COMPOSITE BODY

Abstract: The invention relates to a composite body comprising a first part of ceramic material connected to a second part of metallic material, of ceramic material or of a mixture of metallic and ceramic material by means of a melting ceramic which comprises besides A1₂O₃ either one or more alkaline earth metal oxides or one or more rare earth metal oxides or a combination of these two oxides. According to the invention, the melting ceramic contains a filler material which is distributed over its volume and occupies at least 20% and at most 50% of the volume of the melting ceramic.

Description: The invention relates to a composite body comprising a first part of ceramic material connected to a second part of metallic material, of ceramic material or of a mixture of metallic and ceramic material by means of a melting ceramic which comprises besides Al₂O₃ either one or more alkaline earth metal oxides or one or more rare earth metal oxides or a combination of these two oxides. The term "ceramic material"is to be understood to mean in this description and the appended Claims a material of crystalline oxidic substance which contains at least alumina. This may be, for example, monocrystalline sapphire. Another possibility is densely sintered polycrystalline alumina or yttrium aluminate garnet. These materials, which are suitable to be used as a wall material for discharge vessels of high-pressure discharge lamps, can be heated for a long time at temperatures up to about 1300°C and have at such high temperatures a high resistance to attack by alkali metals and halides. Heating for a long time above 1300°C generally leads to a considerable evaporation of the relevant materials. Heating for a short time to 1750°C to 1800°C is possible, however, without unfavourable consequences. However, heating above 1750°C to 1800°C, even for a short time, leads to the aforementioned evaporation to a very great extent and even to spontaneous formation of ruptures. The term "rare earth metal oxides" is to include the following oxides: Sc₂O₃, Y₂O₃, La₂O₃, Ce₂O₃, Pr₂O₃, Nd₂O₃, Sm₂O₃, Eu₂O₃, Gd₂O₃, Tb₂O₃, Dy₂O₃, Ho₂O₃, Er₂O₃, Tm₂O₃, Yb₂O₃, Lu₂O₃. A melting ceramic means in this description and the appended Claims a connection material having a lower softening temperature than the parts of the composite body, which may be present in the vitreous phase and in the crystalline phase as well as in a combination of these two phases. A composite body of the kind mentioned in the opening paragraph is known from European Patent Application EP-A1-0060582 (PHN 9968 EU). Such a composite body may be a high-pressure discharge lamp provided with a discharge vessel having a wall of ceramic material, through which a current lead-through member is passed, which is connected thereto in a gas-tight manner by means of the melting ceramic. Frequently used high-pressure discharge lamps are high-pressure sodium lamps and high-pressure metal halide lamps. In lamps of both types, in the operating condition the wall of the discharge vessel is subjected to a high temperature and the filling of the discharge vessel is ionized in part. The melting ceramic used in such lamps therefore should be resistant to attack by sodium and metal halides at high temperatures. The gas-tight connection, which provides a high resistance to attack by sodium and halides, can be obtained by means of the known melting ceramic. In general a gas-tight connection is obtained by a fine crystallization of the melting ceramic. It should be noted that in general the thermal coefficients of expansion of the interconnected parts are different so that stresses can occur in the melting ceramic. In case of a fine crystallization, the possibility of crack formation is smaller than in case of coarse crystalline melting ceramic. The term "coarse crystalline" is to be understood herein to mean that 80 % of the crystals have dimensions of 10 µm or more. A melting ceramic is considered to be fine crystalline when the dimensions of 80 % of its crystals lie between 1 and 5 µm. A further advantage of a fine crystalline melting ceramic is that during crystallization process the possibility of occlusions and micro-cracks is considerably smaller than in the case of coarsely crystallized melting ceramic. It has been found that the known melting ceramic, although it is fine crystalline for the major part, nevertheless under given circumstances forms large crystals ( > 20 µm) and moreover can comprise bubbleshaped cavities. The bubble-shaped cavities are due to gas desorption during the processing of the melting ceramic. The presence of large crystals is in practice always associated with micro-cavities. Micro-cavities and bubbles are detrimental to the strength and resistance of the melting ceramic. Another problem arising in practice is that during the sealing-in process β aluminate crystal structures can be formed. The possibility of β aluminate crystal structures being formed is found to increase considerably at higher values of the maximum temperature during the sealing-in process. β aluminate crystal structures are very sensitive to attack by especially sodium and are therefore very undesirable in the case of the use of a melting ceramic in a lamp having as filling constituent sodium or sodium halide. The invention has for its object to provide a means by which the connection between the parts of the composite body remains substantially free from both large crystals and β aluminate crystal structures whilst maintaining