DE2731665A1 - Semiconducting refractory oxide(s) - with controlled electrical conductivity, for use in electrically heated glass melting furnaces - Google Patents

Abstract

A refractory material with controlled conductivity consists of a lattice of a metallic and a non-metallic element doped with an element whose atomic radius is similar to that of the atoms forming the lattice so that it diffuses into the lattice and is not precipitated at the grain boundaries. The concn. of the dopant does not significantly exceed its solubility in the lattice. The refractory is in general free from fluxing elements from gps. I and II. The lattice consists of pure oxides of one of the elements from gps. IV, V or VI. The dopant has a different valency from the matrix element. A pref. matrix element is Cr and the pref. dopants are Ti, Zr,Hf, V, Nb, Ta, Mo, W, Ru,Os, Ir, Si, Ge or Sn. Used as cladding or electrode material in electrically heated glass melting furnaces.

Description

Refractory materials and processes for their

Manufacturing Description The invention relates to on refractory materials, on electrically heated furnaces that contain such materials included and on processes for the manufacture of refractory materials.

For lining the internal heating chambers of furnaces with which materials refractory materials are used. In many cases you have to The refractory materials have very specific properties in order to reactivity to withstand the materials to be heated and / or the furnace atmosphere. So there there are problems, for example, in electric furnaces used for melting glass, because the glass acts as a flux on many types of refractories and because the refractory materials, which are resistant to the flux property have a greater electrical conductivity than the molten glass. Similar There are problems with other types of furnaces, so that in many cases there is a need for refractories It consists of materials that have a higher electrical resistance than already known have refractory materials. There are also refractory materials that act as contact electrodes can be used for the molten glass.

In these cases it is necessary to check the electrical conductivity of the Refractory material to increase the electrodes in order to extend their life. There are also other uses where there is an increase or decrease the electrical conductivity of the refractory materials used Benefit can be drawn.

According to the invention, the electrical conductivity of a refractory Material can be increased or decreased without this having a noticeable effect the ability of the refractory material to withstand the attack of the substances, to which the refractory material is exposed. So is the conductivity, for example of pure chromium (III) oxide at the melting temperature of glass higher than that of the molten glass. It was found, surprisingly, that the lattice structure of the Chromium (III) oxide existing refractory material can be modified so that the chromium (III) oxide is essential greater electrical resistance gets, reducing the power consumption or the loss of electrical energy of a electrically heated glass melting furnace can be reduced.

This is done by diffusing a dopant into the lattice structure of essentially pure chromium (III) oxide particles with a reduction in the amount on electron carriers in the lattice structure, which are available for the transport of electricity stand, achieved.

According to the invention it has been found that the refractory materials behave as insulators at room temperature, at the elevated operating temperatures of the furnaces in which they are installed show noticeable semiconductor properties.

When using a dopant that has an atomic radius, which is approximately as large as the atomic radius of the metal atoms in the crystal lattice of the Particles of the refractory material, the dopant can enter the grid diffuse without wiping at the grain boundaries when the particles are deposited the purity of the lattice structure is controlled so that it is only what is desired Contains dopants, and considering the presence of fluxes on the Excludes grain boundaries, a body is produced that can operate at elevated temperatures has changed electrical conductivity. One found e.g. during sintering Mixture of essentially pure chromium (III) oxide particles and a small one Amount of titanium oxide that the electrical resistivity of the thus obtained refractory material is noticeably increased, so that electric glass melting furnace now are possible for commercial use. It was found that the amount of to be used Dopant means the amount contained in the crystal lattice of the particles of the refractory Material diffuses, must not significantly exceed, and that this amount in the generally the solubility of the dopant in the particles of the refractory Material at the operating temperature.

Most of those known to the inventor in the prior art Refractories have been called sintering aids, sometimes shrinking agents or flux is used to either lower the sintering temperature or the To increase the density of the refractory material produced. It has now been found that these sintering aids not only are they harmful, but also the synergistic effects taking place in accordance with the invention Prevent effect. Impurities therefore have with respect to those used according to the invention Materials have a decisive influence, and the amount of undesirable impurities must be kept within very narrow limits.

Since refractories have been found to be semiconductors at their elevated operating temperature, the following theory proved useful. The electrical conductivity of the p-type semiconductors is due to defects in the metal atoms of the lattice, and in the case of the refractory material made of chromium (III) oxide, these defects are negative, triple-ionized chromium vacancies, which are denoted in the following with #### are designated. Since oxygen is the other lattice material, the oxygen in the lattice is controlled by the crystal-vapor equilibrium, whereby 0 means oxygen on an oxygen site and h represents 0 positive holes.

The equilibrium constant of the above equation is therefore given by where pO2 is the partial pressure of oxygen, p is the concentration of positive holes and the concentration of defects.

