Analytical Electron Microscopy Study of a ZnO-NiO Solid Solution

Goran Dra�zi�c� and Darja Lisjak

`̀ Jozef Stefan'' Institute, Jamova 39, 1001 Ljubljana, Slovenia

Abstract. Results of an analytical electron micro-

scopy study of a binary ZnO-NiO system are reported

and discussed. Emphasis was placed on the determina-

tion of Ni concentration (solubility) in the ZnO grains

using quantitative TEM-EDXS. The in¯uence on the

results of beam diameter, foil thickness and corrections

used are described and discussed. During the study

small precipitates, presumably NiO, were found in the

ZnO grains of the ZnO-NiO samples with different

ZnO/NiO ratios. In TEM, the precipitates exhibited

image contrast only at certain orientations and were

normally invisible during the EDXS analysis. The

presence of the precipitates too small to be seen using

scanning electron microscopy could explain erroneous

results for the Ni concentration in a ZnO solid-solution

phase obtained previously using SEM-EDXS. Quanti-

tative EDXS analyses were performed on ZnO grains

using different electron beam diameters. In each sam-

ple, the spread of the results was correlated to the beam

diameter (analysed volume). It was found that when

the average number of precipitates was less than one

per analysed volume the measured points that included

precipitates could easily be identi®ed on the basis of

their deviation from the mean value of the Ni content.

Key words: Nickel oxide; zinc oxide; PTCR; analytical electronmicroscopy; energy dispersive X-ray spectroscopy.

A positive temperature coef®cient of electrical

resistivity (PTCR) was recently reported for ZnO-

NiO two phase ceramics [1, 2]. ZnO is an n-type

semiconductor and has a defect crystal structure with

excess Zn2� ions which occupy interstitial sites, while

NiO has a defect structure with vacancies at Ni sites

and is a p-type semiconductor [3]. Both oxides form a

series of solid solutions, the solubility of Ni in ZnO is

much smaller than the solubility of Zn in NiO. In all

cases, solubility increases with increasing temperature

[4]. The PTCR anomaly (ZnO and NiO individually

exhibit a negative temperature coef®cient of resistiv-

ity) has been explained as a consequence of the

different resistivities and a large difference in linear

expansion coef®cients of the phases [2, 5, 6]. A

percolation model was used to correlate the electrical

resistivity with phase composition [1]. ZnO grains

with dissolved Ni ions (ZnOss) are the conductive

phase in the ZnOss-NiOss system (where NiOss

represents NiO phase with dissolved Zn ions). The

concentration of the Ni in the ZnO phase is expected

to have a strong in¯uence on the electrical properties

of the system. The degree of solubility of Ni in ZnO

as a function of ®ring temperature and ZnOss/NiOss

ratio in the two-phase system was measured by SEM-

EDXS, but unexpected results were obtained [7]. The

concentration of Ni in the ZnOss phase was found to

be a function of the ZnOss/NiOss ratio in the mixture,

where all mixtures were ®red at the same temperature.

According to thermodynamics one should expect that

maximum solubility is a function of the temperature

and not of the amount of a secondary phase. In the

present work results of an analytical electron micro-

scopy study of a ZnOss-NiOss system are reported

and discussed. Emphasis was on the determination of

Ni concentration in ZnO using quantitative TEM-

EDXS. The in¯uence on the results of beam diameter,

foil thickness, and the corrections used are described

and discussed.

Experimental

Two sets of solid solutions with compositions Zn0.97Ni0.03O(ZnOss) and Ni0.60Zn0.40O (NiOss) were separately prepared from

Mikrochim. Acta 132, 289±294 (2000)

� To whom correspondence should be addressed

ZnO (Chemetall, Wien, grade Pharma A) and NiO (Inco Ltd.,grade F). ZnO and NiO powders were mixed in proper ratios andcalcined at 1050 �C in the case of ZnOss and 1300 �C in the case ofNiOss for 2 h. After the ®rst calcination the powders were milledand recalcined twice more. Bulk samples, with various ratios ofZnOss/NiOss were prepared from ZnOss and NiOss calcinates bymixing, pressing and ®ring in air. All samples were sintered at1420 �C for 2 h. The phase and chemical composition of thesintered samples were analysed by X-ray powder diffractometry(Philips), scanning electron microscopy combined with energydispersive X-ray spectroscopy (Jeol 840A with Tracor Series II)and transmission electron microscopy (JEOL 2000 FX with a LinkX-ray system).