the favourable properties of the melting ceramic. According to the invention, a composite body of the kind mentioned in the opening paragraph is for this purpose characterized in that the melting ceramic contains a filler material which is distributed over its volume and occupies at least 20 % and at most 50% of the volume of the melting ceramic. The term "filler material" is to be understood to mean in this description and the appended Claims a constituent of the melting ceramic which is present as a separate phase between the remaining phases of the melting ceramic. The filler material is distributed (preferably substantially homogeneously) over the volume of the melting ceramic. The average particle size of the filler material can be chosen in a wide range slightly dependent upon the dimensions of the space available for the melting ceramic. The size of the particles of the filler material is preferably chosen so that at least 80 % is smaller than 10 µm in the case of dimensions of the available space between 10 µm and 100 µm. Moreover, the size distribution is chosen so that the specific surface area of the filler material is smaller than 60 m²/cm³ on the basis of the theoretical density. It has been found that the occurrence of large crystals in the ceramic material provided with filler material is avoided substantially entirely. It is a surprise to find that the formation of bubble-shaped cavities is also avoided. It is moreover of advantage that the softening temperature remains below 1750°C. It further appears that the possibility of the formation of β aluminate crystal structures is considerably reduced. It has been found that with a content of the filler material smaller than 20 % by volume the formation of gas desorption bubbles and micro-cavities is not counteracted to an acceptable extent. The possibility of formation of β aluminate crystal structures also remains substantially unchanged in these conditions. The filler material also influences the viscosity of the melting ceramic and hence the transport of the melting ceramic in the space to be filled between the two parts of the composite body. With a percentage of filler material of at most 50 % by volume, it appears that under widely different circumstances a good transport of the melting ceramic containing filler material is guaranteed. Higher degrees of filling lead to hindrance to the transport of the melting ceramic, which may result in thixotropic effects, such as inter alia demixing of melting ceramic and filler material. The use of a material as an addition in a melting ceramic is known per se from German Offenlegungsschrift 2,032,277. In this case, the use of additions being dissolved in the melting ceramic is concerned. On the one hand, a rule or an instruction for determining the required quantity of addition cannot be derived from the said Offenlegungsschrift. On the other hand, in fact, with the use of these additions, a new melting ceramic is obtained having its own specific properties with respect to crystallization, resistance to metal vapours, adhesion, etc. Preferably, in a composite body according to the invention, the filler material consists of one or more of the following metal oxides: alumina, zirconium oxide, hafnium oxide, thorium oxide, uranium oxide. These metal oxides have in a wide range of filling percentages no or substantially no tendency to be dissolved in the melting ceramic. Moreover, the addition of these materials as filling material only leads to a small increase of the softening temperature of the melting ceramic. More particularly, a composite body is to be preferred, in which the filler material consists of HfO₂, the quantity X of which satisfies the relation 20 % by volume ≦ X ≦ 40 % by volume. It has been found that a melting ceramic containing 20 to 40 % by volume of HfO₂ as filler material mostly yields optimum connections between the parts of the composite body. Embodiments of composite bodies according to the invention will be described more fully with reference to a drawing. The drawing shows an end of a high-pressure sodium lamp. The lamp is known per se , from, for example, US-A-3,716,743 and will not be described here further. The lamp end shown is a composite body comprising a first part consisting of a tube 1 and a disk 2, both of densely sintered alumina (PCA). A second part consisting of a hollow niobium sleeve 3 is sealed in a gas-tight manner into the disk 2. An electrode 4 consisting of a tungsten pin provided with a tungsten helix is secured by means of titanium solder in the sleeve 3. The sleeve 3 is connected in a gas-tight manner to the disk 2 and the tube 1 by means of a melting ceramic 5 according to the invention. Examples . A large number of discharge vessels for high-pressure sodium lamps of the kind shown in the drawing are provided with a gas-tight connection between the Nb sleeve and the PCA disk with the use of a melting ceramic provided with a filler material. For a good relative comparison, discharge vessels of equal dimensions are used. The most important dimensions are: inner diameter of tube 1 5.1 mm inner diameter of disk 2 2.08 mm outer diameter of sleeve 3 2 mm height of disk 2 3 mm A first group of connections is obtained with the use of a melting ceramic which, apart from filler material, comprises Al₂O₃ and mainly alkaline earth metal oxides, i.e . according to the composition expressed in mol.