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Porous plug made of tungsten wires and its conductance
Vacuum/volume 42/number Printed in Great Britain
15/pages 979 to 982/I 991
0042-207x/91$3.00+.00 @ 1991 Pergamon Press plc
Porous plug made of tungsten conductance Jae-Young Research received
Leem and Kwang-Hwa
Institute,
for publication
Chung
PO Box 3, Taedok Science 22 April
wires
and its
Jhung, Pressure and Vacuum Laboratory, Korea Standards Town, Daejeon, Chungnam 305-606, Korea
1997
A porous plug suitable for uhv systems was fabricated and its conductance was measured under various working conditions. The porous plug was made by inserting tungsten wires into a small hole. The porous plug with four wires inserted into a small hole of 138 pm radius and 0.15 cm long was shown to have a conductance of the order of 10m4 Is-’ for nitrogen, argon and helium, and to be stable for repeated bakeouts up to a temperature as high as 400°C. The relative conductance for a number of gases, nitrogen, argon and helium, was inversely proportional to the square root of the gas molecular weight. The conductance value of the porous plug can easily be varied by changing the number of tungsten wires and dimensions of the hole.
1. Introduction Accurate control of gas flow into a vacuum system is important in many experimental techniques. This is especially true for accurate calibration of vacuum gauges and mass-spectrometers which depend on accurate control of gas flow rate into a vacuum system. Leak elements have long been used for admitting gases into a vacuum system at measurable flow rates. However, variable leak valves used for the control of flow rate in the conventional vacuum system, cannot be used to calibrate ionization gauges or mass-spectrometers due to the lack of reproducibility after bakeout. In the calibration system, porous plugs are used as a basic control element of gas flow in the range 10-3-10-8 torr I SC’. The porous plugs are made of silicon carbide, platinum, and sintered glasslm5. Such elements consist of a large number of tubes with very small cross-sectional areas, permitting molecular flow of gas at relatively high pressures applied at the inlet. Pore sizes are about several micrometres and produce free molecular flow over a range of supply pressures exceeding 1000 torr6. An advantage of having these porous materials is the ability to produce a wide range of gauge calibration pressures in a relatively short time, while retaining the ability to measure gas pressure P accurately. However, these porous plugs do not satisfy all of the ideal characteristics proposed by Weaver’. The authors have fabricated new porous plugs and a conductance measurement system. The porous plug was made by inserting tungsten wires into a small hole and the conductance of the fabricated porous plug could be controlled by tungsten wires.
Hole
Figure 1. Cross-sectional view of the porous plug.
has a small hole, which is drilled through by a diamond bit less than 300 pm in diameter. The pore of the porous plug is generated by inserting tungsten wires into a small hole. The tungsten wires inserted into the hole were etched in an aqueous solution (KOH : 640 g l- ‘, CuSO, : 0.25 g l- ‘, H,O : 1 1) by applying 10 V ac at the electrode. The diameter of the tungsten wire depended on the electrical etching time. Figure 2 is a schematic diagram of the conductance measurement system. It consists of a gas chamber into which test gas can be introduced at any predetermined pressure from IO- 3 to 1 torr. A capacitance-type diaphragm manometer of I-torr range and a spinning rotor gauge, both calibrated, are used for the pressure measurement in the gas chamber in the determination of leak conductance. The volume of the gas chamber is 4.8 1. The conductance of the plug was measured by observing the pressure in the gas chamber over a relatively long period of time.
2. Porous plug and measurement system Figure 1 is a cross-sectional view of the porous plug. The plug adaptor is constructed by the double-sided zero length conflat reducing flange made up of a mini-flange (1.33 in. dia) on one side and a 2.75 in. flange on the other and a porous plug element is composed of the double-sided conflat flange having a miniflange (1.33 in. dia) on both sides. The centre of the mini-flange
3. Results and discussion To measure the conductance C of the porous plug, the rate of the decrease of the gas pressure in the gas chamber (volume V) was measured as this chamber was pumped through this conductance. Figure 3 is a plot of the pressure In P in the gas chamber vs time for the porous plug with no wire at the centre 979
Jae-Young
Leem and Kwang-Hwa
Chung Jhung: Porous
plugs
Gas Inlet
-28
-4.6
-5.2
L 20
0
40 TIME(X1000
60
80
set)
Figure 4. Plot of In P vs I for Nz, Ar and He as the gas leaks through
the
porous plug with three wires.
