Crucible Electron Beam Evaporation Of Aluminum

Abstract

A crucible liner is provided for a water-cooled copper crucible for evaporating aluminum, the lining being electrically conductive but having a poor thermal conductivity. Also, the lining which does not react to a great extent with hot aluminum is and remains in intimate contact with the copper. The inner layer 22 of the lining is a composite of titanium diboride and boron nitride which is cemented to the water-cooled copper body of the crucible by a second layer operating as a cement comprising titanium or tungsten or molybdenum or a cement such as molybdenum silicide.

Description

BACKGROUND

In evaporation depositing of aluminum, the aluminum is usually melted and vaporized by the application thereto of an electron beam. The aluminum is melted and vaporized in a cavity of a water-cooled copper crucible. When the aluminum melts, it wets the copper whereby much of the heat provided for the evaporating of the aluminum is actually used up in heating the copper crucible and the cooling water for the crucible. If a crucible liner is used, the aluminum soon wets over the lip of the liner to the crucible whereby the copper crucible again carries away heat that was intended for vaporizing the aluminum. The hot aluminum and the liner may react at high temperatures of the molten aluminum. This reaction of aluminum with the crucible can contaminate the aluminum or the molten aluminum will stick to the hot crucible and if aluminum is left in the crucible upon cooling, the contraction of the residual aluminum will fracture the crucible requiring a new one be used for each evaporation. Since the electron beam is used for providing the heat, the liner must be electrically conductive. To conserve heat, the liner should have low heat conductivity. The liner should stick to the crucible and it should react very little or not at all with hot aluminum.

It is an object of this invention to provide an improved crucible for vaporizing aluminum.

It is another object of this invention to provide a crucible for vaporizing aluminum in which the part that is in contact with the hot aluminum is not very reactive therewith, is electrically conductive but has low thermal conductivity and is not easily separated from the body of the crucible.

SUMMARY

In accordance with this invention, a liner is provided for the water-cooled copper crucible which has low thermal conductivity and good electrical conductivity and which does not react to a great extent to the hot aluminum. The liner may be of an intermetallic composite such as titanium diboride and boron nitride. It is stuck to the copper crucible by a cement comprising titanium or tungsten or molybdenum or a cermet such as molybdenum silicide. The disclosed cement will stick to the copper of the crucible. While the cement and intermetallic composition are being applied as by flame spraying, there will be sufficient mixing of the cement and the composition so that the composition will be held in place in the cavity in the copper crucible. The lining is sufficiently conductive to act as a connection for the electron beam and is of sufficiently low thermal conductivity so that neither it nor the copper crucible gets very hot, whereby the efficiency of the application of heat to the aluminum is greatly increased. Due to the fact that the liner sticks to the crucible, molten aluminum cannot get between the liner and the copper crucible. Since the liner does not get too hot, the chemical interaction between the aluminum and the liner is minimized whereby the liner lasts a long time and the evaporated aluminum is nearly pure.

The invention will be better understood upon reading the following description in connection with the accompanying drawing, the sole FIGURE of which illustrates a cross section of a crucible in accordance with this invention.

DESCRIPTION

As shown in the FIGURE, a copper crucible 10 is provided having a cavity 11 for receiving a charge 14 to be melted and also having water-cooling passages 12 therethrough. The cavity 11 in the crucible 10 is provided to receive a charge of aluminum 14 to be evaporated by an electron beam 16 projected thereon by an electron gun 18. The aluminum 14 is shown as being in a molten state and if the liner comprising the layers 20 and 22 (which will be described) are not provided, the hot aluminum 14 would wet the copper 10 providing a good thermal coupling thereto, whereby a great deal of the heat provided by the beam 16 is conducted to the copper crucible 10 and is removed by the cooling water in the passageways 12. An inner liner 22 of an intermetallic composite such as titanium diboride and boron nitride, which has been used for making resistance heating evaporation boats, may be provided in the cavity 11 of the copper crucible 10. Such an inner liner however does not make a good thermal contact with the surface of the cavity 11. Therefore, after a short while, the molten aluminum will run over the edge of the composite 22 because of the poor thermal conductivity of the composite with the cavity 11 and the molten aluminum will wet the adjacent surfaces of the liner and of the cavity 11, whereby the aluminum is again thermally coupled to the crucible and the heat is carried away by the cooling water as before. Furthermore, as the lining 22 gets hot enough, the aluminum and the inner liner 22 will interact chemically and the inner liner 22 will be damaged or destroyed and the evaporated aluminum will not be pure. A cement 20, which may be of titanium or tungsten or molybdenum or a cermet such as molybdenum silicide is provided to cement the inner liner 22 in place in the cavity 11 of the copper crucible 10. Then, the outside of the inner liner 22 sticks to the inside of the cavity 11 and the aluminum 14 cannot get between the inner liner 22 and the crucible 10. The inner liner 22 acts as a poor thermal conductor between the aluminum 14 and the copper crucible 10 and the heat provided by the beam 16 is used more efficiently in evaporating the aluminum 14. The liner comprising the inner liner layer 22 and including the cement 20 provides sufficient electrical conductivity to act as an electrode for the beam 16. The inner liner 22 does not get hot enough to react to any great extent with the aluminum 14, whereby the inner liner 22 lasts a long while and the vaporized aluminum is not contaminated with material of the inner liner 22.

In providing the liner, the cement 20 is flame sprayed into the cavity 11 and then the composite material 22 is also flame sprayed into the cement lined cavity. It is noted that both the cement 20 and the inner lining 22 extend for a short distance over the top of the crucible beyond the edges of the cavity 11. The cement 20 is chosen to stick to the inside of the cavity 11 (and to the top of the crucible) and is of one of the materials noted above. The flame spraying of the liner 22 causes sufficient intermixing of the inner liner material with the cement material to make the inner liner 22 stick in the cavity 11 and along the margin thereof.

Claims

What is claimed is:

1. A crucible comprising:

a body of copper having a cavity in a surface thereof,

a liner for said cavity comprising a first layer of material taken from the group consisting of titanium, tungsten, molybdenum and a cermet contacting the surface of said cavity and a second inner layer of an intermetallic material comprising boron nitride and titanium diboride whereby, said first layer adheres to said cavity and acts as a cementing agent for holding said second inner lining in place.

2. The invention of claim 1 in which said block is water-cooled.

3. The invention of claim 1 in which means are provided for vaporizing a material to be vaporized that is deposited in said cavity.

4. The invention of claim 3 in which said vaporizing means includes a source of a beam of electrons.