 5.4 0.3 1.9 0.1 13.4 0.4 4.5 0.3

4.1 0.4 8.5 0.4 3.7 0.3 5.6 0.3 2.3 0.2 13.3 0.5 2.9 0.5

The expanded uncertainty UΔm is obtained by multiplying uΔm by a coverage factor (k), usually equal to two, which corresponds approximately to a 95% level of confidence. Thus: UΔm ¼ 2uΔm

(6)

To verify the goodness of the method, Δm is compared with UΔm, such that if Δm ≤ UΔm, there is no significant difference between the measured value and the known value. The results of this comparison are presented in Table 6. Almost all the results obtained show that Δm ≤ UΔm is obeyed, so that there are no significant differences, which validate the developed measurement method. For Si3N4 Δm ≤ UΔm is not obeyed. The reason for that could be that this material has not decomposed completely as it is considered a very stable hard ceramic. Measurement of materials from different nature Two nitrates, four sintered materials and a nitrogen bearing silicon carbide were analysed. The results obtained, together with their uncertainty (U), calculated from expression (3), are presented in Table 7. No significant differences were found in all materials analysed except in the one referenced as S7 when analysing samples by both pieces of equipment. This material is nitrogen bearing silicon carbide, which cannot be completely decomposed at the temperature that TruSpec works, being TN-400 the only option to analyse this type of materials by elemental analysis. Determination by SEM-EDS Method I: quantitative analysis (demonstration of matrix effects for nitrogen) Figure 2 shows the correlation between the known or certified nitrogen concentration, and the EDS net intensity obtained as Table 6. Comparison of the results of the elemental analysis measurements of the validation standards Validation standards

398

Merck NaNO3 Merck KNO3 Merck LiNO3 Alfa Aesar Si3N4 BCS-CRM No. 359 nitrogen bearing silicon carbide BCS-CRM No. 360 sialonbonded silicon carbide

TN-400

TruSpec

Δm

UΔm

Δm

UΔm

0.3 0.2 0.0 4.4 0.4

0.6 0.8 1.2 3.4 1.1

0.7 0.0 0.9 — —

1.0 1.0 2.8 — —

0.15

1.18

—

—


Figure 2. Nitrogen net intensity obtained by energy dispersive spectrometry versus the known nitrogen concentration.

the average of ten measurements for all the reference materials, chemical reagents and synthetic standards analysed, to draw a calibration curve to obtain a quantitative method. This net intensity is defined as the peak area subtracting the background, expressed as number of counts.[18] It may be observed that the results obtained do not match a linear equation. When analysing each material separately, it can be observed that the different results obtained can be separated in three groups depending on their major composition. Red coloured results are the results obtained in the measurement of nitrides, whereas results coloured in blue are the ones obtained in the measurement of nitrates, and the ones coloured in green are the ones obtained in the measurement of the synthetic standards (C1–C8), prepared from the mixtures of reference materials and chemical reagents. It can be noted that nitrates and nitrides show a good correlation when treated separately, which can denote the existence of a matrix effect, which increases the intensity of the element, because of the fluorescence effect, or decreases it if the signal is absorbed.[19] About the synthetic standards (green coloured), it can be seen in Fig. 2 that they do not show a good correlation when plotted. Table 8 presents the individual results obtained in the measurement of nitrogen by EDS of the different synthetic standard prepared. As it can be observed, there is a huge dispersion among the ten measurements made in different areas of each synthetic standard as the high values of relative standard deviation show. When analysing these standards at the microscope, it could be


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Nitrogen determination by SEM-EDS and elemental analysis Table 8. Individual net intensities obtained by EDS of the synthetic standards

Individual net intensities (cps)

1 2 3 4 5 6 7 8 9 10 Relative standard deviation (RSD) (%)