: 2 - PorousPlug ; 3 - Isolation Valve : : 5. 6, 7, 11 - Flexible Connection
1 .- Gas Chamhrr 4 - Dosing
Valve
8 - Capacitance 10 - Ion Gauge 14 - RDtary
Diaphragm
Gauge : 9 - Spinning
; 12 - Turbo-molecular
Pump
Rr)tr,r Gauge
;
; 13 - Trap ;
Pump.
Figure 2. Schematic
diagram
of the conductance
measurement
hole of a mini-flange. The radius (a) of the hole is 0.0138 cm and length 0.15 cm, and the dimensions satisfy the Knudsen condition for molecular flow. Figures 4 and 5 show plots of In P vs time for porous plugs with three and four wires inserted into the small hole, respectively. In the figures the solid sloping line represents least squares fit of the In P vs t values. From the least squares fit the standard deviation is below 5.0 x IO- 3 and the correlation coefficient is above 0.9999. The characteristics of Figures 3, 4, and 5 are shown in Table 1. From the table, we can see that the It-lative conductances for a number of gases, nitrogen, argon and
system.
-2.8 r
-2.8
k ; -4.0 a z J -4.6
-5.2 t 0
2
4 6 8 TIME(XlOOOsec)
10
Figure 3. Plot of In P vs f for N’, Ar and He as the gas leaks through porous plug with no wire.
Table 1. Parameters
Gas M(g) No wire Three wires Four wires
980
28.01 39.95 4.00 28.01 39.95 4.00 28.01 39.95 4.00
of porous Radius of hole 0 (cm)
0.0138
1 20
-5.2' 0
12
the
I I 40 60 TIME (XlOOOsec)
Figure 5. Plot of In P vs I for NZ, Ar and He as the gas leaks through porous plug with four wires.
plug and values of conductance Length of hole 1 (cm)
0.15
Temp T WI
Experimental conductance cql (1 s- ‘1
Theoretical conductance ClhW (1 s- ‘)
Ratio K = (GJC,,,“)
291.0 290.8 291.0 297.0 297.5 296.0 297.0 297.0 297.1
1.16x 9.84 x 3.00 x 1.37 x 1.16x 3.69 x 1.07x 9.06 x 2.80 x
l.25x1o-1 1.05 x IO_’ 3.35 x 1om3 1.78 x 10m4 1.49 x 1om4 4.71 x 1om4 1.24 x lo-” 1.03 x 1om4 3.27 x 10m4
0.93 0.94 0.91 0.77 0.78 0.78 0.86 0.88 0.85
1O-2 1om4 1om2 10-j 10m4 10m4 1o-4 lo-’ 1O-4
11 the
Jae- Young Leem and Kwang-Hwa
Chug
Jhong:
Porous plugs
helium, are inversely proportional to the square root of the gas molecular weight (M). In this table the experimental conductance CeXpof the porous plug was determined using equation (1) In P,-In
P = Cex,t/V,
(1)
where P, is the pressure at time zero and t is the time in seconds for the pressure to drop from P, to P8. On the other hand, new concepts of the transmission probability proposed by Santeler’ were used to calculate theoretical conductance Ctheo. Clausing”’ proposed an equation for the more general case of any component connected between two large volumes c ,heo = 11.43(T/M)
“2LZ.2tX,
(2)
