HERMETIC SEALING OF THERMOELEMENTS CONSISTING OF THIN WIRES, CAPABLE OF WITHSTANDING HIGH PRESSURES AND LOW TEMPERATURES

N. P. Kurin

The work now being carried out by us, connected with calorimetric investigations of gases, posed the problem of finding a hermetic method of sealing both simple and compound thermoelements consisting of thin copper and constantan wires \((d = 0.07—0.15\ \mathrm{mm})\). The following requirements were imposed on the sealing material: it must withstand 1) high pressures (150 atm), 2) low temperatures (down to the temperatures of liquid air), 3) be nonconducting, since otherwise the wires of the thermoelement would be short-circuited with one another, and 4) the length of the sealing plug must not exceed \(12\ \mathrm{mm}\) (the latter requirement is determined by the dimensions of our apparatus).

The literature on this question contains only extremely scant material (especially on the strength of cements under high pressures and low temperatures), and therefore we had to test experimentally a considerable number of substances which, from the point of view of electrical insulation, might be suitable as sealing materials. The cements were investigated in the following way: into a metal tube (Fig. 1), with an internal diameter of \(4\ \mathrm{mm}\), 20 thin wires were introduced, and the substance under investigation was poured in to a depth of \(12\ \mathrm{mm}\). After the seal had hardened, the tube was connected by means of a nut (K) to an air compressor, and then the seal (B) was tested for tightness at various pressures and reduced temperatures \((20.0, -10, -20^\circ\mathrm{C}\), and in some cases at the temperatures of liquid air). To obtain the desired temperature, the tube was immersed at room temperature in water, at \(0^\circ\mathrm{C}\) in water and ice, at \(-10\) and \(-20^\circ\mathrm{C}\) in a mixture of ice with common salt, and at \(185^\circ\mathrm{C}\) in liquid air. In this way the following substances were tested.

Fig. 1.

FUSIBLE CEMENTS

Resins. The following were tested as sealing materials: ordinary rosin, dammar, zanzibar, Novolach resin, red and blue sealing wax, button shellac, and ordinary shellac. Rosin in the molten—

in the molten state—a mobile liquid—filled the tube well and filled the spaces between the individual wires of the thermoelement introduced into it, but upon cooling and solidification it cracked to such an extent that the seal was destroyed.

Dammar and Zanzibar, when heated in the molten state, foam strongly as a result of distillation; consequently, on solidifying, a spongy porous gas-permeable plug is obtained. New-lac resin (made of plastics), red and blue sealing wax also proved unsuitable because of cracking on cooling.

Tests of button shellac showed that it is less susceptible to cracking on cooling (in comparison, for example, with rosin), and has high mechanical strength, but it is not applicable because of its tendency to decompose with evolution of a gas phase during melting and because of the high viscosity of the shellac melt. The latter two circumstances invariably led to the free spaces between the 20 individual wires of the composite thermoelement being filled insufficiently well with molten shellac, and the seal obtained in this way was not hermetic. In attempts to create good hermeticity of the seal by repeated reheating and melting of the shellac already poured into the tube, it was established that the shellac gradually thickens, ceases to melt (apparently polymerizes), and, when an attempt is made to melt it by further heating, is completely destroyed, forming a cork-like, porous, brittle mass.

Ordinary shellac, for the same reasons as button shellac, did not give a hermetic seal.

Bitumens. As a putty for sealing thermoelements, several grades of Bakhilovo bitumens were tested.

Bakhilovo bitumen with a melting temperature of 175°C and a content of 10% mineral additives gave a good seal at room temperature (it withstood 150 atm), but upon cooling to 0°C and below the seal ceased to be hermetic.

Bakhilovo bitumen with a melting temperature of 100°C and a filler in the form of fine quartz sand holds pressure well up to 100 atm at room temperature, but the seal is destroyed upon cooling to 0°C.

