ELSEVIER International Journal of Mass Spectrometry and Ion Processes 142 (1995) 133 137

Mass Spectrometry and Ion Processes

Thermionic oxide sources of A1 +, Ga +, In +, and T1 +

T.L. Tan, Z.L. Zhou, P.P. Ong*

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

Received 28 November 1994; accepted 22 January 1995

Abstract

Oxides (A1203, Ga203, In203, and T1203) of Group IIIA metals were coated on tungsten filaments to form thermionic emitters of purely ground state A1 +, Ga +, In + and T1 + ions respectively. Variations of total ion emission current and ion purity with source filament temperature, and with time were studied. A very pure (99.9%) source of Al+(IS0) could be obtained from the A1203-coated filament when heated to 2200°C but the filament was short lived. A stable ion current and a purity between 92% and 96% of A1 + could be produced at a lower temperature of 2000°C after 100 min of heating. At 1900°C, the Ga203-coated filament could produce Ga+( l So) ions of 98% purity but its ion current was unstable with time. However, in the lower range of 1600 1820°C, a more stable Ga + ion current of about 80% purity could be obtained shortly after heating. For the In203-coated filament, the purity of In+(Is0) ions reached a maximum of 90% at 1580°C while the total ion current increased to a maximum of 2900 nA at 1690°C. A very stable In + ion current could be obtained at 1580°C. The Tl203-coated filament using tungsten or tantalum wires was found to be unsuitable as a source of T1 + ions.

Keywords." Thermionic emission; Thermionic oxide sources

1. Introduction

Studies on aluminosilicates as a thermionic material for the emission of Li +, Na +, K +, Rb + and Cs + ions have been quite elaborate [1-9]. These experiments reported that large quantities of alkali metal ions of high purity and stability could be produced over a long period of time using/%eucryptite-coated fila- ments heated to temperatures of 1000- 1200°C. The important merit of thermionic sources is that they produce only single spe- cies of ground state ions, provided the ground

* Corresponding author.

state ion is a singlet. Despite this advantage, studies on the thermionic emission of other metal ions with singlet ground states seem to be scarce. High melting points and the chemi- cally inert nature of metal oxides at high tem- peratures rendered them good possibilities as thermionic emitters of metal ions of a purely single species. Experimental studies on oxides as thermionic emitters of various metal ions were first reported by Blewett and Jones [1]. In a minor part of their work, the character- istics of the ion emission of A1 +, Ga + and In + ions were briefly studied.

The purpose of this work is to investigate systematically the ion emission characteristics

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134 T.L. Tan et al./International Journal of Mass Spectrometry and Ion Processes 142 (1995) 133 137

of oxides of metals from Group IIIA, i.e. A1203, Ga203, In203 and T1203 as sources of ground state AI+( 1 So), Ga+( 1 So), In+(So) and Tl÷(1So) ions respectively. The dependence of total ion emission current and of percentage ion purity on temperature, together with the stability of these two properties over time were studied. The results of these experiments should have direct relevance to typical atomic collision studies [10-12] where stable, long- lived and single species ion beams are required.

2. Experimental procedure

High purity (99.99%) A1203, Ga203, In203 and T1203 in fine powder form were used as the ion-emitting material. The oxides were mixed with distilled water to form a slurry. Tungsten wire of 0.25mm diameter was wound into a filament which was in the form of a helix 5mm long, of internal diameter 2mm and of about 12 turns. The entire fila- ment was dipped into the slurry so as to trap the oxide within the helical coil. The coated filament was immediately placed in the vacuum chamber which was then evacuated to below 5 × 10 -5 Pa.

The total ion emission current, ion purity levels, and mass spectra of the emitted ions were measured using a specially designed apparatus which was fully described by Hogan et al. [7]. The total ion current was measured by an electrometer which was con- nected to a Faraday cup inside the vacuum chamber. The quadrupole mass spectrometer, which was software controlled, allowed various types of time-dependent mass spectra to be measured. The measurements at various tem- peratures were made at intervals of 15-45 rain, over a continuous period of 3 h. The surface temperature of the filament was monitored and measured using an optical pyrometer, and was corrected for the transmittivity of light through the quartz window of the

vacuum chamber. All the experiments were repeated several times using a newly coated filament each time.

