4. The Construction of the X-ray Tube
4.3. ACTIVE ALLOY BRAZING
The active alloy soldering process employs titanium and zirconium as diffusion materials that show great affinity for glass and ceramics. At elevated temperatures reactions between titanium and zirconium and the ceramic crystal lattice take place more vigorously. In the presence of other metals at the interface with which titanium forms solid solutions, a bond is established between the ceramic body and the metal member with which the ceramic was put into contact during the assembly of the tube [19]. Titanium oxide and zirconium oxide are highly refractory ceramics, and it is a well established fact that ceramics are constituted of a number of different oxides. It therefore follows that titanium and zirconium will, under favourable conditions, reduce some of these oxides and combine with the released oxygen as long as active gases, such as oxygen or nitrogen, are not present in the atmosphere in which the seal is being made. Titanium and zirconium are known as active metals by virtue of their relative chemical activity [34]. Titanium for instance cannot be processed in ceramic crucibles as it will react with the ceramic, and so it is usually processed in its liquid form in water cooled copper crucibles. All that needs to be done to produce the necessary reactions to obtain vacuum tight seals, is to put titanium or zirconium in contact with the two ceramic surfaces, or between a ceramic and a metal surface. As the exclusion of an oxygen atmosphere is essential during the firing process for a seal to be effected, the active alloy seal has to be made under high vacuum, or in a very pure hydrogen, argon or helium atmosphere. The purity of the active metals themselves and the atmosphere is of critical importance [19]. Precautions have to be taken to ensure that oxides are not released from the metal surfaces during the brazing operations. Thus careful cleaning of all components prior to their being joined in an active alloy process must not be overlooked. While one way of applying titanium, or zirconium for that matter, to the ceramic and metal surfaces is to paint them with a titanium or zirconium hydride and nitro-cellulose lacquer mixture in a thin, uniform layer either by brushing or spraying the surfaces in question, and then heating the parts to 900°C to form a tight bond, a far more practicable approach is the use of a titanium alloy in the form of a washer, placing it between the components and then firing them at temperatures of 950°e. These titanium alloys typically contain silver, copper, nickel, tin or lead, and are preferable to pure titanium, which, when used as a brazing medium, would require brazing temperatures of 171 O°e. Hence the approach of using alloys for the active alloy process shall be pursued. The brazing alloy, that shall be used for joining the components of the X-ray tube, consists of titanium, silver, copper and indium. The use of such a brazing alloy promises a most effective and simple method of producing a seal, as the alloy can merely be cleaned and then placed in the form of a washer between the metal and the ceramic. After the components are placed under pressure and heated to a temperature of 950°C a vacuum tight seal is formed. According to Hensley [34] the chemical reaction that actually takes place during the formation of the seal is as follows:
3Ti + Ah03 ~ 3TiO +2Al
While the excess titanium and aluminium are dissolved in the braze filler material, the ceramic material Al203, also known as alumina, and TiO bond together by diffusion. The alloy used for brazing the X-ray tube components is the active solder alloy CH4 from Degussa AG. Below is a
listing of the composition of that particular alloy as well as other alloys belonging to Degussa' s CH family.
Table 4.1: Composition of CH active solder alloys by Degussa [42].
%Cu %Ag %Ti %ln
CH1 19.5 72.5 3.0 5.0
CH2 - 96.0 4.0
-
CH3 6.0 91.0 3.0
-
CH4 26.5 70.5 3.0 -
According to Weise [42] CH4 has a soldering temperature of 950°C and a melting temperature of 803°C to 857°C, and has been used successfully in joining Ah03 to steel containing iron and nickel or iron, nickel and cobalt to form vacuum tight seals.
4.4. STRESS IN JOINTS DUE TO THERMAL EXPANSION
A very important aspect of joining metal to ceramic is the type of metal used. According to Kohl [19] and Weise [42] it is essential to use a metal for the tube components, that has thermal expansion properties similar to ceramic. It is a well established fact, that differences in expansion coefficients between the ceramic and the metal will ultimately lead to stresses in the joint. The best solution therefore is to avoid the development of such stresses in the first place by ensuring that the mismatch in expansion coefficients is as minimal as possible. In Figure 4.1 a plot of the expansion versus temperature is shown for a number of materials. The ceramic, that is used for the manufacture of vacuum envelopes for the X-ray tube, is of the type DEGUSSIT AL23 manufactured by Degussa. According to Degussa's technical data on their ceramic products [43]
AL23 is composed of the following materials and displays characteristics as outlined in Table 4.2 below.
