ELSEVIER International Journal of Mass Spectrometry and Ion Processes 134 (1994) 221-228

Mar Speetrometry and 10” Pmceaaes

Studies of rubidium aluminosilicates Rb’ ions

as thermionic emitters of

T.L. Tan, P.P. Ong*, T.M. Fong, K.A. Soo

National University of Singapore Department of Physics, Faculty of Science, Lower Kent Ridge Road, Singapore 0511. Singapore

(Received 15 February 1994; accepted 25 March 1994)

Abstract

Filament sources of Rb+ ions were created using 90%Pt-lO%Rh gauze coated with synthetic rubidium alumino- silicate compounds of various compositions. The performance characteristics of these compounds as thermionic emission sources of Rb+ ions based on total emission current and purity of ion emission as functions of time were studied and compared. In addition, these characteristics were investigated of /3-eucryptite (RbzO - A1203 - 2Si02) and spodumene (RbzO - Al203 - 4Si02) samples at four different filament surface temperatures. It was concluded that the p- eucryptite composition of rubidium aluminosilicate gives the most satisfactory results as a Rb+ ion emitter at a temperature of 1100°C.

Key words: Rubidium aluminosilicates; Thermionic emission; Alkali ion sources

1. Introduction

Thermionic sources have been found to be ideal in providing continuous, stable, long- lived ion beams such as are required in charged particle-neutral particle collision .experiments [l-3].

Aluminosilicate in the form of the mineral spodumene Liz l A120s l 4Si02 was first used as a Li+ ion emitter by Hundley [4]. Later, Bainbridge [5] used spodumenes and similar minerals for the studies of Li+, Na+, K+, and Cs+ ions. Blewett and Jones [6] prepared filament sources of ions of all the alkali metals by heating synthetic

* Corresponding author.

aluminosilicates and concluded that the composition Liz0 l A1203 l 2Si02 which is known mineralogically as P-eucryptite was the most satisfactory ion emitter. A compre- hensive study of the manufacture and emission characteristics of all alkali metals and thallium ion emitters using aluminosilicates in the form of natural or synthetic eucryptites and spodu- menes was made by Feeney et al. [7]. Among other studies using the aluminosilicate systems [8-111, Hogan et al. [12] established that the ,0-eucryptite analogs were the best choice for sources of Na+ and K+ ions, based on the total emission current and its stability, and on the purity of the ion content. To our knowledge, similar systematic studies of a source of Rb+ ions have not been reported.

0168-l 176/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI 0168-l 176(94)03990-H

222 T.L. Tan et al./Int. J. Mass Spectrom. Ion Processes 134 (1994) 221-228

The purpose of this paper is to investigate the ion emission characteristics of five different compositions of rubidium aluminosilicates which were prepared from melts using syn- thetic materials. The characteristics of the aluminosilicates as thermionic sources of Rb+ ions were the total ion currents (or emissivities) and the impurity ion levels, and their stability over a period of time. In addi- tion, the effect of filament temperature on these characteristics was investigated for both the ,0-eucryptite and spodumene species of rubidium aluminosilicate.

2. Experimental procedure

The rubidium aluminosilicate was prepared by first mixing stoichiometric amounts in various ratios of Rb2C03, A1203, and SiOz of high purity (99.99%, 99.999%, and 99.995% respectively). Seven samples of aluminosilicate (Rb2C03 - A1203 - SiOZ) were mixed with com- position ratios of 1 : 1 : 2, 1 : 1 : 4, 1: 1: 6,2 : 1: 5, 3 : 1 : 3, 1 : 0.12 : 1.6, and 1 : 0 : 1 respectively, identical to those prepared by Blewett and Jones [6]. In this paper, samples are labelled according to their composition ratios, e.g. 1: 1 : 4.

After mixing, the composite was placed in a high-purity graphite crucible and fired at 1200-1500°C using an rf vacuum furnace. The procedure used in producing aluminosili- cates of good quality from melts was described in detail previously [12].

Ion emitters were made using strips of 90%Pt-lO%Rh 80 mesh gauze, coated with a layer of the prepared aluminosilicate, according to the method of Feeney et al. [7]. The strips were cut to a size of about 25mm x 2mm. Other details of the construc- tion of the filaments were given by Hogan et al. [ 121. Of the seven samples, two (1: 0 : 1 and 1 : 0.12: 1.6) could not be studied because they were too hygroscopic and could not be fused onto the Pt-Rh strips.