Low-melting Bakhilovo bitumen (melting temperature +35°C), like the preceding two, is, in the molten state, an easily mobile liquid, adheres well to metal, and completely fills the space between the individual wires of the thermoelement.

A seal made from this substance remained hermetic at 0°C up to 100 atm; with an increase in pressure to 150 atm, the seal begins to pass gas, at first slowly, then faster. At a temperature of 20°C, hermeticity was destroyed at considerably lower pressures (50 atm).

Pitches. Several grades of coal-tar pitches were also tested as sealing substances. All of them, in the molten state, are mobile liquids that fill well the intermediate spaces between the individual wires and the walls of the tube. At room temperature, a plug made of this material did not pass gas up to 50 atm; when this pressure was exceeded, hermeticity was destroyed, and this occurred especially upon cooling to 0°C.

An attempt was made at plasticization, i.e., imparting greater elasticity to the coal-tar pitches described. For this purpose, the latter were fused with a small amount (several percent) of plasticizers. Triphenyl phosphate and tricresyl phosphate were used as plasticizers. By adding various amounts of plasticizers it was possible to obtain different degrees of hardness and elasticity—

properties (at room temperature), but none of them withstood high pressures, especially at reduced temperatures. In general, in the properties we investigated, plasticized pitches resemble bitumens.

Pitch. A seal made from vacuum sealing compound—pitch—remained hermetic at room temperature up to 100 atm; at 0°C the hermeticity was preserved only up to 30 atm.

Elemental sulfur and silver chloride were also tried as sealing substances.

Molten sulfur filled all the voids in the space being sealed well, but upon solidification, like rosin, it cracked and the hermeticity was destroyed.

Silver chloride in the molten state is an easily mobile liquid (melting point 444°C); after solidification it resembles a hard horn-like mass.

Preliminary experiments established that in the solid state AgCl is not electrically conducting. A seal made from silver chloride also proved unsuitable, since in the liquid state it adheres poorly to the insulated conductors and to the walls of the tube.

Thus, among fusible sealing compounds we were unable to find the required sealing substance. However, the prospects of such a seal are attractive, since hermeticity of the seal that has been impaired for any reason could easily be restored, while preserving the thermoelements, by simply reheating and melting the compound again.

It should be noted, however, that if, under the conditions of the experiment, the length of the seal can be increased beyond 12 mm, then such substances as pitch, Bakhilov bitumens, and plasticized pitches can be successfully used for the indicated purpose. Thus, with a seal length of 60 mm and a diameter of 4 mm, the indicated substances easily withstood 150 atm.

Nonfusible sealing compounds

Liquid glass. In the literature¹ there are indications of the extraordinary temperature stability of sealing compounds consisting of liquid glass and various fillers. In particular, a compound consisting of liquid glass and talc withstands both high and low temperatures (the temperature of liquid air) well. A paste consisting of 70% liquid glass and 30% talc was ground for a long time in an agate mortar. The resulting dough-like mass was used to fill the space to be sealed in a tube through which a bundle of thermoelement wires had been passed, the wires having first been coated with the same paste. After sealing, the seal remained at room temperature for 10 days and, despite such a long period of time, the paste only became very thick, but did not become completely hard.

For final hardening of the paste, the seal was dried for 3 days in a cabinet at a temperature of 80°C.

The test showed that such a compound cannot serve as a gas-tight sealing substance. Even at a pressure of 5 atm, gas passed both at the thermoelement wires and the walls of the tube, and through the very thickness of the plug itself, indicating the porosity of the latter.

Similar results were obtained in testing a seal made from 50% liquid glass and 50% special batch. Composition of the batch: quartz—75%, feldspar—22%, Na₂SiF₆—1.5%, CaCO₃—1.5%. Hardening of the compound occurred somewhat more rapidly than in the first case (at room temperature, 4 days, and at 75°C—3 days), but

the resulting seal was likewise unsuitable, as in the case of using a putty made of liquid glass and talc.

