Abstract:
A ceramic material is provided containing 45 to 55% by weight TiB 2  and 40.5 to 54.7% by weight BN and 0.1-1.5% by weight Ca in the form of a compound selected from the group consisting of the calcium oxides and calcium borates and mixtures thereof, which material contains from 0.2 to 3% by weight of a compound selected from the group consisting of the oxides, carbides and nitrides of the elements Al, Si, Zr, Ti and mixtures thereof.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    (1) Field Of The Invention  
           [0002]    The invention relates to a material for aging-resistant ceramic vaporizers.  
           [0003]    The most widely used process for coating flexible substrates with metals, in particular with aluminum, is high-vacuum tape coating. In a metalization installation, the substrate to be coated is passed over a cooled roller and, as it does so, is exposed to the aluminum vapor, which is deposited on the substrate surface as a thin metal layer.  
           [0004]    To generate the constant vapor stream required, ceramic vaporizers are heated to about 1450° C. by direct passage of electric current. Aluminum wire is continuously fed in, is liquefied on the ceramic surface and is vaporized at a vacuum of approximately 10 −4  mbar.  
           [0005]    In metalization installations, a series of vaporizers are arranged in such a way that a uniformly thick aluminum layer is deposited over the entire width of the tape. The vaporizers are held by cooled copper clamping jaws, both end-side and lateral clamping being possible.  
           [0006]    The width and length of the vaporizer are selected according to the vaporization rate required (g Al min −1 ). The usual cross section of vaporizers is rectangular. As well as vaporizers with a rectangular cross section, a range of other cross sections are also known. Details of these are to be found, for example, in U.S. Pat. No. 4,089,643 or DE 19823908 (corresponds to the U.S. Application bearing Ser. No. 09/315,320).  
           [0007]    The resistance R of the vaporizer in the electric circuit can be calculated from the geometry of the vaporizer and the electrical properties of the ceramic material (characterized by the resistivity R spec ) in accordance with equation (1).  
           
         R=R 
         spec 
         ×L/A 
       
           [0008]    L: Length of the vaporizer  
           [0009]    A: conductive cross section  
           [0010]    In the electric circuit, the vaporizer behaves as a metallic resistor, i.e. its resistance increases as the temperature rises. Therefore, it is additionally necessary to know the ratio between resistance at operating temperature (R HT ) and the resistance at room temperature (R RT ) in order to accurately characterize the electrical properties. For ceramic vaporizers, the ratio R HT /R RT  is between 2.3 and 6.0.  
           [0011]    A distinction is drawn between 2-component vaporizers (predominantly comprising TiB 2  as conductive phase and BN as insulator) and 3-component vaporizers (predominantly comprising TiB 2  as conductive phase and a mixture of BN and AlN as insulator), according to the number of principal components in the composition of the vaporizers. In the 3-component vaporizer, AlN replaces some of the BN for economic reasons, since AlN is considerably less expensive. Therefore, in general the 3-component vaporizer contains 15-20% AlN. Such compositions are described, inter alia, in patents (e.g. U.S. Pat. No. 3,915,900, column 1, lines 10 and 11 and U.S. Pat. No. 4,089,643, column 2, line 5).  
           [0012]    Since 2-component vaporizer materials have a lower high-temperature bending strength than 3-component vaporizer materials, they generally also contain 0.1-5% by weight CaO.  
           [0013]    After production, 2-component vaporizers are generally packaged in bags made from metalized film, in order to protect the vaporizers from taking up moisture during transport or storage prior to use. This is because it has been found that 2-component vaporizer material is damaged in a characteristic way by the uptake of moisture. The damage becomes apparent from a strongly rising resistance during heating of the vaporizer, which takes place by the direct passage of current in metalization installations. In practice, this means that a vaporizer made from the damaged material cannot be heated in metalization installations, since its resistance is too high for conventional power supplies (10-15 volts secondary).  
           [0014]    Despite this drawback, 2-component vaporizers are frequently used, since they can be operated at higher temperatures and can therefore reach vaporisation rates which are up to approximately +50% higher than 3-component vaporizers. Furthermore, they have a higher service life than 3-component vaporizers.  
         SUMMARY OF THE INVENTION  
         [0015]    The object of the invention is to provide a ceramic material which is suitable for the production of 2-component vaporizers and is better able to withstand atmospheric humidity and is therefore more resistant to aging than known materials for 2-component vaporizers.  
