GB2098199A

GB2098199A - Fused silica tubing

Info

Publication number: GB2098199A
Application number: GB8111440A
Authority: GB
Original assignee: GEORGE HENRY HERBERT GEE
Current assignee: GEORGE HENRY HERBERT GEE
Priority date: 1981-04-10
Filing date: 1981-04-10
Publication date: 1982-11-17

Abstract

Method of threading (3) fused silica tubing (1) by evacuating the interior and heating the tubing so that it collapses onto a threaded alumina former. The method is used for the production of a needle valve having tap barrel 1 made of fused silica and internally threaded at 3 to receive a P.T.F.E. plunger 4. <IMAGE>

Description

SPECIFICATION Fused silica tubing The present invention relates to fused silica tubing. In particular, it relates to a method of internally threading fused silica tubing and to the products which can be made by using the method.

Silica SiO2 is very widely present in the earth's crust and is usually crystalline. The most common allotropic form of crystalline silica is quartz, such as siliceous sand and rock crystal; other forms are tridymite and cristobalite. When fused at 2000"C, all types yield an amorphous and isotropic vitreous material. Depending upon the raw material employed-typically quartz in the form of sand or rock crystal-and the melting process, this vitreous material is either opaque, translucent or transparent. The material referred to herein as "fused silica" has a high degree of purity, being composed of more than 99% silica and preferabiy more than 99.99% silica, and is usually completely transparent, although the inclusion of minute gas bubbles in the melt may make it translucent or even opaque.Such pure transparent fused silica may be derived from melting rock crystal or from high temperature hydrolysis of silicon halides.

Fused silica is a very pure, extremely stable material with excellent refractory properties even at high temperatures. Softening commences at about 1200-1300"C and up to that temperature the material does not undergo physical deformation and is not sensitive even to abrupt temperature changes, so that a specimen at red-heat may be plunged into cold water without damage. Fused silica also has excellent resistance to most chemi cais; the only acid which will attack it at low temperatures is hydrofluoric, although it may be damaged by phosphoric acid at about 400"C. In addition, at ambient temperature it exhibits higher electrical resistivity than almost any other material.

These properties make fused silica particularly suitable for use in the carriage or transfer of high purity fluids, corrosive chemicals and gases such as are encountered in the field of nuclear energy and reseach, high vacuum techniques and processes employed in the semi conductor industry where contamination is undesirable and high temperatures may be required. However, a problem arises with regard to the taps and valves which are necessary in such equipment. Heretofore, grease or some form of lubricant has been necessary, or "O''-rings have had to be provided, in connection with such taps and valves, and this has inevitably increased the risk of contamination.The present invention seeks to overcome this difficulty by making it possible to manufacture items such as needle valves and stopcocks of fused silica without any lubricants or "O"-rings being necessary. The key factor in the invention is an appreciation of the advantages secured by internally threading fused silica tubing, and a workable method of doing so.

The present invention provides a method of internally threading fused silica tubing which comprises inserting a threaded alumina former into a length of fused silica tubing having a clean interior surface, evacuating the interior of the tubing, heating the tubing in the area of the thread on the former until the fused silica becomes malleable and collapses onto the threaded former due to the pressure difference between the outside and the inside of the tubing, cooling the tubing and unscrewing the former.

Prior to the present invention, there had not to my knowledge been any attempt to intrernally thread fused silica tubing and little if any appreciation of the advantages of doing so.

Fused silica tubing has been threaded externally by the use of cutting wheels or forming a thread by hand, but any thread cut with a diamond or carborundum cutting wheel needs to be fire polished to re-create the glaze on the surface and eliminate the rough edge left from the cutting. There are severe practical difficulties in entering a tube with a flame to effect fire polishing, particularly if the tube is 20 mm. or less in diameter, so internally threading fused silica tubing by cutting is hardly a practical possibility and the idea of internally threading has been ignored.

After the idea had ocurred to me of internally collapsing a length of heated fused silica tubing onto a threaded former by evacuating the interior of the tubing, I was left with the problem of finding a material for the former which would withstand the very high temperature required to make the fused silica tubing sufficiently malleable; the temperature concerned is in the region 1200"C-1300"C and with extremely pure silica (99.99% SiO2) it is nearer 1300"C. I have found alumina to be a suitable material, and my preferred threaded alumina former is made from a material known under the trade name DERANOX 975.

This has the approximate analysis: Alumina 97% Silica 1+% Lime i% Magnesia i% All percentages being by weight, and the remaining *% being made up of trace elements including potash, sodium, ferrous oxide and titanium. For every size bore fused silica tubing to be internally threaded a corresponding bore externally threaded alumina rod is used as a forming tool, e.g. 20 mm. bore silica tubing would require a 20 mm. outside diameter threaded alumina rod.