where T (Kelvin) is the temperature, M (g) is the weight of the gas molecule, a (cm) is the orifice radius, and M.is the transmission probability that a molecule, which passes through the inlet plane of the hole, will be transmitted through the inlet plane of the hole. The transmission probability CIwas derived by Santeler’ for the circular cross-section. For the porous plug with no wire, the ratio K of CeXpand Ctheo is in the range 0.91-0.94. The ratio of CeXp and Ctheo is in the range 0.77-0.78 and 0.85-0.88 for plugs with three and four wires, respectively. Tables 2 and 3 are the characteristics of the porous plugs having different hole sizes. In summary, the ratio K of CeXpand Cthco is K = 0.91-1.00 for no wire, K = 0.73-0.78 for three wires, and K = 0.85-0.88 for four wires. The porosity of the plug generated by inserting three and four wires in a small hole with a constant cross-section has a
Table 2. Parameters
of porous
Gas
Radius of hole
M(g)
a (cm)
28.01 39.95 4.00 28.01 39.95 4.00 28.01 39.95 4.00
0.0246
Table 3. Parameters
of porous
No wire Three wires Four wires
Gas M(g) No wire Three wires Four wires
28.01 39.95 4.00 28.01 39.95 4.00 28.01 39.95 4.00
Radius of hole a (cm)
0.0330
complicated geometrical shape. However, equation (2) can be applied when the cross-section has a circular shape. If the crosssection differs from a circle, an experimental correction must be made’ ‘. Thus, in this study, the ratio K corresponds to the experimental correction factor. The porous plug should be baked after it is first installed in order to drive out any adsorbed gases and moisture. In order to investigate the reproducibility after bakeout, the conductance measurement system incorporating the porous plug assembly, was baked in steps from 30 to 400°C. Figure 6 shows the conductance variations as a function of baking temperature for the porous plug having four wires into a small hole. In the present
1.22
XlB4
5 ; 1.12 v f t, 2
.
.
1
I
8
.
.
1.02 -
5 0.92
0
I
I
I
100 200 300 TEMPERATURE (“C
I
I 400
1
Figure 6. Conductance variations as a function of baking temperature.
plug and values of conductance Length of hole 1 (cm)
0.10
Temp T WI
Experimental conductance cql (1 s- ‘1
Theoretical conductance c,,,, (1 s- ‘1
K = (C,&t,,J
302.2 301.6 302.0 291.0 291.2 291.0 295.0 295.5 295.0
7.92 6.62 2.10 9.50 8.02 2.52 9.16 7.68 2.42
7.98 6.68 2.11 1.28 1.07 3.37 7.97 6.68 2.08
0.99 0.99 1.00 0.74 0.75 0.75 0.87 0.87 0.86
x IO-’ x lo-’ X 10-Z x 10m4 X 1om4 X 10-j x 10m4 x tom4 x 10-l
x 10m3 x IO_ 3 X lo-2 x 10-j x 1om3 X lo-) x 1om4 x 1om4 x IO-’
Ratio
plug and values of conductance Length of hole 1 (cm)
Temp
0.10
T W)
Experimental conductance c,,, (1 s- ‘1
Theoretical conductance &eo (1 s- ‘1
Ratio K = (C~&,~m)
302.0 302.1 302.1 293.0 292.5 293.0 294.0 294.0 293.6
1.69 x 1.40 x 4.42 x 2.12x 1.78 x 5.57 x 1.80 x 1.54x 4.71 x
1.70 x 1.43x 4.50 x 2.88 x 2.41 x 7.61 x 2.08 x 1.74x 5.49 x
0.99 0.98 0.98 0.74 0.74 0.73 0.87 0.88 0.86
IO-* 10-Z lO-2 IO_’ lo-’ 1om3 10-j lo-’ 10-l
10-2 lo-’ 1o-2 lo-’ 10m2 lo-’ lo-’ lo-’ lo- 3
981
Jae-Young
Leem and Kwang-Hwa
0.92
I
3
Chung Jhung:
17
10
Porous
plugs
24
TIME(hour)
Figure 7. Conductance
variations as a function of bakeout time at a
temperature of 400°C.