TEM samples were prepared by mechanical thinning (down to120 mm), dimpling (20 mm) and ion milling using 3.8 keV argonions. The ion milling time was 5 h. Samples were examined with atransmission electron microscope (TEM), operated at 200 kV. Thechemical composition was investigated using a Link AN-10000EDXS system with an Ultra Thin Window Si(Li) detector,mounted perpendicularly to the electron beam. The samples weretilted at an angle of 31� during the EDXS spectrum collection. TheCliff ± Lorimer [8] method and absorption and fluorescencecorrections [9±12] were employed for quantitative analysis. Weused anhydrous NiSO4 and ZnSO4 as standards for k-factordetermination. Sample thickness, needed for absorption andfluorescence corrections, was estimated using the contaminationspot method. The reproducibility of the measurements (relativestandard deviation of zinc and nickel wt%) was better than or equalto 2%. The concentration of oxygen was calculated from thestoichiometry.

Results and Discussion

Microstructure

In Fig. 1, a TEM micrograph (bright ®eld) of a grain

boundary area between ZnOss and NiOss grains in a

sample with 40 wt% ZnOss and 60 wt% NiOss

(0.4ZnOss ± 0.6NiOss) is shown. Dislocations in the

ZnOss grains and the stress ®eld at the grain boundary

are clearly seen. The presence of stress at the grain

boundary is most probably due to a large difference

(a factor of 2) in the linear thermal coef®cients of ZnO

and NiO [3]. There were no visible defects inside the

NiOss grains found even when the grain was tilted to

other orientations. A TEM-EDXS microstructural

study of samples of composition 0.8ZnOss ± 0.2NiOss

and pure ZnOss reveals the presence of very small

precipitates of a Ni rich phase in the ZnOss grains.

The precipitates are visible only in certain orientations

of the ZnOss grains.

In Fig. 2, a TEM micrograph (bright ®eld) of a

grain in the pure ZnOss sample is shown. Small black

dots, more or less uniformly distributed over the

whole micrograph are most probably artifacts due to

the ion milling damage. The features with dark

contrast, around 20 nm in diameter are strain ®elds

around precipitates. The exact dimensions of the

precipitates could not be measured directly from the

micrograph. The strain ®eld around a precipitate is the

consequence of the crystal lattice mismatch between

the matrix lattice of ZnO (hexagonal, wurtzite type

structure) and the precipitate. From the EDXS

analysis which revealed a higher concentration of Ni

in the area with precipitates and from the selected area

electron diffraction patterns it was concluded that the

precipitates were probably NiO (cubic). Incompat-

ibility between the hexagonal, wurtzite type structure

(a� 0.3249 nm, c� 0.5205 nm) and the cubic NaCl

type structure (a� 0.4177) is most likely the reason

for the high strain ®elds around the precipitates which

Fig. 1. TEM micrograph (bright ®eld) of a grain boundary areabetween ZnOss and NiOss grains in a0.4ZnOss ± 0.6NiOss sample.Note dislocations (D) in the ZnOss grain and stress ®elds (S ) at thegrain boundary (GB)

Fig. 2. TEM micrograph (bright ®eld) of a grain in a pure ZnOsssample. (P) ± precipitates with characteristic strain ®eld aroundthem

290 G. Dra�zi�c and D. Lisjak

prevents them from growing. No precipitate larger

than a few nm was found.

The concentration of precipitates varied with the

ZnOss/NiOss ratio in the samples. In the case of

samples composed of pure ZnOss the number of

precipitates was one order of magnitude smaller than

in the 0.8ZnOss ± 0.2NiOss samples. The difference in

the number of precipitates is clearly seen in Figs. 3a

and 3b, where TEM micrographs of ZnOss grains in

pure ZnOss and 0.8ZnOss ± 0.2NiOss are shown.

Small black dots are again ion-beam damage, black

features 10 to 20 nm in diameter are strain ®elds

around the precipitates.

The presence of precipitates too small to be

detected in the scanning electron microscope was

found to be the reason for incorrect measurements of

the solubility of Ni in ZnO using SEM-EDXS. The

error arises because both dissolved Ni in the ZnO

matrix and nickel in the NiO precipitates contribute to

the overall measured concentration of Ni. Conse-

quently, we endeavored to determine the correct

degree of solid solubility of Ni in ZnO using TEM-

EDXS analysis which offers a much higher spatial

resolution.

