Copyright ©JCPDS-International Centre for Diffraction Data 2006 ISSN 1097-0002
236
NEW OBSERVATIONS ON LiBr
RELEASING AGENT LAYER IN FUSION BEADS
Fernand Claisse Ph.D.
Fernand CLAISSE inc. 2780, boulevard de Monaco
Quebec QC, G1P 3H2 Canada
Abstract
The objective of this paper is to describe how LiBr is distributed in fusion beads. Theoretical
calculations based on models and compared to measured intensities of beads containing cement
confirms the existence of a layer of bromides at the surface of the bead. The composition of the
layer is not pure LiBr but a mixture of bromides of Li and metallic ions from the sample. A small
quantity of bromides are present inside the bead as droplets.
Introduction
For years, XRF analysts add lithium bromide (and other bromides or iodides) to the fusion
mixture to prevent fused glass to stick to crucibles and moulds. Everybody knows that bromine is
a volatile element, so that the Br lines are not as constant as those of other elements. As a result,
the absorption by Br affects all emission lines to a variable degree (Fig. 1), and that should be
taken into account in calculation of concentrations. But how?
Fe2O3 Ka (kcps)
MgO Ka (kcps)
2,55
2,5
2,45
2,4
2,35
0
20
40
60
Br Ka (kcps)
80
16,2
16,15
16,1
16,05
16
15,95
15,9
15,85
15,8
15,75
0
20
40
60
80
Br Ka (kcps)
Fig. 1 Variation of Mg and Fe Kα lines from a bead containing cement,
as a function of the observed intensity of Br in the fusion bead.
An answer to that question requires some knowledge on the state of LiBr in fusion beads. The
author already made calculations on that subject, but partial results only were published (Claisse
and Blanchette 2004). In the actual presentation, the same experimental data were used, i.e. eight
curves similar to those in Fig. 1, for each element of a cement sample prepared as fusion beads
with different amounts of LiBr. These curves contain secret information on how LiBr is
distributed in the fusion bead. The objective of this research is to extract as much information as
possible from those curves, and describe a model for the distribution of LiBr in the fusion bead,
so that calculation procedures can be developed to correct for the effect of LiBr on line
intensities.
This document was presented at the Denver X-ray
Conference (DXC) on Applications of X-ray Analysis.
Sponsored by the International Centre for Diffraction Data (ICDD).
This document is provided by ICDD in cooperation with
the authors and presenters of the DXC for the express
purpose of educating the scientific community.
All copyrights for the document are retained by ICDD.
Usage is restricted for the purposes of education and
scientific research.
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Copyright ©JCPDS-International Centre for Diffraction Data 2006 ISSN 1097-0002
237
Fraction of Intensity at "no" LiBr
Experimental
Twelve fused beads were made from a cement sample. They were all identical, 1g cement with
6g lithium tetraborate and some LiBr as releasing agent. The amount of LiBr was variable but did
not exceed about 75 to 80 mg. After X-ray measurements, measured intensities were plotted as a
function of the BrKα line intensity which is an approximate measure of the residual Br content
after fusion. Eight curves were obtained from the eight more easily measurable elements. The
curves were extrapolated to Br intensities of 0 and 80 kcps, and the I Br Kα = 80 kcps / I Br Kα = 0 kcps
intensity ratio was plotted a function of the Kα wavelength of each element (Fig 2).
1
Ca
0,98
Fe
S
Si
0,96
K
0,94
0,92
Mg
0,9
Sr
Na
0,88
0,86
0
2
4
6
8
10
Ka wavelength (Angstrom)
12
14
Fig. 2 Intensity ratio of Kα line intensity of elements,
from lines similar to those in Fig. 1, at Br Kα intensity of 0 and 80 kcps.
The more striking observation is that the effect of Br may reach about 10 % loss of intensity for
Na, Mg and Sr, and vary in a strange manner for elements between Na and Sr.
Calculation of fluorescence intensities from beads.
To compare various models of LiBr distribution in fused beads, theoretical intensities had to be
calculated for each model. The Sherman equation (Eqn. 1) was used to calculate the Kα
theoretical intensities of elements in the cement bead containing no LiBr. Calculations were made
at each 0.02 Ǻ interval of a Rh tube spectrum, and summed to obtain the total intensity. In
absence of LiBr,
I BrKα =0 = Σ (Sherman Eq. * Δλ)
[1]
This equation was applied to the composition of the fusion bead. Later on, Eqn. 1 will be
modified according to each model, and new theoretical intensities will be compared to these
intensities.