experiments, bakeout time was held for 8 h, and after bakeout, the conductance was measured at a temperature of 297 K regardless of the baking temperature. These values are in agreement to within + 1 per cent. The conductance variations as a function of bakeout time at the temperature of 400°C are shown in Figure 7 and in no case have variations of more than f0.5 per cent ever been observed. 4. Conclusion As the basic element for the control and measurement of gas flow into a vacuum system for the calibration of an ionization gauge or mass-spectrometer, a porous plug of a new type has been fabricated. The porous plug was made by inserting tungsten wires into a small stainless hole. The conductance of a porous plug
982
with four wires inserted into a small hole was shown to be of the order of lo-“ 1 s-’ for nitrogen, argon and helium, and to be stable with bakeout up to a temperature of 400°C over a time of 24 h. The relative conductance for a number of gases, nitrogen, argon and helium, was inversely proportional to the square root of the gas molecular weight (M). The ratio K of CcXpand Clhco is K = 0.91-1.00 for no wire, K = 0.73-0.78 for three wires, and K = 0.854.88 for four wires. The porous plugs satisfy ideal conductance characteristics : (I) the absolute value of the conductance would depend only upon the molecular weight and temperature of the gas up to a pressure of several torr; (2) it is repeatedly bakeable up to 400-C : (3) it has a small internal surface area on which gas could be adsorbed ; (4) it can be degassed quickly; (5) it is reproducible and independent of bakeout procedure. In addition to the above characteristics, the porous plug using tungsten wires is insensitive to shock and rough handling and easy to clean and inspect. The conductance of the fabricated porous plug could also be controlled easily by tungsten wires and hole dimensions.
References ‘A J Mathews, Evaluation of Porous Materials as Molecular Lerrks, AEDC-TR-64-94 (1964). ‘C L Owens, J Vuc Sci Tech&, 2, 104 (1965). ‘R G Christian and J H Leek, J Scient Instrum, 43,229 (1966). “P Fowler and F J Brock, J Vat Sci Technol, 7,507 (1970). ‘J K N Sharma and P Mohan, Vacuum, 38(7), 51 I (1988). ’ W W Hultzman and L N Krause, J Vuc Sci Trchnol, 11(5), 889 (1974). ’ D E Weaver, J Vat Sci Technol, 8, 752 (1971). ’ R G Christian, J H Leek and J G Werner, Vacuum, 16( 1I), 609 (1966). “D J Santeler, J Vat Sci Technol, A4(3), 338 (1986). “I P Clausing, Ann Phys, 12,961 (1932) ; republished in J Vat Sci Tech&, 8, 636 (1971).
” A Guthrie and R K Wakerling, Vucuum Equipment and Techniques. McGraw-Hill, New York (1949).
15/pages 979 to 982/I 991
0042-207x/91$3.00+.00 @ 1991 Pergamon Press plc
Porous plug made of tungsten conductance Jae-Young Research received
Leem and Kwang-Hwa
Institute,
for publication
Chung
PO Box 3, Taedok Science 22 April
wires
and its
Jhung, Pressure and Vacuum Laboratory, Korea Standards Town, Daejeon, Chungnam 305-606, Korea
1997
A porous plug suitable for uhv systems was fabricated and its conductance was measured under various working conditions. The porous plug was made by inserting tungsten wires into a small hole. The porous plug with four wires inserted into a small hole of 138 pm radius and 0.15 cm long was shown to have a conductance of the order of 10m4 Is-’ for nitrogen, argon and helium, and to be stable for repeated bakeouts up to a temperature as high as 400°C. The relative conductance for a number of gases, nitrogen, argon and helium, was inversely proportional to the square root of the gas molecular weight. The conductance value of the porous plug can easily be varied by changing the number of tungsten wires and dimensions of the hole.
1. Introduction Accurate control of gas flow into a vacuum system is important in many experimental techniques. This is especially true for accurate calibration of vacuum gauges and mass-spectrometers which depend on accurate control of gas flow rate into a vacuum system. Leak elements have long been used for admitting gases into a vacuum system at measurable flow rates. However, variable leak valves used for the control of flow rate in the conventional vacuum system, cannot be used to calibrate ionization gauges or mass-spectrometers due to the lack of reproducibility after bakeout. In the calibration system, porous plugs are used as a basic control element of gas flow in the range 10-3-10-8 torr I SC’. The porous plugs are made of silicon carbide, platinum, and sintered glasslm5. Such elements consist of a large number of tubes with very small cross-sectional areas, permitting molecular flow of gas at relatively high pressures applied at the inlet. Pore sizes are about several micrometres and produce free molecular flow over a range of supply pressures exceeding 1000 torr6. An advantage of having these porous materials is the ability to produce a wide range of gauge calibration pressures in a relatively short time, while retaining the ability to measure gas pressure P accurately. However, these porous plugs do not satisfy all of the ideal characteristics proposed by Weaver’. The authors have fabricated new porous plugs and a conductance measurement system. The porous plug was made by inserting tungsten wires into a small hole and the conductance of the fabricated porous plug could be controlled by tungsten wires.