For constant pO2, K = [####] 2 p6 applies. If the pure Cr O3 grid is doped by a foreign atom with a higher valence than Cr, e.g. by T., the condition for electrical neutrality is given by n + 3 [####] = p + 2 [###] + [###], where n is the concentration of electrons, [###] the concentration of the positive double ionized oxygen vacancies which determine the concentration of the positively charged dopant located on a Cr site means Ti.

Using Brower's approximation method for the electrical neutrality condition, the following modified equation is obtained: 3 [####] = p + [###], since K '= [####] 6 p6, consequently It can be seen immediately that with increasing concentration of the dopant, [Ti ##], p must become smaller in order to keep K 'constant. Since the electrical conductivity is directly proportional to p, the electrical conductivity of the p-type Cr 2 O 3 refractory material decreases when dopants such as Ti ++++, which have a higher valence than the base metal Cr +++, are used.

What has been said for titanium also happens when using any other Materials with a higher valence than Cr +++, provided that these materials have an atomic radius sufficiently similar to the atomic radius of chromium is so that the material in the chromium (III) oxide Diffuse grids can. What is more, the theory presented above applies to any semiconducting material p-type.

In the case of chromium, the following materials can accordingly be used as dopants -to increase the specific resistance of the refractory made of chromium oxide Materials used: titanium, zirconium, liafnium, vanadium, niobium, tantalum, molybdenum, Tungsten, ruthenium, osmium, iridium, silicon, germanium and tin. Here are silicon, Germanium and tin elements of group IV, titanium, zirconium and hafnium elements of the Group IVa, vanadium, niobium and tantalum elements of group Va, molybdenum and tungsten Group VIa elements and ruthenium, osmium and iridium group VIII elements.

In the event that one wishes, the conductivity of a refractory To increase p-type materials, one applies the following theory. The condition of the electrical neutrality is fulfilled if n + 3 [####] + [Mg ##] = p + 2 [V ##], where n is the concentration of electrons, rMgCr] is the concentration of the on Cr sites negatively charged dopant Mg and [V ##] the concentration of the positive double ionized oxygen vacancies.

Using Brower's approximation method, we get: consequently This equation shows that p, the number of positive holes 1 must increase with increasing concentration of the dopant in order to keep K 'constant. The electrical conductivity of all p-type refractories is therefore increased by adding dopants of a lower valence than the grid metal.

The electrical conduction in n-type refractories occurs basically by means of ionic defects in the oxygen or in the negative element of the refractory grid. The main ionic effect is therefore referred to as V0. The following equation applies to the crystal equilibrium: where e 'represents electrons.

The following applies to the equilibrium constant of the above equation: K = V ##] n2 P02 1/2, where n is the concentration of electrons, [V ##) the concentration of the positive double ionized oxygen vacancies.

Accordingly, K '= [V ##] n² applies to constant PO2.

Let X be the positive element (valence +3) of the lattice and let Mg the dopant. The condition of electrical neutrality is fulfilled if: n + 3 [Vx '' '] + Mgx'] = p + 2 [VO ++].

Brower's approximation method gives: This equation shows that as the concentration of the dopant increases, n must become smaller in order to keep K 'constant. For refractory materials of the n-type, therefore, the electrical conductivity is reduced by the addition of dopants, such as Mg, which have a lower valence than the metal of the lattice of the refractory material.

Materials that have a higher valence than the metal "X" of the refractory material, such as titanium, are referred to below as Ti ++++. When doping refractory materials of the n-type with the materials Ti just described, the following equation applies to the condition of electrical neutrality: n + 3 [Vx '''] = p + 2 [VO ++] + [Tix +] According to the approximation method from Brower we get: consequently This equation shows that as the concentration of the dopant increases, n must decrease in order to keep K 'constant. In the case of refractory materials of the n-type, the electrical conductivity therefore increases by adding dopants such as Ti with a higher valence than the metal of the lattice of the refractory material.

According to the invention it was confirmed that refractory materials, at least qualitatively, actually in agreement with the one presented above Theory behave and that it is possible to determine the electrical conductivity of refractories Increase or decrease both p-type and n-type materials. The following Examples are intended to provide a more detailed explanation of this surprising finding.

Description of the Preferred Embodiments Example 1 This Example shows what improvement with a small amount of the dopant can be achieved provided that the grid of refractory material in the is essentially pure.