3. Results and discussion

3.1. Aluminium ions

Fig. 1 shows the variation with source filament temperature of the percentage ion emission composition of the A1203 emitter. The purity of A1 + ions was 4.5% at 1310°C and increased monotonically to 99.9% at 2200°C. All the five alkali metal ions were found to be present in significant quantities at lower temperatures. The presence of Rb + ions as the main impurity (42% at 1310°C) from the heated filament was quite surprising because past studies on thermionic sources [5,7-9] reported K + ions as the main impu- rity. This could be attributed to the Rb ele- ment which was present as a trace impurity in the A1203 compound or some contamin- ation of the vacuum chamber by Rb com- pounds. The results show that the AlzO3- coated filament when heated above 2000°C produced A1 + ions of purity better than 90%. However, at temperatures above 2200°C, the

100 : • R b ÷ ~ - • N a +

, - . 80 • C s ÷ g •

0 6o

0

0

c 20 _o

i i i i t

1200 1400 1600 1800 2000 2200

Temperature(°C)

Fig. l. Percentage ion emission composition of the A12 03-coated tungsten filament as a function of source filament temperature.

T.L. Tan et al./International Journal of Mass Spectrometry and Ion Processes 142 (1995) 133-137 135

filament was short lived, and was usually burnt after 15 min of heating. This could be caused by some chemical reaction between A1203 and the tungsten filament at such high temperatures.

Below 1300°C, the total A1 + ion current from the A1203-coated filament was negligible. At this temperature, the current was 88 nA and increased about 30-fold to 2640 nA at 2200°C as shown in Fig. 2.

The variation of percentage ion purity of A1 + ions at 2000°C with time, up to 165 min, is illustrated in Fig. 3. The ion purity fluctu- ated in the range 92-96%, reaching the maxi- mum after 75 min of heating. Fig. 4 shows that the total ion current from the A1203-coated filament at 2000°C decreased rapidly in the first 30min and then stabilised to about 150nA after 100min of heating. From these experiments, it can be concluded that the A1203-coated filament is the best thermionic source of A1 + ions after 100min of heating between temperatures of 2000 and 2200°C.

3.2. Gallium ions

At 1600°C, the purity of Ga + ions from the Ga203-coated filament was already 80% as shown in Fig. 5. A purity of 98% was attain- able at 1900°C. The impurities present were

10000 • A I ÷

• G a ÷ A In*

e,-

.¢' lOOO I , . .

: 3

¢. , .

o

"6 I-- 100

i i i i i

1200 1400 1600 1800 2000 2200

Temperature(°C) Fig. 2. Total ion emission current from AI203, Ga203 and In203 filaments as a function of source filament temperature.

100 • AI + • G a ÷

9 8 n*

96 __>, f , -

~. 94 C

o 9 2

9O

i i i i i i i i i ,

0 20 40 60 80 100 120 140 160 180

T i m e ( m i n )

Fig. 3. Percentage ion emission purity from A1203(2000°C), Ga203(1900°C) and In203(1580°C) filaments as a function of time.

K--, Na + and Li + ions in order of magni- tude. Fig. 2 shows that copious amounts of ions were already emitted from the Ga203- coated filament at 1600°C. The ion current then decreased with increasing temperature, and finally it increased to 2750nA when heated to 1900°C.

Both ion purity of Ga ÷ ions and total ion current from the Ga203-coated filament heated at 1900°C fluctuated significantly with time for the 120min studied as shown in Figs. 3 and 4. This could be the result of the variation of the emitting surface with time at high temperatures, irregular diffusion of atoms

10000

t . -

1000

(J C

o 100

O I -

• A I *

• G a ÷

• In+

1 0 i t , i , i i , i ~ , ~ , i

0 20 40 60 80 100 120 140 160 180

T i m e ( m i n )

Fig. 4. Total ion emission current from A1203(2000°C), Ga203(1900°C) and In203(1580°C) filaments as a function of time.