           [0016]    The object is achieved by a ceramic material containing 45 to 55% by weight TiB 2  and 40.5 to 54.7% by weight BN and 0.1-1.5% by weight Ca in the form of a compound selected from the group consisting of the calcium oxides and calcium berates and mixtures thereof, which material contains from 0.2 to 3% by weight of a compound selected from the group consisting of the oxides, carbides and nitrides of the elements Al, Si, Zr, Ti and mixtures thereof. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 depicts the resistance curves as a function of time with a linear increase in power before and after storage for a conventional vaporizer.  
         [0018]    [0018]FIG. 2 depicts the resistance curves as a function of time with a linear increase in power before and after storage for a vaporizer of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]    The ceramic material preferably contains 0.2 to 1.5% by weight of a compound selected from the group consisting of the oxides, carbides and nitrides of the elements Al, Si, Zr, Ti and mixtures thereof.  
         [0020]    The ceramic material particularly preferably contains 0.2 to 0.5% by weight of a compound selected from the group consisting of the oxides, carbides and nitrides of the elements Al, Si, Zr, Ti and mixtures thereof.  
         [0021]    The compound selected from the group consisting of the oxides, carbides and nitrides of the elements Al, Si, Zr, Ti and mixtures thereof is preferably a compound selected from the group consisting of the nitrides of the elements Al, Si, Zr, Ti and mixtures thereof.  
         [0022]    The compound selected from the group consisting of the oxides, carbides and nitrides of the elements Al, Si, Zr, Ti and mixtures thereof is particularly preferably AlN.  
         [0023]    Particularly preferably, the material according to the invention consists of the above mentioned components.  
         [0024]    The vaporizer material according to the invention does not differ from conventional 2-component vaporizer material in terms of its properties which are relevant for use (wetting with liquid aluminum, vaporisation rate at operating temperature, service life), but it does not present any time-dependent change in the materials properties. In particular, the resistivity does not change in the way which is known for 2-component vaporizer material.  
         [0025]    The material according to the invention can be produced in the known way for conventional 2-component vaporizer materials. By way of example, production can take place by hot-pressing of homogenous powder mixtures in graphite molds using ram pressure, temperatures of at least 1800° C. being required in order to achieve solid bodies with sufficient density (U.S. Pat. No. 4,528,120, column 1, lines 36-47).  
         [0026]    The compounds selected from the group consisting of the oxides, carbides and nitrides of the elements Al, Si, Zr, Ti and mixtures thereof are introduced in finely distributed form into a conventional TiB 2 /BN/CaO mixture. This preferably takes place as early as during mixing of the components mentioned first.  
         [0027]    TiB 2  and BN are preferably used in the form of fine powders, as is known in the prior art (for example U.S. Pat. No. 4,528,120, column 5, table above Example 1).  
         [0028]    The compounds selected from the group consisting of the oxides, carbides and nitrides of the elements Al, Si, Zr, pi and mixtures thereof are preferably used as powders with a mean particle size of from 5 to 15 μm, preferably from 8 to 12 μm.  
         [0029]    Such powders are commercially available, for example under the names SiC Grade UF-10, TiC High Vacuum 250, ZrC Grade A, AlN Grade A and Grade B, Si 3 N 4  Grade B7 from H. C. Starck (Goslar). They can also be produced in a manner known per se from commercially available powders. For example, the AlN which is particularly preferably used can also be produced by nitriding of aluminum powders and subsequent comminution in, for example, ball mills.  
         [0030]    The material according to the invention is particularly suitable for the production of vaporizers which are used to coat flexible substrates with metals. The production of the vaporizers from a block of the material according to the invention takes place as is known in the prior art. This may, for example, be carried out by sawing out the desired shape.  
         [0031]    The invention thus also relates to vaporizers which consist of the ceramic material according to the invention.  