Models of the LiBr state in fusion beads
Model 1: LiBr is present as dissolved in the fusion bead
The first model that should be considered is LiBr dissolved in the fusion bead, which is how LiBr
is often thought to exist. In the calculations, it is only sufficient to consider that LiBr is part of the
flux. An amount of 22 mg was used to illustrate the calculated effect shown in Fig. 3. If that
amount of LiBr is changed, the curve passes above or below the dot representing Sr, and the rest
of the curve does not essentially move.
This results from the fact that the emission wavelength of Sr is just below the absorption edge of
Br and is strongly absorbed. The emitted lines at longer wavelengths are only slightly affected as
Fraction of Intensity at "no" LiBr
Copyright ©JCPDS-International Centre for Diffraction Data 2006 ISSN 1097-0002
238
1
Ca
0,98
Fe
S
Si
0,96
K
0,94
0,92
Mg
0,9
Sr
Na
0,88
0,86
0
2
4
6
8
10
Ka wavelength (Angstrom)
12
14
Fig. 3 LiBr (22 mg) is assumed to be present as a solution in the fusion bead
the curve shows. No different quantity of LiBr could make the curve match all the measured
intensity values. It is clear that this model is not that which describes the state of LiBr in borate
glass correctly.
Model 2. LiBr is present as a film on the surface of the fusion bead
As shown in Fig.4, both the excitation radiation and the emitted radiations from the bead must
pass through the film before the sensor detects the radiations emitted by the elements.
Fig. 4 Both excitation and fluorescence rays are absorbed by the film
The measured intensity expression in Eqn. 1 must be modified for absorption through the film
(Lachance-Claisse 1995, p. 212) :
Theoretical “measured intensity” = Eq. 1 * exp(-μ f * ρt)
[2]
where μ f * is the effective absorption coefficient of the film, and ρt is the film thickness
expressed in g/cm2
Setting the concentration of LiBr in the film as a 0.2 (temporarily) and no other element present,
the effect on emission lines of the elements is shown in Fig. 5, which is closer to the reality than
in Model 1.
Remembering that we still keep the 22 mg of LiBr in solution, the first model in combination
with the second model, now seem to confirm that LiBr can be present inside the fusion bead, but
that cannot be as individual dissolved atoms, because halides are insoluble in borate glass. The
bromide is probably present as very fine droplets that were trapped inside the bead just after
casting when the glass had become viscous.
Fraction of Intensity at "no" LiBr
Copyright ©JCPDS-International Centre for Diffraction Data 2006 ISSN 1097-0002
239
1
Ca
0,98
Si
Fe
S
0,96
K
0,94
0,92
Mg
0,9
Sr
Na
0,88
0,86
0
2
4
6
8
10
Ka wavelength (Angstrom)
12
14
Fig. 5 Effect of assuming 20% LiBr (and nothing else) in a
surface film on the bead
Model 3. Cations from the bead are present in the surface film
Loubser (ca. 1998) reported that Li atoms of lithium fluxes behave as free Li+ ions. It is likely
that all oxides dissolved in the melt are also present as positive and negative ions, and move
freely in the glass, temperature permitting. On the other hand, the Boron oxide structure is
flexible but its atoms are not free to move out of that structure.
Fraction of Intensity at "no" LiBr
LiBr in the film is an ionic compound and its Li atoms are therefore free to move. Consequently,
it seems possible that Li+ in the LiBr film and positive ions of the dissolved cement in the glass
can mutually exchange their positions. There is no barrier between the surface film and the glass
that prevents ions to move to or from either side. In order to show what the effect of a major
metallic element of the cement might have on the absorption of the emission lines of cement, the
effect of adding a 64% concentration of Ca+2 ions in the film is shown in Fig. 6. The addition of
the metallic element Ca affects particularly the light elements, and makes the theoretical
calculations closer to the observed intensities.