136 T.L. Tan et al./International Journal of Mass Spectrometry and Ion Processes 142 (1995) 133 137

100

80 o~ t -

O 60

0 0.. 40 E o L) c 20 o

• Ga* • K +

• Li ÷ • Na*

I I J I I I I

1550 1600 1650 1700 1750 1800 1850 1900 1950

T e m p e r a t u r e ( ° C )

Fig. 5. Percentage ion emission composit ion of the Ga203- coated tungsten filament as a function of source filament temperature.

from the interior to the surface as surface atoms were quickly depleted by ion emission, and phase changes of the material on the fila- ment [7]. It appears that the GazO3-coated filament could be best used in the lower tem- perature range of 1600-1800°C where the ion purity and current were more stable with time but at the expense of ion purity which was 76% at 1710°C.

3.3. Indium ions

For the heated In203-coated filament, the purity of In + ions increased from 84% to 90% when the temperature was raised from 1380 to 1580°C (Fig. 6). However, the In + purity decreased sharply at higher tempera- tures, to 38% at 1750°C, in contrast to those of A1 + and Ga + ions. At 1750°C, there were almost as much K + ions as In + ions. The other impurities observed were Na + and Rb + ions.

Fig. 2 shows that the total ion current from the InzO3-coated filament increased to a maxi- mum of 2870nA at 1690°C before decreasing sharply to 279 nA at 1750°C. The purity of In + ions increased gradually from 90% to a steady value of 93% after 120min of heating at 1580°C (Fig. 3). The total ion current from InzO3-coated filament at 1580°C was found

100

.o 80 : n ,.-,0 • K+

EO 40 • Na ÷ ¢..) • Rb+ ."

0 ~ " "

t i i , ~ , i ~ i i

1350 1400 1450 1500 1550 1600 1650 1700 1750 1800

T e m p e r a t u r e ( ° C )

Fig. 6. Percentage ion emission composition of the In203-coated tungsten filament as a function of source filament temperature.

to be very stable at 1100nA throughout the whole 165 min of heating. It appears that the opt imum conditions in terms of ion purity and current for the In203-coated filament could be attained after 120 min of heating at 1580°C.

The relative isotopic abundances of the Ga+-69 and Ga+-71 ions were found to be in the ratio of 1.53 : 1, in excellent agreement with the natural abundance of Ga-69 and Ga-71 (1.51 : 1). However, the ions of In-l13 and In- 115 were produced in the ratio of 0.082:1, greater than the natural isotopic abundance of 0.045 : 1.

3.4. Thallium ions

At 1400°C, the T1203-coated filament began to emit a very high total ion current in the range of few #A but the current decreased to a negligible value after about 5 min. At this temperature, the purity of T1 + ions never exceeded 20%, and the alkali metal ions dominated the ion composition. An examin- ation shows that almost all the T1203 com- pound had evaporated from the filament. This was perhaps caused by the volatile nature of T1203. In a separate experiment using tantalum wire as the filament, the same effect was observed.

It can be concluded that a thermionic

T.L. Tan et al./International Journal of Mass Spectrometry and Ion Processes 142 (1995) 133 137 137

emitter using T1203 compound was unsuitable as a source of TI + ions. A better source of T1 + could be thallium aluminosilicates as reported by Feeney et al. [5].

filaments showed that the present results were reproducible within the limit of 4- 5%.

References

4. Conclusions

A stable source of high purity (92.0 99.9%) Al+(1S0) ions could be obtained from a A1203- coated tungsten filament in the temperature range of 2000 to 2200°C after 100 min of heat- ing. In the 1600-1820 ° range, a GazO3-coated tungsten filament could produce a stable Ga+(1S0) ion current of about 80% purity shortly after heating. Pure (93%) and very stable In+(Is0) ions could be obtained from In203-coated tungsten filament at 1580°C after 120 min of heating. Because of the vola- tile nature ofTl203 with the heated tungsten or tantalum filament wire, T1203 was found to be unsuitable as a source of T1 + ions. Repeated experiments for all four types of oxide-coated

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