Fused silica is in a metastable state and above 1 000 C may be transferred into cristo balite; this allotropic transformation is pro moted by impurities, particularly at the surface, and subsequent cooling causes devitrification. The purer the silica, the slower the process. Therefore it is necessary thoroughly to clean the interior of the fused silica tubing prior to heating so as to avoid incipient devitrification. Hydrofluoric acid is used for this purpose.

As an example of an article in the form of a tap or valve which can be made using the method of the invention, a leakproof needle valve will now be briefly described with refe rence to the accompanying drawing, in which Figure 1 is a somewhat schematic plan view of the tap barrel; and Figure 2 is a similar view of the tap insert or plunger.

Figure 1 shows the usual T-shaped tap barrel 1 with a restriction 2 for recieving the needle of the needle valve plunger. The tap barrel is made of fused silica which is internally threaded at 3 by the following process.

The fused silica tubing is first cleaned using hydrofluoric acid at 10% concentration, after which it is washed with water and distilled water. A threaded former made of Deranox 975 is then inserted in one end, and the interior of the tubing is evacuated. While the fused silica tubing is revolving in a small lathe, the flame from an oxygen-propane burner is played on the area 3 in the vicinity of the thread on the former. The area involved may be no more than 3 inch along the axis of the tubing, and the burner may play upon this area for no more than ten minutes, but the silica during this time should reach a temperature at least in the range 1200"C-1300"C so that it becomes malleable and collapses onto the threaded former due to the pressure difference between the outside and the inside of the tubing.The tubing is then cooled and the former removed by unscrewing it from the internally threaded fused silica tubing.

The needle valve plunger 4 shown in Fig. 2 again has a generally conventional form, being composed of a turning head 5 at one end and a needle 6 at the other, for co-operation with restriction 2 in the tap barrel, together with a mid-portion 7 which is externally threaded to screw into the internally threaded area 3 of the tap barrel. The thread may for example be 4 Whitworth, and the diameter 6 4 mm. But the important factor about the plunger 4 is that it is made of P.T.F.E. (polytetrafluorethylene).

The advantages of a leakproof tap or valve made from inert material are numerous, the inert nature of the P.T.F.E. plunger being added to the extremely pure (preferably 99.99% S,O2) character of the fused silica to give a component of ultra purity. The threads on the tap barrel and plunger can be made to fit extremely well, so that no lubricant is necessary, no "O"-rings are required and there is no "dead space" for trapping gases or liquids. The method of internally threading the fused silica tubing makes it unnecessary to provide further polishing, so that contamination is eliminated, and the size of the article can be kept down because nothing further is required to make it leakproof. Such taps or valves can be used in high vacuum processes and doping processes and in conditions of very high temperatures. They can be employed in the carriage or transfer of high purity materials with absolute ease of use and without contamination, since no grease is needed. There is no limitation in size, and stopcocks up to 1 inch may be manufactured without any need for the usual grease or "O"- rings.

Claims

1. A method of internally threading fused silica tubing which comprises inserting a threaded alumina former into a length of fused silica tubing having a clean interior surface, evacuating the interior of the tubing, heating the tubing in the area of the thread on the former until the fused silica becomes malleable and collapses onto the threaded former due to the pressure difference between the outside and the inside of the tubing, cooling the tubing and unscrewing the former.

2. A method according to claim 1, wherein the threaded alumina former has the composition 97% alumina, 13% silica, i lime, i% magnesia, the remaining 4% being trace elements.

3. A method according to claim 1 or claim 2, wherein the interior of the fused silica tubing is first cleaned using hydrofluoric acid.

4. A method according to any one of claims 1 to 3, wherein the fused silica is extremely pure, being composed of more than 99.99% Six2.

5. A method according to claim 4, wherein the tubing in the area of the thread on the former is heated to substantially 1 300"C.

6. An article incorporating internally threaded fused silica tubing formed by the method according to any one of the preceding claims.

7. An article according to claim 6 in the form of a valve incorporating an internally threaded fused silica tap barrel and a tap insert or plunger threaded to cooperate with the thread in the tap barrel.

8. An article according to claim 7, wherein the tap insert or plunger is composed of P.T.F.E. (polytetrafluorethylene).

9. A method according to claim 1 of internally threading fused silica tubing, substantially as hereinbefore descibed.

1 0. A needle valve incorporating internally threaded fused silica tubing, substantially as hereinbefore described with reference to the accompanying drawing.