Measurements of the total ion emission current, ion purity levels, and mass spectra of the emitted ions were conducted using a specially designed apparatus which is fully described by Hogan et al. [12]. Measurements were made at intervals of a half to several hours, over a continuous period of 2 days. The desired surface temperature of the fila- ment was monitored and measured using an optical pyrometer.

3. Results and discussion

Figure 1 shows the variation with time of the positive total ion current for each of the five samples tested at the filament surface tempera- ture of 1000°C. Current values were normal- ised in order to take the different ion emitting surface areas into account. The /3-eucryptite sample with the constituent ratio 1: 1: 2 pro- duced initially a total ion current of about 230nA, and then the current rose steadily over the next 15 h to .a constant value of about 400nA. The spodumene counterpart (1 : 1 : 4) began with a relatively high current of 1.35 PA, which then dropped drastically over the next 5 h to a value of 230nA. The current then continued to fall more gradually over the next 10h until it stabilised around 25 nA.

The 1: 1: 6 sample also began with a current of over 1 PA, which dropped sharply over the next 5 h to about 940 nA.Over the entire 2-day period, the current fluctuated between 830 and 920nA. The largest current was manifest by the 2 : 1 : 5 sample. After 25 h, the current alter- nately rose and fell sharply, but with a general increase. In contrast, the 3 : 1: 3 sample pro- duced a current which varied over the narrow range 450-520 nA.

During the first several hours after the samples were initially heated, the current emitted was often observed to be unsteady. This could be explained in terms of phase

T.L. Tan et al./Int. J. Mass Spectrom. Ion Processes 134 (1994) 221-228 223

$

0 \ 6 I I I I III1 1 III1 Ill11 I I I I

0 10 20 30 40 50 lime (ht

Fig. 1. Total ion emission current of Rb+ ions as a function of time for various rubidium aluminosilicate compositions: 1 : 1 : 2 (0);

changes in the material caused by ion extraction [7] and changes in the exposed surface area of the aluminosilicate coating when the filament was heated for the first time [12].

It can be seen from Fig. 1 that, with the exception of the 2 : 1: 5 sample, all the other samples produced fairly steady total ion currents. Although, both 1: 1: 2 and 1: 1: 4 samples gave the most stable currents after 15 h of heating time, the 1: 1: 4 sample produced an ion current of the order of about 20 times lower than that of 1: 1: 2. Therefore, it appears that the 1: 1: 2 sample is the best choice for producing a stable emission current.

Table 1 lists the current densities or emis- sivities of the samples based on measurements

made after the samples had been in continuous operation at 1000°C for 2 days. Some physical characteristics are also included. The emissivity values of the various rubidium aluminosilicate samples were found to be generally higher than those of sodium aluminosilicates, measured by Hogan et al. [12]. This shows good agreement with the typical current densities recorded by Feeney et al. [7] for alkali metal ion emitters.

For the purpose of comparison, Fig. 2 shows the variation with time over 2 days of the percentage of Rb+ ions for all five samples. It can be seen that the percentage of the 1: 1: 2 sample fluctuated the most, falling sharply to about 96.4% after about 4 h of heating, and later reaching a high of 98.7%. Despite this fluctuation, it still showed the highest overall

224 T.L. Tan et al./lnt. J. Mass Spectrom. Ion Processes 134 (1994) 221-228

Table 1 Emissivities of rubidium aluminosilicates, measured after approximately 2 days of operation at lOOO”C, and some physical properties of the samples

Ratio of Emissivity Colour Level of Ratio of constituent (Am-‘) hygroscopicity percentage compounds *‘Rb+/*‘Rb+ RbrO : AlrOs : SiOr

1:1:2 0.21 White Very low 2.64 1:1:4 0.012 White Very low 2.41 1:1:6 0.46 White Very low 2.50 2:1:5 1.09 White Slight 2.65 3:1:3 0.27 Grey Slight 2.69

1:O:l a White Very high 1:0.12: 1.6 a White Very high

a The emissivity of the sample was not measured.

percentage of Rb+ ions. The sample with the all samples, the impurity ions were mainly lowest percentage of about 96.4% was 1: 1: 4. K+ which constituted about 1.3-2.1%, fol- The 2 : 1: 5 and 1: 1: 6 samples showed the lowed by both Na+ and Cs+ ions (about steadiest percentage levels of about 97%. In 0.4%) and Li+ ions (about 0.1%). Potassium

99 I ” ” ” ” ” ” ” ” ” ” “I’ I

96 i 1 1 1 I I I I I 1 I I I I I I I I I I I I

0 10 20 30 40 50 Time (h)

Fig. 2. Percentage ion emission purity of Rb+ ions as a function of time for various rubidium ahnninosilicate compositions: 1: 1 : 2 (0); 1:1:4(0);1:1:6(~);2:1:5(~);3:1:3(+).