Table 4.2: Properties of DE GUS SIT AL23 [431.
DEGUSSIT AL23, Sintered aluminium oxide
Chemical composition >99.5% Ah03
Structure dense
Melting point, °C 2030
Working temperature, °C 1900
Thermal expansion 0-1000 DC, m/m °C 8.1xI0-6
Compressive strength, kg/cm2 30000
Bending strength, kg/cm2 3800
Tensile strength, kg/cm2 2650
Electrical Breakdown strength, kV 22
RMS/mm (at 20°C)
Table 4.2 (cont.): Properties of DEGUSSIT AL23 [43].
Behaviour in oxidising atmospheres Behaviour in reducing atmospheres Behaviour in high vacuum
Behaviour in andMo
1.8
1.6
1.4
t
1.2.:::::
<l 1.0 c Q
i CI)
c 0.8
Q
il:i
0.6
0.4
0.2 ~
0 0
high vacuum or Hz with W
1. Copper 2. AlSI304 3. MJnel 4. Nickel 5. Iron
6. Forsterile (ALSIMAG 243) 7. Uarium
8. Chromium
9. lircoria (ALSIMAG 508) 10. Tantalum
11.96% Alumina (ALSIMAG 6614) 12. 100% Alumina
13.85% ,A,lumina (ALSIMAG 576) 14. Kovar
15. Molybdenum 1 6. Tungsten
17. Zi-con (ALSIMAG 475) 18. Silica
200 400
DEGUSSIT oxide
600 Temperature
AL23, Sintered resistant resistant resistant resistant
~.---
800 1ooo"C
Fig. 4.1: Percentage elQngation for vacuum materials [19J. Note that Kovar is an alternative designation for Dilver P.
aluminium
From Figure 4.1 it can be seen, that the metal alloy Kovar has a percentage elongation very close to that of 100% alumina, especially for temperatures of up to 700°C. Note should be taken of the fact, that Kovar has so far been generally referred to as Dilver P, the name that it is currently
trading under, and therefore the author shall return to using this designation for that particular type of steel throughout the remaining sections of this chapter. Figure 4.1 also shows that copper for instance is least suitable for joining to ceramics. This has been verified by Hensley [34], who produced experimental copper-to-ceramic seals using the brazing alloy CH4, which, although resulting in mechanically strong bonds formed between the copper and the ceramic, failed to be vacuum tight. Similarly poor results were obtained for nickel and monel, and were also attributed to the stresses formed in the joints. For the purpose of clarity, it is important to note, that actual stresses do not occur in the active braze joint, but in the ceramic adjoining it. Therefore the actual limiting factor in joining metals to ceramic is not the active solder alloy itself, but rather the ceramic. From the above considerations Dilver P can therefore be considered as the ideal metal to be joined to ceramic to form stress free and vacuum tight joints.
4.5. THERMAL SHOCK
Another characteristic related to the thermal expansion coefficient of ceramics is that of thermal shock. Thermal shock refers to the temperature range that a ceramic component can withstand without rupture. The Schott-Winkelmann equation for the sudden cooling of a body is a measure of thermal shock [34][44]:
~T=-
O"g:
·-w
Ea
gChwhere ~T
=
the tolerable temperature range ceC);0"
=
the tensile strength cPa);E =
the elastic modulus (Pa);a =
the expansion coefficient (K \ h=
the thermal conductivity (W m-I KI); g=
the density (kg m-3);cil
=
the specific heat (J kg-I0 c-\
W
=
a shape factor dependent on the geometry of the part;(4.1)
In the case of an X-ray tube thermal shock has to be viewed in a serious light because of the heat developments during its operation, which can be considerable. It is therefore important to establish what temperature range a particular metal-to-ceramic seal can withstand without becoming leaky or undergoing complete rupture. Because ceramics generally tend to be stronger in compression than in tension [19], the effects of thermal shock become noticeable during the cooling down of a joint after having undergone heating. In the next section the joining of Dilver P discs to a ceramic tube with an outer diameter of 30 mm and an inner diameter of 25 mm will be described. The cooling curve profile that the heating cycle will have to follow once the joint has been brazed at a temperature of 950°C will be of special importance if rupture of the ceramic due to an excessively rapid cooling rate is to be avoided.