EP0439775A1 - Molybdenum material, especially for making lamps - Google Patents

Abstract

The molybdenum material has been doped with potassium, silicon and aluminium. An aluminium content of between 140 and 180 ppm, a potassium content of between 150 and 300 ppm and a silicon content of between 300 and 500 ppm are particularly advantageous.

Description

This application is closely related to parallel application No. ... (Az. GR 90 P 5502). The invention is based on a molybdenum material according to the preamble of claim 1.

In the following, the term molybdenum material is to be understood as meaning materials which are used for various purposes, preferably in lamp construction. The end product of molybdenum production, which is initially available as a sintered rod, is then only further processed purely mechanically, so that nothing changes in the chemical composition. The desired primary materials are created by rolling, hammering and drawing. More specifically, wires or pins are initially created in these processes. Tubes or strip material for the film production are then in turn produced from pins or wires as semi-finished products.

The doping of molybdenum material with potassium and silicon in the form of potassium silicate solution has been known for a long time. For example, U.S. Patent 4,419,602 describes these elements as Use additives for molybdenum sealing foils to increase the recrystallization temperature. However, it has been shown that the material properties of the doped molybdenum have a considerable spread, so that, if desired, a material with precisely defined properties has hitherto had to be adjusted subsequently by mixing different components in a very difficult working step.

It is an object of the present invention to improve the quality of the material properties of semi-finished molybdenum, in particular for the lamp industry, and to reduce rejects.

Another object is to simplify the process for producing molybdenum material and to make it more cost-effective.

These objects are achieved by the characterizing features of claim 1. Particularly advantageous embodiments of the invention can be found in the subclaims.

In recent years, the requirements for the thermal and mechanical resilience of the molybdenum material have increased continuously, especially in connection with the development of halogen incandescent lamps and PAR lamps. This initially led to extensive specialization of molybdenum materials for various areas of application. For example, various molybdenum materials for core wires, gas-tight sealing pins, holder wires and sealing foils were manufactured. While with holder wires a high and constant elongation the The most important property is that a high degree of ductility and a high recrystallization temperature are important for sealing foils. On the other hand, with melting pins and core wires, the appropriate combination of a high recrystallization temperature with a high number of bends is the decisive factor. Gap freedom also plays an important role in melting pens.

These different requirement profiles were taken into account by different doping with potassium and possibly also silicon. This made the production of the semi-finished product very complicated and inefficient, because the machines had to be retooled and reprogrammed again and again. There was also the risk of confusing the different materials during further processing.

In addition, there appeared to be an apparently unsolvable problem in the high spread of the respective dopings for a long time. The choice was either to accept a high committee or to process the material despite the poor quality. For example, a high gap increases the risk that the halogen circuit in a lamp is disturbed by impurities, which leads to early failures.

A suitable additional doping with aluminum has now succeeded in overcoming both difficulties. Aluminum chemically binds the potassium and silicon to a high-temperature resistant compound and thus holds the potassium, which is otherwise in an uncontrolled manner during the reduction process would evaporate partially (ie up to 50%!). Through the targeted addition of a certain amount of aluminum, a desired, precisely defined amount of potassium can now be recorded in the molybdenum material. 0.8 to 2.0 times the potassium is particularly advantageous. Without the simultaneous addition of aluminum, the potassium previously had to be over-doped, so that during the manufacturing process a portion that could not be precisely determined evaporated, which in turn led to a dispersion of the material properties. This is now prevented by the addition of aluminum. The same applies to silicon.

This positive property is achieved by adding 80-600 ppm by weight of aluminum; particularly good results are shown when using 100-300 ppm. When a considerably larger amount of aluminum is added (in the range per percent and percent), the potassium-stabilizing effect of aluminum is masked by its gettering properties, in particular for O ₂ (cf. Mikrochimica Acta, 1987, I, pp. 437-444). At the same time, the thermal and mechanical behavior is also affected; in particular, this material is no longer suitable for lamp manufacture.

