DE1170748B - Process for coating a molybdenum wire with a carbon layer by thermal decomposition of volatile compounds - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY

GERMAN

PATENT OFFICE

EDITORIAL

Boarding school Class: C 23 c

German class: 48 b -11/10

Number: 1170748

File number: N 21817 VI b / 48 b

Filing date: July 10, 1962

Lay-out day: May 21, 1964

The invention relates to a method for coating a molybdenum wire with a well-holding one Carbon layer at low temperature (about 1100 ° C) and on one with such a wire provided grid electrode for an electric discharge tube.

It is known that a carbon layer at about 900 to 1000 ° C with a molybdenum underlay reacts so that the carbon disappears into the surface. To prevent this, it has already been suggested an intermediate layer of a carbide, e.g. B. silicon carbide, thereby attaching that the core was coated with silicon oxide and a carbonaceous substance and that then on 1700 is heated to 2000 ° C in an inert atmosphere to reduce the oxides and create a carbide layer to build. A layer of carbon could be applied to this layer, e.g. B. in the form of powdery graphite. It turns out, however, that the molybdenum core as a result of the strong heating becomes brittle. In general, the electrodes must first be formed in the known method before applying the carbide layer and the carbon layer. There was always one strong heating required, otherwise no intermediate carbide layer would be formed. Were the carbides attached as such on the electrode, heating to temperatures above 2000 ° C was necessary, to melt the carbides to achieve good adhesion of the carbide layer.

Contrary to all expectations, however, it has been found that the formation of a well-adhering intermediate layer under a carbon layer on a molybdenum wire is already possible at a temperature of about 1100 ° C, so that the coated molybdenum wire completely retains its mechanical properties and is easily processed into grid electrodes can be if, according to the invention, the molybdenum wire is first passed through an atmosphere of silicon chloride (SiCl ₄ ) at a temperature of about 900 ° C. and then at about 1100 ° C. through an atmosphere of carbon tetrachloride (CCl ₄ ).

It then turns out that after this treatment the molybdenum core is covered with a very well adhering intermediate layer of silicon with a carbon layer on it and that this core can easily be wound into grids, since the applied layers are so thin that they impair the mechanical properties of the molybdenum essentially do not affect. It is believed that the good adhesion is obtained by methods of coating a molybdenum wire with a carbon layer
thermal decomposition of volatile compounds

Applicant:

N.V. Philips' Gloeilampenfabrieken, Eindhoven

(Netherlands)

Representative:

Dr. rer. nat. P. Roßbach, patent attorney,

Hamburg 1, Mönckebergstr. 7th

Named as inventor:

Popko Reinder Dijksterhuis,

Hans Melsert,

Poppe Zuidema, Eindhoven (Netherlands)

Claimed priority:

Netherlands of July 14, 1961 (267 095)

that the following layers are present from the Mo core to the outside: MoSi ₂ ; Si; SiC; C. It is surprising that the reaction between Si and C, which generally only takes place at high temperature, can now occur at around 1100 ° C., probably as a result of a cleaning effect of the carbon tetrachloride.

The coating can be done very quickly. A stay of IV2 seconds in any gas atmosphere is sufficient to the wire with a structure of the layers mentioned with a total thickness of about μ to overlay. The wire can namely a speed of about 1000 to 2000 m in the Hours, while the length of the ovens in which the gas atmospheres prevail is about 1 m.

With a lower throughput speed, the layers naturally become thicker.

The invention is explained in more detail with reference to a drawing in which

Fig. 1 schematically illustrates the method and

Figure 2 shows a grid made with a molybdenum wire coated in accordance with the invention.

In FIG. 1, 1 denotes the Mo wire which runs from a supply reel 2 to a supply reel 3 passing ovens 4 and 5. The heating

409 590/254

of the wire is carried out by passing current through the wire 1, for example by means of sliding contacts 6. A suitable speed at which the wire passes the ovens is about 2000 m / hour, the thickness of the layer on the wire being about 1 μ. In furnace 4 silicon _{tetrachloride (SiCl 4} ) and in furnace 5 carbon tetrachloride (CCl ₄ ) are fed in gas form. The wire thickness can be 40 to 150 μ.

The grid 9 of FIG. 2 has good heat radiation and low primary and secondary emissions due to the black carbon layer 8 on the surface of the wire 1. The silicon intermediate layer is denoted by 7. The boundary layers, not shown, between the core and of layer 7 and between layers 7 and 8 probably consist of the compounds MoSi., And SiC, as previously indicated.

Claims (1)

Claim: Method for coating a molybdenum wire with a well-adhering carbon layer by thermal decomposition of volatile compounds, in particular for the production of grid electrodes for electrical discharge tubes, characterized in that the molybdenum wire is initially exposed to an atmosphere of silicon tetrachloride (SiCl ₄ ) and at a temperature of about 900 ° C is then passed through an atmosphere of carbon tetrachloride (CCl ₄ ) ^{at about 1100 ° C.} 1 sheet of drawings 409 590/254 5.64 © Bundesdruckerei Berlin