Nanoscale composition inhomogeneity in silica-aluminas prepared by various methods, страница 4

measurement immediately after.

We did not measure directly the beam diameter, but

a reliable estimate can be made on the following

grounds. According to [11], the standard diameter

of the beam in a JEM-100-CX-TEMSCAN microscope

operated in our microanalysis conditions is

about 6 nm. That was matching with another estimate

(5±10 nm) made for the beam diameter in our working

conditions [12]. On the other hand, an approximate

measurement of the specimen drift made for each of

the analysed points showed that 70% of the drift

lengths ranged from approximately 0 to less than

100 nm. Therefore, by taking into account the upper

limit of 10 nm as the beam diameter for all our

measurements and the value of 100 nm as the drift

of all measured ``points'' (which is evidently an overestimation),

we can reliably estimate that 70% of our

measured Si/Al values correspond to a specimen surface

area of less than 103 nm2. This estimation can be

considered as including with a good approximation

also the increase of the analysed volume due to the

beam broadening effect in the specimen, which is

another unavoidable effect occurring in microanalysis

(see Fig. 1).

For the accurate determination of the Si/Al

ratio according to Eq. (1) it is necessary both to

measure as accurately as possible the X-ray peak

intensities and to know the actual value of the KAlSi

parameter of the spectrometer. The spectrometer

resolution power did not permit the complete separation

of the K
 peaks corresponding to Al and Si, these

being always partially superposed. We had therefore

to use a carefully designed method for the peaks

integral evaluation, based on using only the Ærst

half-peak for Al and the second half-peak for Si.

Details of the procedure, which is not original, are

given elsewhere [13].

The accurate measurement of the KAlSi parameter

was performed by using as standards a series of

minerals of well-known Si/Al compositions. The

homogeneity of the Si and Al distribution at a micrometer-

scale was tested by several statistically consistent

measurements performed with a scanning beam

WDS X-ray spectrometer with an error less than 1%.

The calibration curve of our spectrometer is shown in

Fig. 2. The best linear Ætting curve corresponding to

these according to the least squares method was

calculated. In this way we determined for KAlSi the

value of 1.390.07. This is taken for calculating the

Si/Al weight concentration ratio according to Eq. (1).

3. Results

The Si/Al weight concentration ratio was measured

on at least 50 randomly selected points, on randomly

selected particles, for each of the specimens listed in

Table 1. The results are shown graphically in Figs. 3±

7. The Ægures are grouped in a way similar to that used

for the table, so that a comparison between laboratory

prepared and industrial samples of similar composition

is easy. An exception is Fig. 5 corresponding to

laboratory samples for which no ``matching'' industrial

sample was available and where, consequently,

Fig. 2. Si/Al calibration curve of the EDS X-ray spectrometer.

C. SaÃrbu, B. Delmon / Applied Catalysis A: General 185 (1999) 85±97 89

Fig. 3. Nanoscale fluctuations of the Si/Al weight concentration ratio measured in specimens with a very low content of alumina (about 10%

alumina at the most). A is the value of the Si/Al ratio calculated on the basis of the global silica/alumina content. B is the average value

calculated on the basis of the Si/Al ratio values experimentally measured on all the analysed points (except for the extreme values that could

not be taken into account for calculation). See text for the physical meaning of the A value. Comparable specimens are shown on the same row

in all figures.

90 C. SaÃrbu, B. Delmon / Applied Catalysis A: General 185 (1999) 85±97

Fig. 4. Nanoscale fluctuations of the Si/Al weight concentration ratio measured in specimens with a low content of alumina (from 10% to

25% at the most). For the meaning of A and B see Fig. 1 and text.

C. SaÃrbu, B. Delmon / Applied Catalysis A: General 185 (1999) 85±97 91

Fig. 5. Nanoscale fluctuations of the Si/Al weight concentration ratio measured in specimens with a balanced content of alumina. Three