Quantitative TEM-EDXS Analysis

The main problem during the EDXS analysis was that

during the measurement the precipitates were not in

contrast, so we were unable to identify how many, or

indeed, if any precipitates existed in the analysed

volume. We tried to overcome this problem in the

following way. If we performed EDXS analyses at

many points with suf®cient precision and lateral

resolution we could distinguish between measure-

ments where precipitates were present and those

where precipitates were absent on the basis of

statistics (deviation from the median value). To

enhance the precision of the method we performed

an optimisation of the analytical parameters.

Optimisation of the Analytical Parameters

Compromises were made among the parameters of

lateral spatial resolution, counting statistics, spectrum

collection time and foil thickness. To estimate how

many precipitates are present in the analysed volume,

the geometry of the electron beam and the foil

thickness need to be known. We used several different

beam diameters in this study; 20, 50, 100 and 500 nm.

The shape of the smallest electron beam used was

ellipsoidal, with diameters 20 and 30 nm. According

to an equation proposed by Reed [11] the beam

diameter broadening for foils with different thickness

was calculated and the results are shown in Fig. 4.

Experimentally it was found that during a period of

5 min the drift of the sample was a few nm, and the

sample contamination was reasonably low. For longer

collection times, carbon build-up was found to be

quite high, especially at small beam diameter. If 5 min

is the longest time for EDXS spectra collection and

with a requirement for at least 2� 104 counts for the

smallest peak (Ni K� in this case), for an uncertainty

less than 2% (1.4% in the case of 2� 104 counts at a

95% con®dence limit) [13] then the foil thickness at

the measuring point should be around 200 nm. At this

thickness the in¯uence of the amorphous contamina-

tion layer, originating from the ion erosion process, is

also low. The foil thickness was determined using the

contamination spot method [13]. Because the foil is

Fig. 3. TEM micrographs of precipi-tates in ZnOss grains in (a) ± pureZnOss and (b) ± a 0.8ZnOss ± 0.2NiOsssample

Analytical Electron Microscopy Study of a ZnO-NiO Solid Solution 291

relatively thick the need to perform absorption and

¯uorescence corrections were critically assessed. In

the Ni-Zn-O system the mass absorption coef®-

cients for the Ni and Zn lines are relatively small

[10] hence for a thickness of around 200 nm the

correction factor is very close to 1 (1.002). This

means that absorption corrections need not necessa-

rily be employed. The ¯uorescence correction was

made using to the equation proposed by Nockolds et

al. [12]. In Fig. 5, the calculated ratio of the

¯uorescence intensity to the primary intensity for

2 wt% Ni in ZnO is plotted as a function foil

thickness. It is evident from the diagram, that if the

errors are to be kept to a minimum (below 1%) the

¯uorescence correction should be used.

Concentration of Ni in ZnOss Phase

Using optimised analytical parameters and procedures

described above we performed a series of point

analyses on ZnOss grains in pure ZnOss and

0.8ZnOss ± 0.2NiOss samples using different beam

diameters. A TEM micrograph of an area of a ZnOss

grain with contamination spots where 10 points were

analysed using a 20 nm beam is shown in Fig. 6. In

Fig. 7 a diagram of the Ni concentrations measured at

different points in a pure ZnOss sample is displayed.

The spread of the results was relatively low except for

three measurements where a large increase in Ni

content indicated that one or more precipitates were

present in the analysed volume. In Fig. 8 a similar

diagram for the sample 0.8ZnOss ± 0.2NiOss is

shown. The average concentration of Ni and the

Fig. 4. Calculated beam broadening vs sample thickness forsamples with 2 wt% Ni in ZnO

Fig. 5. Ratio of ¯uorescence intensity to primary intensity (in%) vsfoil thickness, calculated for an approximate concentration of2 wt% Ni in ZnO

Fig. 6. TEM micrograph of an area in a ZnOss grain (pure ZnOsssample) where 10 points were analysed using a 20 nm beamdiameter

Fig. 7. Ni concentration at different analysed points in a pureZnOss sample. Shaded bars represent measurements where NiOprecipitates were present in the electron beam path (Spt stands forspectrums collected at different points)

292 G. Dra�zi�c and D. Lisjak

spread of the results were much higher than in the

case of the pure ZnOss sample. The question was,

how many NiO precipitates contributed to the total Ni

concentration in the measured points. From the TEM

micrographs shown in Figs. 3a and 3b the numerical

density of precipitates (number of precipitates per unit

volume) in both samples was estimated. From the

beam diameter (for a 20nm beam), beam broadening

and sample thickness, the analysed volumes were

calculated and the number of precipitates per analysed

volume estimated. It was found that in the pure ZnOss

sample approximately 0.1 precipitate per analysed

volume and in the case of the 0.8ZnOss ± 0.2NiOss

sample 2±10 precipitates per analysed volume were

present. In Table 1 the results of quantitative EDXS

analyses performed on pure ZnOss and 0.8ZnOss ±

0.2NiOss samples using different beam diameters are

given.