Hole
Figure 1. Cross-sectional view of the porous plug.
has a small hole, which is drilled through by a diamond bit less than 300 pm in diameter. The pore of the porous plug is generated by inserting tungsten wires into a small hole. The tungsten wires inserted into the hole were etched in an aqueous solution (KOH : 640 g l- ‘, CuSO, : 0.25 g l- ‘, H,O : 1 1) by applying 10 V ac at the electrode. The diameter of the tungsten wire depended on the electrical etching time. Figure 2 is a schematic diagram of the conductance measurement system. It consists of a gas chamber into which test gas can be introduced at any predetermined pressure from IO- 3 to 1 torr. A capacitance-type diaphragm manometer of I-torr range and a spinning rotor gauge, both calibrated, are used for the pressure measurement in the gas chamber in the determination of leak conductance. The volume of the gas chamber is 4.8 1. The conductance of the plug was measured by observing the pressure in the gas chamber over a relatively long period of time.
2. Porous plug and measurement system Figure 1 is a cross-sectional view of the porous plug. The plug adaptor is constructed by the double-sided zero length conflat reducing flange made up of a mini-flange (1.33 in. dia) on one side and a 2.75 in. flange on the other and a porous plug element is composed of the double-sided conflat flange having a miniflange (1.33 in. dia) on both sides. The centre of the mini-flange
3. Results and discussion To measure the conductance C of the porous plug, the rate of the decrease of the gas pressure in the gas chamber (volume V) was measured as this chamber was pumped through this conductance. Figure 3 is a plot of the pressure In P in the gas chamber vs time for the porous plug with no wire at the centre 979
Jae-Young
Leem and Kwang-Hwa
Chung Jhung: Porous
plugs
Gas Inlet
-28
-4.6
-5.2
L 20
0
40 TIME(X1000
60
80
set)
Figure 4. Plot of In P vs I for Nz, Ar and He as the gas leaks through
the
porous plug with three wires.
: 2 - PorousPlug ; 3 - Isolation Valve : : 5. 6, 7, 11 - Flexible Connection
1 .- Gas Chamhrr 4 - Dosing
Valve
8 - Capacitance 10 - Ion Gauge 14 - RDtary
Diaphragm
Gauge : 9 - Spinning
; 12 - Turbo-molecular
Pump
Rr)tr,r Gauge
;
; 13 - Trap ;
Pump.
Figure 2. Schematic
diagram
of the conductance
measurement
hole of a mini-flange. The radius (a) of the hole is 0.0138 cm and length 0.15 cm, and the dimensions satisfy the Knudsen condition for molecular flow. Figures 4 and 5 show plots of In P vs time for porous plugs with three and four wires inserted into the small hole, respectively. In the figures the solid sloping line represents least squares fit of the In P vs t values. From the least squares fit the standard deviation is below 5.0 x IO- 3 and the correlation coefficient is above 0.9999. The characteristics of Figures 3, 4, and 5 are shown in Table 1. From the table, we can see that the It-lative conductances for a number of gases, nitrogen, argon and
system.
-2.8 r
-2.8
k ; -4.0 a z J -4.6
-5.2 t 0
2
4 6 8 TIME(XlOOOsec)
10
Figure 3. Plot of In P vs f for N’, Ar and He as the gas leaks through porous plug with no wire.