Powders with a particle size of approximately one v4m become one Mixture produced, the mixture of 99.5% of a Cr 203 material with a 99.9% purity and 0.5% titanium dioxide, also with a purity of 99.9%. After thorough mixing of the powder, pour the powder into toluene a slip is produced using so much toluene that the toluene is approximately Makes up 20 percent by volume of the mixture. This material is made from fired in a mold Plaster cast (2.5 cm x 15.2 cm) and first at room temperature, then in a drying cabinet at 1490 C dried. The material is then placed in a kiln and the kiln is slowly heated up over a period of 24 hours, until it has reached a temperature of 16000 C. The kiln is open for 24 hours kept this temperature. The sample is then allowed to cool in the oven, whereupon it is cut into test cubes 2.54 cm on a side. Each other Opposite cube faces are dusted with J-alloy platinum powder. J-alloy foil sheets are then placed on top of the powder and individual platinum wires welded to the corresponding J-alloy foils. The test specimen is then placed in a furnace and the temperature increased until the J alloy melts.

Then the specimen is removed from the oven and the excess Foil is cut from the (2.54cm) 2. measuring end faces of the cube. In the middle between the foil layers will be a hole of laterally through the cube 6.4 mm drilled. A thermocouple is inserted into the hole. Extension wires made of platinum wire are connected to the two leads made of platinum wire. The test specimen is placed in an oven and conductivity measurements are made using a Wheatstone bridge at different temperatures by means of the thermocouple recorded. The conductivity of the iron is shown in table 1 next to the sample name "Example 1".

For comparison, the procedure described above was repeated, only that a test piece was made which was made from essentially pure Cr203 powder was made without the addition of 0.5% dopant. The electrical conductivity this pure sample is indicated under the sample designation "pure Cr203".

Example 2 The procedure of Example 1 is repeated except that instead of the dopant Ti02 0.5% MgO powder with a purity of 99.9% be used. The electric Conductivity of this specimen is given in Table 1 under the sample designation "Example 2".

Example 3 The procedure of Example 1 was repeated except that the powder mixture used to make the specimen, 2.7 percent by weight Contains TiO2. The decrease in the electrical conductivity of this specimen is approximately three times the reduction achieved by using the 0.5% TiO is achieved as a dopant in Example 1 2.

Example 4 The procedure of Example 1 is repeated except that the powder mixture contains 5.2 percent by weight TiO2. The electrical conductivity this material is associated with an increase in resistance that approximates is five times the increase in resistance obtained in Example 1 by adding 0.5% TiO2 is caused as a dopant.

Example 5 The procedure of Example 1 is repeated except that the powder mixture 5.0 percent by weight Nb205 with a purity of 99.9% and 95.0 Contains percent by weight Cr203 with a purity of 99.9%. Dds specimen this Example has approximately the same electrical conductivity as the material of Example 4.

Example 6 The procedure of Example 5 is repeated except that 5 percent by weight Ta205 can be used. This material indicates an increase electrical conductivity corresponding to that of the material 2 of Example 5.

Example 7 The procedure of Example 1 is repeated except that the powder mixture of 96 percent by weight SnO2 with a purity of 99.9% and consists of 4 percent by weight of Nb205.

The electrical conductivity is approximately more than 10% greater than the conductivity of a known material with the following formulation: 97 Sn02, 1% CuO, 1% U03 and 1% Sb203.

Table 1 Electrical conductivity of the refractory bricks (Ohm l Meter 1) Sample 12040C 126ooC 13160C 13710C 14270C Pure Cr203 5.26 5.41 5.78 6.25 6.90 Example 1 1.84 2.56 3.55 4.54 5.81 Example 2 13.51 13.33 13.16 12.99 12.82 E-glass 0.36 0.66 1.14 1.89 2.60 Fig. 1 represents the isometric view of a electrically heated glass melting furnace, its temperature by means of current flow is maintained by the molten glass.

Fig. 2 is an incomplete cross-section through the masonry approximately along line 2-2 of FIG. 1. In Fig. 2, numeral 10 denotes a layer of stones of the composition given in Example 4. These stones are approximate 15 cm thick. Numeral 12 denotes a support layer made from Harbison Walker Superduty firebricks with a thickness of approximately 30.5 cm. The electrodes 14 are made of one material with the composition that is in Example 7 was written formed from bars made by slip casting. These rods extend up through annular insulators 16 made of Harbison-Walker corundum were produced by means of Schliker casting. The chrome masonry has a slightly smaller one Conductivity than the E-glass that is melted in the furnace and the conductivity the electrode is noticeably higher than that of the known electrodes used so far, which essentially consist of pure SnO2.

Fig. 3 of the drawings shows a refractory chrome brick that the described above, the difference being that the Dopant was only applied to the inner surface of the stone before the brana, so that only the inner surface of the stone that is in contact with the molten glass comes, has the higher electrical resistance. This stone can be used in place of the prominent one can be used.

It can now be seen that the principles of the invention to increase or used to reduce the conductivity of refractory materials can without losing the resistance of these materials to attack to change other materials. It is further seen that the principles of the invention are not limited to refractories containing oxide grids, but rather that they can also be used on refractory materials made of carbide, nitride, sulphide, of boride, silicate, etc. for use in atmospheres made with such materials are compatible, can apply.