From Table 1 we can see that for both samples the

smallest relative standard deviation of measurements

was obtained when the largest beam diameter

(500 nm) was used. In the pure ZnOss sample the

standard deviation of the measurements using a 20 nm

beam was very close (2.2%) to that of the measure-

ments performed with a 500 nm beam (1.8%). In the

case of the pure ZnOss sample, where the number of

precipitates was relatively low, we were able to

determine the correct concentration of Ni in solid

solution using the described procedure. In the case of

the 0.8ZnOss ± 0.2NiOss sample the relative standard

deviation of the measurements using a 20 nm beam is

much higher (3.4%) than with a 100 nm beam (1.7%)

or a 500 nm beam diameter (1.4%). The large spread

in the results (20 nm beam) was in accordance with

the estimated number of precipitates per analysed

volume (2 to 10 precipitates). When larger beam

diameters were used (100 and 500 nm) the relative

variation in the number of precipitates in the analysed

volume was much smaller, resulting in a lower

relative standard deviation. With 20 nm, the smallest

beam diameter achieved in our microscope (with

reasonable count rate, etc.), we were unable to

determine the correct concentration of Ni in the

ZnOss solid solution. Perhaps using dedicated ®eld

emission gun (FEG) microscopes with beam dia-

meters an order of magnitude smaller than in our case

and using a foil thickness below 100 nm, so that

number of precipitates per analysed volume would be

smaller than 1, it would be possible to determine the

correct concentration of Ni using the described

procedure.

Summary

Using analytical electron microscopy, nano-size,

presumably NiO, precipitates were found in ZnOss

grains in ZnOss-NiOss samples with different

ZnOss/NiOss ratios. Precipitates were in contrast just

in certain orientations, so normally during the EDXS

analysis they were not visible. The presence of the

precipitates, too small to be seen in the scanning

electron microscopy could explain the erroneous

results for the Ni concentration in the ZnOss phase

obtained using SEM-EDXS.

Optimising analytical parameters, taking into

account beam diameter, foil thickness, beam broad-

ening, acquisition time, peak intensities and absorp-

tion and ¯uorescence corrections, the reproducibility

of measurements of Ni in ZnO was equal to or better

than 2% (relative standard deviation).

Fig. 8. Ni concentration at different analysed points in a0.8ZnOss ± 0.2NiO sample (Spt stands for spectrums collected atdifferent points)

Table 1. Average concentration of Ni in ZnOss grains in pure ZnOssand 0.8ZnOss---0.2NiOss samples as a function of beam diameter

Beam Average Ni Rel. stand.Sample diameter

(nm)conc.(wt%)

deviation(%)

Pure ZnOss 500 2.2 1.820� 2.1 2.2

0.8ZnOss ± 0.2NiOss 500 3.1 1.4100 3.1 1.720 3.0 3.4

� Measurements with a clear indication of the presence ofprecipitates were not taken into account (see Fig. 7).

Analytical Electron Microscopy Study of a ZnO-NiO Solid Solution 293

Quantitative EDXS analyses were performed on

ZnOss grains using 20, 100 and 500 nm electron beam

diameters on samples consisting of pure ZnOss and

0.8ZnOss ± 0.2NiOss. In each sample, the spread of

the results was correlated to beam diameter (analysed

volume). It was found that in the case when the

number of precipitates was less than one per analysed

volume (pure ZnOss sample) the measured points

where a precipitate (or precipitates) was included

could easily be determined on the basis of the

deviation from the mean (or better median) value.

Using this procedure, the correct (without the

in¯uence of the precipitates) degree of solubility of

Ni in ZnOss could be measured. When the number of

precipitates per analysed volume was greater than 1

we were not able to avoid the in¯uence of precipitates

on the measured concentration of Ni.

Acknowledgements. The ®nancial support of the Ministry of Scienceand Technology of the Republic of Slovenia is gratefully acknowl-edged. The authors wish to thank Mrs. Medeja Gec for TEM samplepreparation.

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294 Analytical Electron Microscopy Study of a ZnO-NiO Solid Solution