Surprisingly, however, it has been shown that the properties of the molybdenum material can be considerably improved with the economical doping with aluminum given above. A molybdenum material can be obtained which is superior to all specific molybdenum materials available to date. This has even made it possible to pass through the various molybdenum materials mentioned above to replace a uniform and improved type of molybdenum, which lowers the cost of production. In addition, with such new molybdenum types there is the possibility of energy savings of up to 25%, since sintering in direct current passage (high sintering) can now be dispensed with (cf. C. Agte / J. Vacek, Wolfram and Molybdenum, Akademie -Verlag, Berlin, 1959, especially chapter 6). Instead, the sintering process can now be carried out in push-through furnaces at considerably lower temperatures (approx. 1700 ° C compared to approx. 2000 ° C).

Two exemplary embodiments are to be explained in more detail below.

A first type of molybdenum uses a doping of approx. 160 ppm aluminum, 275 ppm potassium and 500 ppm silicon. The gap is less than 1%, while the number of bends is 11.5. These values are measured on a wire with a diameter of 600 µm.

A second type of molybdenum uses a doping of approximately 150 ppm aluminum, 150 ppm potassium and 300 ppm silicon. The clearance is around 8%, while the number of bends is 6, again measured on a wire with a diameter of 600 µm.

Each of the two exemplary embodiments is suitable on its own for encompassing the wide range of applications that have hitherto been covered by various types of molybdenum.

Conversely, the invention can also be targeted can be used to optimize the crystal structure of the molybdenum material with regard to a very specific application, since the type of structure largely determines the properties of the material.

The particularly preferred molybdenum materials (column I) described in the two exemplary embodiments have the following properties as wire compared to conventional materials (column II shows the best available value of the conventional material):

This table shows the improvement of the properties, in particular the reduction in the variation in the potassium content, in an impressive manner.

The process for producing the molybdenum material runs in principle according to the Coolidge process: The starting material for the production of the molybdenum products is MoO₃ with a purity of 99.97% by weight. In two steps the MoO₃ is reduced to Mo via MoO₂ at temperatures of approx. 500 - 600 ° C (1st step) or 1000 - 1100 ° C (2nd step). These reductions in molybdenum oxide are carried out in a manner known per se using an H₂ / N₂ mixture and pure H₂ gas. A rotary kiln is advantageously used instead of a feed oven to be equipped with boats. The molybdenum trioxide, which is initially present as a powder, is added in a manner known per se as an aqueous potassium silicate solution either before (as was the case in embodiment 2) or after (as in the case of embodiment 1) the first reduction as dopants. At the same time, the aluminum is added as nitrate (Al (NO₃) ₃). It would also be conceivable to use another unstable aluminum compound, for example AlCl₃. In contrast, a compound of high stability, for example Al₂O₃, is unsuitable since the aluminum would not be released in spite of the high temperatures during the reductions.

In order to be able to produce the desired ductile materials, the molybdenum powder is pressed on hydraulic presses in steel matrices. Presintering may be advantageous at this point. Then the usual high sintering takes place in the direct current passage (5000 A) in a sinter bell at temperatures up to 2000 ° C. This process is used more with higher doping (embodiment 1). Alternatively, this process can now be carried out in a push-through furnace to increase capacity and save energy. which is mainly used for lower doping (embodiment 2). The sintered rod formed is then processed into molybdenum wire by rolling, hammering and drawing. This wire can now be used as a power supply, holder pin or so-called electrode (eg for automotive halogen light bulbs) or as a core wire for the manufacture of tungsten filaments. The strip material for the foils can be obtained from the molybdenum wire by further rolling, while the tubes are produced by rolling the wire and then longitudinally bending it into a "tube".