Examples of other refractory materials are: SiC, A1P, AlAs, GaP, AlSb, GaAs, InP, GaSb, InAs, InSb, Alumina-Zircon, etc.

For use as a dopant to increase conductivity of chromium (III) oxide, the following materials are suitable: lithium, magnesium, manganese, Iron, cobalt, nickel, copper and zinc. Lithium. Is an element of the group copper a Element of group Ia, zinc an element of group IIa, magnesium a Group II element, manganese a Group VIIa element, and iron, cobalt and nickel are Group VIII elements.

The dopants should generally be used in amounts which are the limit of their solubility in the lattice of the refractory material do not exceed the operating temperature. The effective amounts are usually between 0.5 percent by weight and 5.5 percent by weight, based on the weight of the refractory material.

In summary, according to the invention, refractory materials created that have special semiconductor properties at elevated temperatures. The electrical conductivity of the refractory materials is determined by varying the Defects of the metal or non-metal ions of the lattice changed and the purity is controlled to eliminate the effect of flux at the grain boundaries. Man can according to the invention with any semiconducting, refractory materials the electrical Increase or decrease conductivity, as the theory applies to both materials in which defects occur in the metal ions of the grid, as well as on such Materials in which defects occur in the non-metal ions is applicable. L. e e r e i t e

Claims (16)

Claims 1. Refractory material with controlled conductivity, consisting of refractory particles with a crystal lattice made of a metallic and a non-metallic element, characterized in that the grid is the particles described above containing a dopant, wherein the dopant has an atomic radius which is sufficiently close to the atomic radius of the lattice forming metallic element, so that the dopant diffuses into the lattice without to deposit at the grain boundaries between the particles that the amount of dopant its solubility in the grid does not significantly exceed that of the refractory Material at the grain boundaries is generally free of elements that act as flux of Groups I and II, and that the particles form a refractory body are sintered together, as well as a granular sintered body from the above bcachreebenen refractory material. 2. Refractory material according to claim 1, characterized in that that the particles forming the lattice are essentially pure and oxidic Have crystal lattices from an element of groups IV, V or VI and that the lattice of the above particles a dopant mpt one of which the lattice forming metallic element of different valence contains .. 3. Refractory material according to claim 2, characterized in that that the metallic lattice element is chromium and that the dopant is made of titanium, Zircon, hafnium, vanadium, niobium, tantalum, molybdenum, tungsten, ruthen, osmium, iridium, Silicon, germanium or tin. 4. Refractory material according to claim 2, characterized in that that the metallic grid element is chromium and that the dopant is titanium consists. 5. Refractory material according to claim 4, characterized in that that the titanium in an amount of 0.5 percent to approximately 5 percent by weight, based on the weight of the refractory material. 6. Refractory material according to claim 2, characterized in that that the metallic lattice element is zirconium and that the dopant consists of ruthenium, Lithium, cobalt, sodium, stage, calcium, platinum, magnesium, palladium, scandium, Nickel, yttrium, copper, chromium, zinc, manganese, cadmium, iron or indium. 7. Refractory material according to claim 2, characterized in that that the metallic lattice element is zirconium and that the dopant is made of manganese consists. 8. Refractory material according to claim 7, characterized in that that the manganese in an amount of 0.2 percent by weight to approximately 5.9 percent by weight, based on the weight of the refractory material. 9. Refractory material according to claim 2, characterized in that that the metallic lattice element is tin and that the dopant is vanadium, Niobium, tantalum, chromium, molybdenum, tungsten, bismuth or uranium. 10. Refractory material according to claim 2, characterized in that that the metallic lattice element is tin and that the dopant is made of niobium consists. 11. Refractory material according to claim 10, characterized in that that the niobium in an amount of 0.3 percent by weight to approximately 5.5 percent by weight, based on the weight of the refractory material1. 12, refractory material according to claim 2, characterized in that that the metallic grid element is chrome and that the dopant consists of tantalum. 13. Refractory material according to claim 12, characterized in that that the tantalum in an amount of 0.45 percent by weight to approximately 5 percent by weight, based on the weight of the refractory material. 14. Refractory material according to claim 2, characterized in that that the metallic lattice element is chromium and that the dopant is made of niobium consists. 15. Refractory material according to claim 14, characterized in that that the niobium in an amount of 0.25 percent by weight to approximately 5 percent by weight, based on the weight of the refractory material. 16. Application of the refractory material according to claims 1 to 15 at high temperatures, in particular as a lining material or as an electrode material in electrically heated glass melting furnaces, the material being converted into a semiconductor passes from the n- or p-type and as a result of the addition of a dopant the electrical The conductivity of the crystal lattice is effectively changed by adding the dopant. leads to holes in the lattice in the case of an n-type semiconductor, while in the case of a semiconductor of the comp-type, free electrons are produced.