C1

C2

C3

C4

C5

C6

C7

C8

4.86 4.69 7.8 5.85 7.54 3.21 8.05 4.42 3.14 2.73 39.50

3.61 6.39 4.86 4.81 3.29 3.58 5.77 4.63 5.67 3.71 23.15

8.29 9.28 8.71 8.54 9.87 6.22 10.57 6.89 8.03 9.57 15.47

13.48 16.53 12.48 16.71 17.15 14.65 12.34 16.55 17.08 13.64 12.96

0.25 0.77 0.73 0.32 0.16 0.44 0.42 0.40 0.28 0.38 47.30

0.74 0.59 0.98 1.31 1.53 1.05 1.28 0.94 0.71 1.41 30.34

2.09 1.47 1.24 3.46 3.83 2.62 2.05 3.15 2.74 3.54 34.02

17.48 20.59 18.01 14.45 17.65 20.62 18.14 14.64 21.12 20.59 13.27

observed that these mixtures were not homogeneous for being measured by this technique. Figure 3 shows SEM micrographs (using the signal of backscattered electrons) of synthetic standards C2 and C6, which correspond to the size of the scanned area for spectrum acquisition (100 100 mm2). Individual particles of the different reference materials and chemical reagents mixed were identified in both synthetic standards: NaNO3, sodium borate and kaolin in synthetic standard C2 and NaNO3, CaF2, Ca(OH)2, Al2O3 and SiO2 in synthetic standard C6. So, no synthetic standards can be used for the quantification by EDS, as no homogeneity is achieved with the means used for their preparation. It can be concluded that it is not possible to obtain a calibration curve that correlates the nitrogen net intensity with the concentration for any type of material. This quantification method could be used if information about the sample to be analysed was available in advance. In that case, a calibration curve built with standards of the same nature as the sample to be analysed could be used.

Figure 4. Comparison of the results obtained by energy dispersive spectrometry using the standardless method versus the known concentration or the results obtained by elemental analysis.

Figure 3. Scanning electron microscopy micrographs of samples C2 and C6.

Table 9. Standard deviations of the different measurements conducted on the samples analysed S2

S3

S4

S5

S6

S7

Merk NaNO3

Merk LiNO3

BCS–CRM No. 359

0.18

0.22

0.21

0.20

0.12

0.29

0.10

0.14

0.15

0.42

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399

Standard deviation (wt%)

S1

M. F. Gazulla et al.

Figure 5. Scanning electron microsopy micrograph of BCS-CRM No. 359 reference material.

To assure that no beam damage occurs, one of this samples was analysed several times in the same area, not observing significant differences in the results obtained, as it was shown in a precious study.[20] Method II: standardless quantitative determination of nitrogen

400

The instrument software is supplied with a quantifying method based on fundamental parameters that do not need the use of standards for this quantification. This method is known as standardless ZAF procedure,[8] and quantify taking into account all the elements displayed in the EDS analysis. At this point, it is necessary to consider that this analysis system detects the elements with an atomic number of 6 or higher (from carbon upwards). So, samples that have Li or B in its composition, for example, could not be quantified by this method. Figure 4 shows the correlation between the data obtained by EDS using this method (as the average of ten measurements acquired on areas of 100 100 mm selected at random) and the results obtained by elemental analysis of some of the materials analysed in this paper, specifically, samples identified as S1, S2, S3, S4, S5, S6 and S7; Merck NaNO3, Merck LiNO3 and BCS-CRM No. 359 nitrogen bearing silicon carbide. Table 9 shows the results of the standard deviation of the ten measurements conducted on each sample, all the samples presenting low values except BCS-CRM No. 359 nitrogen bearing silicon carbide. As it can be observed in Fig. 4, there are two materials, which result by EDS that do not match the known concentration: BCSCRM No. 359 nitrogen bearing silicon carbide and LiNO3. The appearance of BCS-CRM No. 359 standard was checked by SEM. A micrograph of this sample is shown in Fig. 5, in which two types of particles were identified: irregular particles of silicon carbide, of 100 mm approximately, coated by silicon nitride particles of