Moreover, the doping of the molybdenum with potassium, silicon, aluminum (eg 275 ppm K) according to the invention has nothing to do with the coincidentally similar doping of the tungsten with the same substances (eg 75 ppm K). While according to the invention the doping in the case of molybdenum brings about an improvement in a whole series of properties, in the case of tungsten this doping is primarily responsible for the growth in the length of the grains, which is ultimately intended to prevent the tungsten wire from sagging. The powder metallurgical behavior of both elements is also not comparable (tungsten is sintered at 2800 ° C). The reactions of molybdenum when doping and reducing differ fundamentally from those of tungsten. The reason is considered to be the considerably weaker binding energy of the molybdenum compounds compared to corresponding tungsten compounds. For example, in contrast to tungsten, in the case of molybdenum, no stable ß-phase is formed during the reduction, which means that the Potassium in the crystal lattice - as happens with tungsten - would allow. The effect of doping with molybdenum can therefore be characterized as a surface effect with respect to the crystal structure. On the other hand, one can speak of a volume effect with tungsten.

The experience gained with tungsten with regard to doping with potassium, silicon and aluminum cannot therefore be transferred to the specific problems in the production of molybdenum.

The molybdenum wire according to the invention is used, for example, in a motor vehicle halogen incandescent lamp which has a cylindrical bulb made of hard glass or quartz glass, in which the two luminous elements for low beam and high beam are held by means of three current leads. A dimming screen may also be provided. Such a lamp is described for example in DE-OS 28 29 677. In a particularly preferred exemplary embodiment, the power supply lines and possibly also the anti-dazzle shield are made from molybdenum wire with the addition of 150 ppm aluminum, 150 ppm potassium and 300 ppm silicon. In the case of a piston made of quartz glass, the molybdenum wire can be used for the holder pins and foils, in the case of a hard glass piston it is used for the continuous power supply (electrodes).

Another area of application is a high-voltage halogen incandescent lamp pinched on one or two sides with a long axial lamp or a halogen lamp pinched on one side with a U-shaped or V-shaped lamp. For support of the filament, in the first case the power supply remote from the base can be supported in the bulb, as described in DE-GM 88 12 010. In the case of a festoon lamp, support brackets for the luminous element can be provided (for example EP-OS 150 503). Finally, in the third case, the U-shaped or V-shaped lamp body can be held at the end remote from the base by a frame (see, for example, EP-OS 173 995). The above preferred exemplary embodiment is also used in these cases.

During the manufacture of the filament, the filament wire is wound onto a core wire made of molybdenum, which is ultimately released again by immersion in an acid.

Claims (12)

Molybdenum material, in particular for lamp manufacture, the molybdenum being doped with potassium, silicon and aluminum, characterized in that the aluminum content is between 80 and 600 ppm by weight. Molybdenum material according to claim 1, characterized in that the weight ratio Al / K is about 1: 0.8 to 1: 2.0. Molybdenum material according to claim 1, characterized in that the weight ratio aluminum / silicon is approximately 1: 1.8 to 1: 3.8. Molybdenum material according to claim 1, characterized in that the aluminum content is between 100 and 300 ppm. Molybdenum material according to claim 4, characterized in that the aluminum content is between 140 and 180 ppm. Molybdenum material according to one of the preceding claims, characterized in that the potassium content is 100-400 ppm and the silicon content is 200-700 ppm (each by weight). Molybdenum material according to claim 6, characterized in that the potassium content is between 250 and 300 ppm and the silicon content is between 400 and 600 ppm. Molybdenum material according to claim 6, characterized in that the potassium content is between 130 and 170 ppm and the silicon content is between 270 and 320 ppm. A process for producing molybdenum material according to claim 1, characterized in that the aluminum is added as an unstable compound, in particular as nitrate, to a pulverized molybdenum compound, which is then reduced. A method according to claim 9, characterized in that the molybdenum compound is MoO₃ with a purity> 99.97 wt .-%. A method according to claim 9, characterized in that the reduced molybdenum is pressed into a rod and then densely sintered at a temperature of approximately 1700 ° C without direct current passage. A method according to claim 11, characterized in that the sintered rod made of doped molybdenum is then processed further to melt-down pins, tubes, core wire, holder wire or strip material.