         [0032]    The following example serves to explain the invention further:  
       EXAMPLES  
     Example 1  
       [0033]    Production and comparison of the resistance to aging of a vaporizer according to the invention (EV) and a comparative 2-component vaporizer (2K-VV) according to the prior art  
         [0034]    Homogenous powder mixtures of the following composition were produced:  
         [0035]    2K-VV: 48% TiB 2 , 51% BN, 1% CaO  
         [0036]    EV: 48% TiB 2 , 49.5% BN, 1% CaO, 1/5% AlN  
         [0037]    These homogenous powder mixtures were each compressed by axial, two-sided hot pressing in graphite molds at temperatures of &gt;1800° C. and a pressure of &gt;25 MPa, to form sintered bodies with a density of &gt;95% of the theoretical density. In each case two vaporizers were machined out of the hot-pressed sintered bodies by means of band saws.  
         [0038]    The vaporizers were rectangular, with a vaporizer geometry of 10×20×120 mm, which is customary for test purposes.  
         [0039]    In each case one vaporizer according to the invention and one conventional vaporizer were heated by a linear increase in the power to a level of 4 kW. This power is necessary in order to reach the working temperature which is typical for vaporizers with the selected vaporizer geometry and material composition.  
         [0040]    The other two vaporizers were initially stored for 12 hours in a desiccator at 100% atmospheric humidity before being heated as described above.  
         [0041]    The results are shown in FIGS. 1 and 2, which illustrate the resistance curve as a function of time with a linear increase in power to a level of 4 kW.  
         [0042]    [0042]FIG. 1 shows the results for a conventional vaporizer (VV).  
         [0043]    Curve  1  shows the measured curve of the resistance for the vaporizer which is heated immediately. The increase in resistance which is typical of undamaged material can be seen. The high-temperature resistance which is established for the selected power of 4 kW is approx. 2800 moc (μ Ohm cm).  
         [0044]    Curve  2  shows the measured curve of the resistance for the material after prior storage for 12 hours in a desiccator under 100% atmospheric humidity at 20° C. It is clear that, as the power is increased, the resistance rises rapidly, reaching levels of &gt;10,000 moc. A vaporizer of this type will fail in a conventional tape coating installation and has to be replaced.  
         [0045]    [0045]FIG. 2 shows the results with the vaporizers according to the invention (EV).  
         [0046]    Curve  1  shows the measured curve of the resistance for the vaporizer which is heated immediately. The increase in resistance which is typical of undamaged material can be seen. The high-temperature resistance which is established for the selected cower of 4 kW is approx. (μ Ohm cm).  
         [0047]    Curve  2  shows the measured curve of the resistance for the material after prior storage for 12 hours in a desiccator under 100% atmospheric humidity at 20° C. It is clear that after storage there is no damage corresponding to that which can be seen in FIG. 1.  
         [0048]    Further tests (results not shown) have demonstrated that even extending the storage time to a few days does not cause any damage. The effect described is even achieved with lower additive levels of 0.3-0.5%.  
         [0049]    Examples for vaporizer material containing ≦0.5% AlN. The vaporizers EV 2 to EV 6 were produced in a similar manner to that described in Example 1.  
                                                                           High-temp.   High-temp.           AlN   TiB 2     BN   CaO   resistance   resistance       Sin-   content   content   content   content   before   after       tered   [% by   [% by   [% by   [% by   storage   storage       body   weight]   weight]   weight]   weight]   [moc]   [moc]                   EV 2   0.4   47.7   50.9   1.0   2725   2730       EV 3   0.3   48.7   50.0   1.0   2470   2480       EV 4   0.4   47.2   51.4   1.0   2830   2840       EV 5   0.5   46.4   52.1   1.0   3380   3300       EV 6   0.3   48.5   50.2   1.0   2290   2300                  
 
       Example 2  
       [0050]    Vaporisation rate and service life of a vaporizer according to the invention compared to known vaporizers  
         [0051]    A vaporizer according to the invention (EV1) and a commercially available 3-component vaporizer (3K-VV) of the same dimensions (commercially available under the name ESK-Trimet® from ESK, Kempten) and the 2-component vaporizer in accordance with Example 1 (2K-VV) were operated in the test metalization installation under standard conditions (P&lt;10 −3  mbar; end-side clamping) using the power specified in Table 1. The measured vaporisation rate and service life are given in Table 1.  
                                                                         Power   Vaporisation               [KW]   rate [g/mincm 2 ]   Service life [h]                                    EV   4   0.35   10       2K-VV   4   0.35   10       3K-VV   3.2   0.25    9 ← economic operation           4   0.35    6           2.4   0.2   10