Table 1. Parameters
Gas M(g) No wire Three wires Four wires
980
28.01 39.95 4.00 28.01 39.95 4.00 28.01 39.95 4.00
of porous Radius of hole 0 (cm)
0.0138
1 20
-5.2' 0
12
the
I I 40 60 TIME (XlOOOsec)
Figure 5. Plot of In P vs I for NZ, Ar and He as the gas leaks through porous plug with four wires.
plug and values of conductance Length of hole 1 (cm)
0.15
Temp T WI
Experimental conductance cql (1 s- ‘1
Theoretical conductance ClhW (1 s- ‘)
Ratio K = (GJC,,,“)
291.0 290.8 291.0 297.0 297.5 296.0 297.0 297.0 297.1
1.16x 9.84 x 3.00 x 1.37 x 1.16x 3.69 x 1.07x 9.06 x 2.80 x
l.25x1o-1 1.05 x IO_’ 3.35 x 1om3 1.78 x 10m4 1.49 x 1om4 4.71 x 1om4 1.24 x lo-” 1.03 x 1om4 3.27 x 10m4
0.93 0.94 0.91 0.77 0.78 0.78 0.86 0.88 0.85
1O-2 1om4 1om2 10-j 10m4 10m4 1o-4 lo-’ 1O-4
11 the
Jae- Young Leem and Kwang-Hwa
Chug
Jhong:
Porous plugs
helium, are inversely proportional to the square root of the gas molecular weight (M). In this table the experimental conductance CeXpof the porous plug was determined using equation (1) In P,-In
P = Cex,t/V,
(1)
where P, is the pressure at time zero and t is the time in seconds for the pressure to drop from P, to P8. On the other hand, new concepts of the transmission probability proposed by Santeler’ were used to calculate theoretical conductance Ctheo. Clausing”’ proposed an equation for the more general case of any component connected between two large volumes c ,heo = 11.43(T/M)
“2LZ.2tX,
(2)
where T (Kelvin) is the temperature, M (g) is the weight of the gas molecule, a (cm) is the orifice radius, and M.is the transmission probability that a molecule, which passes through the inlet plane of the hole, will be transmitted through the inlet plane of the hole. The transmission probability CIwas derived by Santeler’ for the circular cross-section. For the porous plug with no wire, the ratio K of CeXpand Ctheo is in the range 0.91-0.94. The ratio of CeXp and Ctheo is in the range 0.77-0.78 and 0.85-0.88 for plugs with three and four wires, respectively. Tables 2 and 3 are the characteristics of the porous plugs having different hole sizes. In summary, the ratio K of CeXpand Cthco is K = 0.91-1.00 for no wire, K = 0.73-0.78 for three wires, and K = 0.85-0.88 for four wires. The porosity of the plug generated by inserting three and four wires in a small hole with a constant cross-section has a
Table 2. Parameters
of porous
Gas
Radius of hole
M(g)
a (cm)
28.01 39.95 4.00 28.01 39.95 4.00 28.01 39.95 4.00
0.0246
Table 3. Parameters
of porous
No wire Three wires Four wires
Gas M(g) No wire Three wires Four wires
28.01 39.95 4.00 28.01 39.95 4.00 28.01 39.95 4.00
Radius of hole a (cm)
0.0330
complicated geometrical shape. However, equation (2) can be applied when the cross-section has a circular shape. If the crosssection differs from a circle, an experimental correction must be made’ ‘. Thus, in this study, the ratio K corresponds to the experimental correction factor. The porous plug should be baked after it is first installed in order to drive out any adsorbed gases and moisture. In order to investigate the reproducibility after bakeout, the conductance measurement system incorporating the porous plug assembly, was baked in steps from 30 to 400°C. Figure 6 shows the conductance variations as a function of baking temperature for the porous plug having four wires into a small hole. In the present
1.22
XlB4
5 ; 1.12 v f t, 2
.
.
1
I
8
.
.
1.02 -
5 0.92
0
I
I
I
100 200 300 TEMPERATURE (“C
I
I 400
1
Figure 6. Conductance variations as a function of baking temperature.
