Non-sag tungsten wire

Details Non-sag tungsten wire Non-sag tungsten wire

1997-01-15

2001-02-20

Sheehan, John (Department: 1742)

Metal treatment

Stock

Chromium, molybdenum, or tungsten base

C075S235000, C420S430000, C313S315000

Reexamination Certificate

active

06190466

ABSTRACT:

The invention relates to a non-sag tungsten wire used in light sources or heating elements, which tungsten wire is prepared from a tungsten block by a powder metallurgy process with thermomechanical technique, and has an overlapped crystal structure after recrystallization and contains a dopant material. Recrystallization takes place during heat treatment or the first operation of the wire.
Incandescent lamp filaments and heating elements made from tungsten wires are expected to have good vibration resistance both in cold and in hot condition on one hand and good non-sag properties on the other.
It is well known from the literature (see e.g. E. Pink, L. Bartha: The Metallurgy of Doped/Non-sag Tungsten, Elsevier Appl. Sc.) that non-sag properties can be achieved by doping tungsten oxides with aluminium, potassium and silicon compounds. During this process, silicon and aluminium dopants evaporate while the sets of bubbles formed from the potassium vapor produce an overlapped recrystallized structure after heat treatment which structure ensures good non-sag properties, at the same time, however, vibration resistance not always reaches the desired level.
In order to increase vibration resistance, it is a usual method to use ThO
2
dopant material in 0.75-1.0% since in thoriated tungsten an equiaxial crystallite structure (i.e. a structure with no preferred orientation of crystal axes) is formed where the rapid migration of grain boundaries is prevented by the thoria particles on the grain boundaries, and due to this, tungsten wire will be made resistant to vibration; this type of tungsten wire, however, has a tendency to get deformed rapidly at high temperatures. Tungsten wires with this dopant have the disadvantages of having bad sag properties on one hand and of containing radioactive thorium on the other.
In order to combine the good properties mentioned, manufactures are experimenting various solutions. For example, a small percentage of rhenium is added to the tungsten doped with aluminium, potassium and silicon, which results in good non-sag properties together with good vibration resistance. Still, this solution has the disadvantage of being expensive and also, this type of tungsten is difficult to process (has poor workability).
The objective of the invention was to provide a solution that is able to combine the good properties mentioned and is able to eliminate the radioactive thorium and also produces tungsten wires with good workability at an acceptable price.
Based on our recognition, the stated objective can be achieved by a tungsten wire that contains lanthanum(III) oxide or cerium dioxide or a combination thereof. We have recognized that in cases when the tungsten is doped with lanthanum(III) oxide and/or cerium oxide in a determined quantity, the solid second phase being disintegrated in forging and drawing will, similarly to the bubbles of potassium vapor, prevent secondary recrystallization from occurring for a time, and then, above a certain temperature an abrupt grain growth—similarly to the case of potassium-doped tungsten—will take place, which results in an overlapped recrystallized structure similar to that of the aluminium-, potassium- and silicon-doped material.
In accordance with this, our invention is a non-sag tungsten wire for light sources or heating elements, which tungsten wire is prepared from a tungsten block by powder metallurgy process with thermomechanical technique, and has an overlapped crystal structure after recrystallization and contains a dopant material, and this dopant material contains at least one of the following additives:
lanthanum(III) oxide,
cerium dioxide.
We have found that by making use of lanthanum(III) oxide and/or cerium dioxide additives the objective set can be achieved most successfully in case of small additive concentrations, namely if the quantity of additive does not reach 0.6 weight percents and its value is preferably 0.475 weight percents or less.
We have found that the lower limit of additive quantity just ensuring the desired effect is 0.2, preferably 0.3 weight percents. wires with good properties. A further advantage of the tungsten wire according to the invention over those doped with aluminium, potassium and silicon is that its electron work function is substantially lower, which enables it to be used e.g. for cathodes in discharge lamps and cathode ray tubes as well.


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