Bakelite. Bakelite was also used for sealing thermoelements in gas tubes. For this purpose the wires of the thermoelement were placed in an experimental tube and filled with liquid bakelite, in stage A, mixed with Petrov’s contact.

To obtain a monolithic plug, the seal was kept at room temperature for 12–18 hours; during this, partial polymerization of the bakelite occurred, associated with its transition from stage A to stage B. Then the temperature was raised to 30–40°C and, over the course of 8 hours, gradually brought to 75–80°C. At this temperature the seal was kept for 8 hours. As a result of this temperature regime, the liquid bakelite, under the influence of the contact, polymerizing, gradually passed from stage A to stage B and finally to stage C. The latter is characterized by the fact that bakelite, being in this stage, is almost insoluble in acids and alkalis, and also does not melt and constitutes a solid mass of relatively high mechanical strength.

Fig. 2.

When the resulting seal was tested, even at low pressures (10 atm.) the gas passed intensively between the bakelite plug and the walls of the metal tube. This indicated that, during polymerization, the bakelite decreases in volume and the body of the plug separates from the walls of the tube, thereby forming a space free for the passage of gas. Thus, for a cylindrical tube, bakelite cannot be used as a sealing substance.

The considerable mechanical strength of the bakelite plug suggested using it for sealing thermoelements by changing the design of the tubes in which the sealing is performed. The new form of these tubes is shown in Fig. 2. Here K is the tube through which the gas flows; A is a conical tube in which the thermoelement is sealed by means of the bakelite plug C. To eliminate the gap formed during bakelitization between the walls of the conical tube A and the body of the plug C, the latter is pressed against the cone by nut B.

Testing of this seal gave positive results: the system remained hermetic at a pressure of 150 atm. and at temperatures of 20°, 0°, −10–20 and at the temperature of liquid air. Thus, this method positively solves the problem of fully hermetic sealing of complex and simple thermoelements at high pressures and low temperatures.

It should be noted that the described seal, despite its simplicity, is associated with delicate mechanical and assembly work caused by the small size of the individual parts.

In addition to the above, we found a second method of hermetic seal-

sealing thermoelements. Specifically, the bakelite plug C (Fig. 2) was replaced by a leather one. For this purpose, a plug, slightly larger in size than the dimensions of cone A, was cut from half-tanned leather; a hole was drilled in the latter ($d = \sim 1\ \mathrm{mm}$), after which the blank was boiled for 15 minutes in molten grease for vacuum cocks, consisting of rubber, vaseline, and paraffin, and then its outer surface was carefully worked with a fine file. To fill all the pores in the silk insulation, the thermoelement wires were boiled for 15–20 minutes in molten sealing wax, after which the excess of the latter was removed and the wires, in the form of a bundle, were introduced into the opening of the plug. After careful crimping, plug C was placed in cone A and, in the heated state, was clamped with nut B. To prevent short-circuiting of the wires with nut B and the narrow part of cone A, the thermoelement was tightly wrapped with thread E. The length of the leather (and bakelite) plug was 12 mm.

The leather seal proved to be completely hermetic at a pressure of 150 atm. and at low temperatures. In comparison with the bakelite one, it is advantageously distinguished by greater speed of manufacture and lower brittleness.

CONCLUSION

Various substances were investigated for hermetic sealing, capable of withstanding high pressures (150 atm. of gas) and low temperatures (down to −185°C), of simple and complex thermoelements consisting of thin wires ($d = 0.07$–$0.15\ \mathrm{mm}$). Of all those tested, bakelite and gland-leather seals proved suitable.

LITERATURE

1) Edited by Acad. Ioffe, Technique of the Physical Experiment, Gosizdat, 1929.
Ostwald, Luther, Drucker, Physicochemical Measurements, ONTI, Part 1, 1935.
Angerer, Laboratory Technique, GTTI, 1934.
2) Zintl and Goubeau, Z. anorg. Chem. 163, 105, 1927.