%: Al₂O₃ 32.9 MgO 10.3 CaO 50.8 BaO 4.2 B₂O₃ 1.8 The melting ceramic is obtained as follows: a mixture of the aforementioned constitutive oxides each having a purity of at least 99.9 % is fused in a high-frequency furnace to a vitreous mass or to a vitreous mass with small crystals. After cooling, this mass is pulverized and then a filler material in the form of crystalline grains is added to the powder thus obtained. After careful mixing, rings of melting ceramic are formed therefrom by pressing. The melting ceramic thus formed proves to contain a small quantity of water varying from 0.04 mol.% to 0.12 mol.%. Table I indicates of a number of examples besides a serial number also the kind and quantity of filler material used. Furthermore, the maximum temperature during the sealing-in process T max is stated. Of the filler material grains added, about 85 % was smaller than 10 µm, while the specific surface area was 25 m²/cm³ for Al₂O₃, 58 m²/cm³ for HfO₂ and 39 m²/cm³ for ZrO2. The connection is obtained in a sealing-in process carried out in the following manner. The parts to be interconnected are first joined in the desired position and are provided at the connection area with a ring of the melting ceramic chosen. This assembly is heated in a furnace from room temperature to T max , which temperature lies between 1250°C and 1550°C, at a rate of about 10°C/s, is held for at most 150 seconds at T max and is then cooled by at least 10°C/s to about 1100°C. This temperature of 1100°C is maintained for at least 110 seconds and for at most 250 seconds, whereupon there is cooled to room temperature by 5°C/s. A second group of examples is obtained by means of a melting ceramic on the basis of Al₂O₃, alkaline earth metal oxide and rare earth metal oxide. The composition in mol.% of the starting ceramic then is Al₂O₃ 35.8 CaO 55.4 SrO 7.0 Y₂O₃ 1.8. The melting ceramic containing a filler material is obtained in the same manner as described for the first group of examples. Of the second group of examples, Table II indicates data about filler material and maximum temperature during the sealing-in process T max . The filler materials Al₂O₃, HfO₂ and ZrO₂ are of the same kind as are used in the first group of examples. Besides, ThO₂ is used as filler material, of which 84 % of the grains was smaller than 5 µm and the specific surface area was 28 m²/cm³. The sealing-in process is analogous to the sealing-in process of the first group of examples, but the range of T max now extends to about 1700°C. As a third group of examples, discharge vessels are formed by means of a melting ceramic which, apart from the filler material, comprises Al₂O₃ and rare earth metal oxides and of which the composition in mol.% is: Al₂O₃ 65.0 Sc₂O₃ 19.25 Y₂O₃ 5.25 La₂O₃ 10.5. The method of preparing the melting ceramic containing filler material is analogous to the method of preparation used in the first group of examples. The sealing-in process for this third group is also analogous to the sealing-in process for the first group, the maximum temperature T max lying within the range of from 1650°C to 1750°C. Table III indicates data about filler material and T max for this third group of examples. The filler materials Al₂O₃, HfO₂ and ZrO₂ are the same as are used in the first and second groups of examples. Moreover, as filler material UO₂ is used of which 84 % of the grains is smaller than 4 µm and the specific surface area is 35 m²/cm³. The discharge vessels obtained were found to have a good gas-tight connection between the Nb sleeve and the disk, while the melting ceramic was found to be fine crystalline with finely divided filler material crystals. In most cases, the gap-shaped space between the PCA disk and the niobium sleeve was entirely filled with melting ceramic, which shows the favourable transporting properties of the melting ceramic. In none of the examples, a β aluminate crystal structure could be found. During the sealing-in process of discharge vessels with the use of a melting ceramic with serial numbers 5, 8, 15, 17 and 18 from the tables, an increased viscosity of the relevant melting ceramic was found, which in a few cases resulted in only a partial filling of the gap-shaped space between the PCA disk and the niobium sleeve. However, thixotropic effects, such as demixing, were not found.

Claims: 1. A composite body comprising a first part of ceramic material connected to a second part of metallic material, of ceramic material or of a mixture of metallic and ceramic material by means of melting ceramic which comprises besides Al₂O₃ either one or more alkaline earth metal oxides or one or more rare earth metal oxides or a combination of these two oxides, characterized in that the melting ceramic contains a filler material which is distributed over its volume and occupies at least 20% and at most 50% of the volume of the melting ceramic which filler material has particles having a size that is for 80% of the particles smaller than 10 µm and having a specific surface area smaller than 60 m²/cm³. 2. A composite body as claimed in Claim 1, characterized in that the filler material consists of one or more of the following metal oxides: alumina zirconium oxide hafnium oxide thorium oxide uranium oxide. 3. A composite body as claimed in Claim 1 or 2, characterized in that the filler material consists of HfO₂, of which the quantity X satisfies the relation 20 % by volume ≦ X ≦ 40 % by volume.

Inventors: OOMEN, JORIS JAN CORNELIS, ROUWENDAL, JAN WILLLEM
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1987
Main classification: C04B37
IPC codes: B23K20, C04B37, H01J61, B23K1