T.L. Tan et aLlInt. J. Mass Spectrom. Ion Processes 134 (1994) 221-228 225

ions were found likewise to be the major impurity ions by Feeney et al. [7]. On the whole, the intensity of impurity ions is found to gradually decrease with time of operation. From our experience of using aluminosilicate sources of alkali ions [2,3], an increase in impurity will occur initially if the material is left unheated for an extended period of time. This phenomenon is probably attributed to the slow but spontaneous cold diffusion of material from inside the bulk region to the surface.

It may be of interest to note that the ratios of the percentages of *‘Rb+ to *‘Rb+ ions recorded during the experiments corre- sponded well with that of the accepted relative natural abundance of these two isotopes. This implies that the slight difference in their

nuclear masses has little effect on their ion formation. The ratios are listed in Table 1. The relative abundances of *‘Rb and *‘Rb which are 72.16% and 27.84% respectively give a ratio of 2.59. This indicates that the manufacturer of the rubidium carbonate used to synthesise the aluminosilicates did not use any isotopic separation in the production of the carbonate.

So far, all the samples had been tested at a filament surface temperature of 1000°C over a period of 2 days. In addition, it is also important to investigate the optimum tem- perature, i.e. the temperature at which the emitter would emit the highest purity ions and the most stable ion current of greatest magnitude, of the thermionic ion source. Temperature studies at 900, 1100, and 1200°C

0 10 20 30 40 50

Time (h)

Fig. 3. Total ion emission current of Rb+ ions as a function of time for the P-eucryptite (1: 1: 2) sample at various filament surface

temperatures: 900 (0); 1000 (0); 1100 (A); 1200°C (X).

226 T.L. Tan et al./Int. J. Mass Spectrom. Ion Processes 134 (1994) 221-228

o’,,, (

I IIll 1 IIll 1 I III 1 I III 1

0 IO 20 30 40 50 3me (h)

Fig. 4. Total ion emission current of Rb+ ions as a function of time for the spodumene (1 : 1 : 4) sample at various filament surface temperatures: 900 (0); 1000 (0); 1100 (a); 1200°C (x).

were therefore conducted for the P-eucryptite (1: 1: 2) and the spodumene (I : 1: 4) samples over 2 days.

Figure 3 shows that the total ion current produced by 1: 1: 2 at 900°C changed drama- tically between 300 and 950 nA in the first 20 h. This can be explained in terms of the uneven heating of the aluminosilicate sample at this temperature. The current from the filament at 1200°C was unsteady, fluctuating around 480nA throughout the whole period of 50 h. Flaking off of the aluminosilicate caused by large expansion of the filament at this high temperature could be a possible reason. A closer study of Fig. 3 shows that the total ion current increases linearly from 900 to 1100°C after about 10h of heating. The current at 1200°C was found, however, to be of the same magnitude as that at 1100°C throughout

the whole period of operation. This suggests that the ion emission current attains a satu- ration value at around 1100°C. Similar trends were found by Heinz and Reaves [l l] in their work on the total emission current of Li+ ions as a function of temperature. It can be seen from Fig. 3 that the filaments heated to 1000°C and 1100°C produced a steady current. Since it was found that any further increase in filament temperature beyond 1100°C fails to produce a higher ion emission, but only causes material flaking, we conclude that 1100°C is the optimum temperature for obtaining a copious and stable ion emission current of the 1 : 1 : 2 sample.