plug and values of conductance Length of hole 1 (cm)
0.10
Temp T WI
Experimental conductance cql (1 s- ‘1
Theoretical conductance c,,,, (1 s- ‘1
K = (C,&t,,J
302.2 301.6 302.0 291.0 291.2 291.0 295.0 295.5 295.0
7.92 6.62 2.10 9.50 8.02 2.52 9.16 7.68 2.42
7.98 6.68 2.11 1.28 1.07 3.37 7.97 6.68 2.08
0.99 0.99 1.00 0.74 0.75 0.75 0.87 0.87 0.86
x IO-’ x lo-’ X 10-Z x 10m4 X 1om4 X 10-j x 10m4 x tom4 x 10-l
x 10m3 x IO_ 3 X lo-2 x 10-j x 1om3 X lo-) x 1om4 x 1om4 x IO-’
Ratio
plug and values of conductance Length of hole 1 (cm)
Temp
0.10
T W)
Experimental conductance c,,, (1 s- ‘1
Theoretical conductance &eo (1 s- ‘1
Ratio K = (C~&,~m)
302.0 302.1 302.1 293.0 292.5 293.0 294.0 294.0 293.6
1.69 x 1.40 x 4.42 x 2.12x 1.78 x 5.57 x 1.80 x 1.54x 4.71 x
1.70 x 1.43x 4.50 x 2.88 x 2.41 x 7.61 x 2.08 x 1.74x 5.49 x
0.99 0.98 0.98 0.74 0.74 0.73 0.87 0.88 0.86
IO-* 10-Z lO-2 IO_’ lo-’ 1om3 10-j lo-’ 10-l
10-2 lo-’ 1o-2 lo-’ 10m2 lo-’ lo-’ lo-’ lo- 3
981
Jae-Young
Leem and Kwang-Hwa
0.92
I
3
Chung Jhung:
17
10
Porous
plugs
24
TIME(hour)
Figure 7. Conductance
variations as a function of bakeout time at a
temperature of 400°C.
experiments, bakeout time was held for 8 h, and after bakeout, the conductance was measured at a temperature of 297 K regardless of the baking temperature. These values are in agreement to within + 1 per cent. The conductance variations as a function of bakeout time at the temperature of 400°C are shown in Figure 7 and in no case have variations of more than f0.5 per cent ever been observed. 4. Conclusion As the basic element for the control and measurement of gas flow into a vacuum system for the calibration of an ionization gauge or mass-spectrometer, a porous plug of a new type has been fabricated. The porous plug was made by inserting tungsten wires into a small stainless hole. The conductance of a porous plug
982
with four wires inserted into a small hole was shown to be of the order of lo-“ 1 s-’ for nitrogen, argon and helium, and to be stable with bakeout up to a temperature of 400°C over a time of 24 h. The relative conductance for a number of gases, nitrogen, argon and helium, was inversely proportional to the square root of the gas molecular weight (M). The ratio K of CcXpand Clhco is K = 0.91-1.00 for no wire, K = 0.73-0.78 for three wires, and K = 0.854.88 for four wires. The porous plugs satisfy ideal conductance characteristics : (I) the absolute value of the conductance would depend only upon the molecular weight and temperature of the gas up to a pressure of several torr; (2) it is repeatedly bakeable up to 400-C : (3) it has a small internal surface area on which gas could be adsorbed ; (4) it can be degassed quickly; (5) it is reproducible and independent of bakeout procedure. In addition to the above characteristics, the porous plug using tungsten wires is insensitive to shock and rough handling and easy to clean and inspect. The conductance of the fabricated porous plug could also be controlled easily by tungsten wires and hole dimensions.
References ‘A J Mathews, Evaluation of Porous Materials as Molecular Lerrks, AEDC-TR-64-94 (1964). ‘C L Owens, J Vuc Sci Tech&, 2, 104 (1965). ‘R G Christian and J H Leek, J Scient Instrum, 43,229 (1966). “P Fowler and F J Brock, J Vat Sci Technol, 7,507 (1970). ‘J K N Sharma and P Mohan, Vacuum, 38(7), 51 I (1988). ’ W W Hultzman and L N Krause, J Vuc Sci Trchnol, 11(5), 889 (1974). ’ D E Weaver, J Vat Sci Technol, 8, 752 (1971). ’ R G Christian, J H Leek and J G Werner, Vacuum, 16( 1I), 609 (1966). “D J Santeler, J Vat Sci Technol, A4(3), 338 (1986). “I P Clausing, Ann Phys, 12,961 (1932) ; republished in J Vat Sci Tech&, 8, 636 (1971).
” A Guthrie and R K Wakerling, Vucuum Equipment and Techniques. McGraw-Hill, New York (1949).