1
Ca
0,98
Si
Fe
S
0,96
K
0,94
0,92
Mg
0,9
Sr
Na
0,88
0,86
0
2
4
6
8
10
Ka wavelength (Angstrom)
12
14
Fig. 6. Effect of Addition of 64% Ca in the film
Silicon is the second major element of the cement, so that we can add 16% Si+4 ions in the
surface film. The effect is less significant than that of Ca on account of the lower absorption of Si
and lower concentration, but the differences between measured and calculated intensities is still
decreasing.
Fraction of Intensity at "no" LiBr
Copyright ©JCPDS-International Centre for Diffraction Data 2006 ISSN 1097-0002
240
1
Ca
0,98
Si
Fe
S
0,96
K
0,94
0,92
Mg
0,9
Sr
Na
0,88
0,86
0
2
4
6
8
10
Ka wavelength (Angstrom)
12
14
Fig. 6 Effect of Addition of 16% Si in the film
Model 5. Addition of the emission contribution from the film
Only three points are net interpreted correctly. Up to now, we have taken into consideration the
absorption by a surface film only. However, the tube radiation the strikes the film also excites
radiations from the film, which add to the calculated radiations from the bead (Fig. 7).
Fig. 7 The film makes a contribution to Ka line intensities
The mathematical expression for the intensity of these radiations is similar to Eqn. 1, modified
for the film thickness (Lachance-Claisse 1995, p. 209).
Intensity emitted by the film = Eq. 1*(1-exp-(-μf* ρt))
[3]
Eqn. 1 applies to the composition of the film and is for the emission of each element. So far, we
have assumed a film composition of 20% Br, 64% Ca, 16% Si by weight. Lithium ions are also
present, but considering that one Ca atom replaces two Li atoms, and one Si atom replaces four
Li atoms, the number of Li ions that remains must be small, their weight is small, and their
absorption of X-rays is low; that makes their contribution negligible, and Li is not considered in
the calculations.
As observed in Fig. 8, the emission contribution of Ca and Si contribute to a decrease in the loss
of absorption due to the film, and the calculated curve now passes nearer the Ca and Si points.
The last point that does not fit the calculated curve is that of Sulfur. Although S is in significant
concentration, it should be discarded. In a fusion containing SO3 (acidic) and Li tetraborate
(acidic), in presence of LiBr (acidic) the probability of losing S as SO2 is great.
Fraction of Intensity at "no" LiBr
Copyright ©JCPDS-International Centre for Diffraction Data 2006 ISSN 1097-0002
241
1
Ca
0,98
Si
Fe
S
0,96
K
0,94
0,92
Mg
0,9
Sr
Na
0,88
0,86
0
2
4
6
8
10
Ka wavelength (Angstrom)
12
14
Fig. 8 The emission radiations fom Ca and Si in the film
improves the calculated curve
Conclusion
The good fit (Fig. 8) between the calculated effect of LiBr on the emission intensities of seven
cement elements in fusion beads, and their measured emitted intensities indicates that the model
describes well the distribution of LiBr in the bead.
The story of LiBr as releasing agent during fusion can now be described as follows: the LiBr
melts at the beginning of heating, but does not dissolves in the molten flux as oxides do; instead it
forms a film at the surface of the melt. The Br remains at the surface at all times, while the Li
ions exchange their positions with ions of the sample dissolved in the molten glass. Then, the
film becomes a mixture of bromides of Li and positive elements from the sample. Apart from
that, a small portion of the film mixes with the molten glass at the time of casting into the mould,
and the fine particles that are formed are trapped in the cooling bead. During the fusion, some of
the ions in the film react with oxygen of the atmosphere and now dissolve in the glass, leaving
Br2 molecules that escape to the atmosphere.
In the actual example of the bead that emits Br radiations with an intensity of 80 kcps, the film
thickness expressed in g/cm2 is 1.6x10-4, or 0.5 micrometer approximately.
Another conclusion is : the correction to make to measured intensities depends on the sample
composition, which is a major difficulty in the case of variable sample compositions. That is the
next problem to solve.
References
Claisse, F. and Blanchette, J. (2004) Physics and Chemistry of Borate Fusion. Fernand Claisse
inc. ed.
Lachance, G.R. and Claisse, F. (1995) Quantitative X-Ray Fluorescence Analysis. Theory and
Application. Ed. Wiley & Sons.
Loubser (ca. 1998) PhD thesis on structure of Lithium borates. French University.