For the spodumene (1 : 1 : 4) aluminosilicate, the current at the various temperatures appeared to be steady after a continuous heating period of 25 h, as shown in Fig. 4. It

T.L. Tan et al./lnt. J. Mass Spectrom. Ion Processes 134 (1994) 221-228 221

96

0 10 20 30 40 50 lime (h)

Fig. 5. Percentage ion emission purity of Rb+ ions as a function of time for the /3-eucryptite (1: 1: 2) sample at various filament surface temperatures: 900 (0); 1000 (0); 1100 (A); 1200°C (x).

can be seen that at 1 lOO”C, the 1: 1: 4 filament gave the highest and most stable ion current.

From Fig. 5, it can be seen that the purity of Rb+ ions from the 1: 1: 2 sample fluctuated greatly at 1000°C. Smaller fluctuations were observed at 900°C. At higher temperatures of 1100°C and 12OO”C, the purity of the Rb” ions remained stable with the filament at 1100°C emitting those of higher purity. According to Feeney et a?. [7], at a higher temperature, an increased diffusion of ions through the com- pound usually led to an accelerated rate of removal of impurity. Therefore this resulted in a higher level of impurity at a higher tem- perature. Studies [7] on Li+ ion emitters showed that a higher level of purity was obtained at 900°C than at 1000°C in the first hour of heating. The observation that the purity of Rbf ions was higher at 1100°C than

at 1200°C probably shows that the enhanced rate of removal of impurity at a higher tem- perature was sustained throughout the whole period of heating. As the total purity of Rb+ ions remained very steady and as high as 98.0% at llOO”C, the optimum temperature for maximum purity of ion emission for the 1 : 1: 2 sample appeared to be 1100°C.

The total purity of Rb+ ions emitted from the 1: 1: 4 sample at all temperatures fluctu- ated sharply between 96.2% and 96.6% after 5 h of heating, as shown in Fig. 6. Of these temperatures, the filament at 1100°C emitted the highest purity Rb+ ions during the period studied. Although the purity of Rb+ ions was higher at 1000°C than at 900°C initially, the purity at these two temperatures was relatively similar after about 10 h of heating. It is interesting to note that such behaviour was

228 T.L. Tan et aLlInt. J. A4a.w Spectrom. Ian Processes 134 (1994) 221-228

96.7

96.6

96.3

96.2 ’ , , , , I I I I III1 1 I1 I I, I I I II

0 10 20 30 40 50 lime (h)

Fig. 6. Percentage ion emission purity of Rb+ ions as a function of time for the spodumene (1: 1: 4) sample at various tilament surface temperatures: 900 (0); 1000 (0); 1100 (a); 1200°C (x).

also observed for Li+ ions using spodumene emitters [7].

5. References

4. Conclusion

For the production of Rb+ ions, the ,f3- eucryptite (RbzO l A1203 l 2Si02) analogue of synthetic rubidium aluminosilicate, operated at the filament surface temperature of 1 lOO”C, was found to be the best thermionic emitter of those studied. Its total ion emission current, though not the highest, was the most stable. Moreover, it yields an average ion purity level of about 98% which is higher than those obtainable from samples of other compositions. Both the ,3-eucryptite (1 : 1: 2) and spodumene (1: 1: 4) samples produce optimum ion emission at 1100°C.

[I] H.R. Skullerud, T. Eide and Th. Stefansson, J. Phys. D, 19 (1986) 197.

[2] M.J. Hogan and P.P. Ong, J. Chem. Phys., 95 (1991) 1973.

[3] T.L. Tan, P.P. Ong and M.J. Hogan, Phys. Rev. E, 48 (1993) 1331.

[4] J.L. Hundley, Phys. Rev., 30 (1927) 864. [5] K.T. Bainbridge, J. Franklin Inst., 212 (1931) 317. [6] J.P. Blewett and E.J. Jones, Phys. Rev., 50 (1936) 464. [7] R.K. Feeney, W.E. Sayle II and J.W. Hooper, Rev. Sci.

Instrum., 47 (1976) 964. [8] F.U. Haq, J. Phys. E, 19 (1986) 275. [9] R.E. Weber and L.F. Cordes, Rev. Sci. Instrum., 37

(1966) 112. [lo] S.K. Allison and M. Kamegai, Rev. Sci. Instrum., 32

(1961) 1090. [l l] 0. Heinz and R.T. Reaves, Rev. Sci. Instrum., 39 (1968)

1229. [12] M.J. Hogan, P.P. Ong, J.L. Ang and K.K. Cheang,

Int. J. Mass Spectrom. Ion Processes, 116 (1992) 249.