DE2236774A1 - CERAMICS RESISTANT TO HEAT SHOCK - Google Patents

Description

Dip !. ⁴ Ing.Heinz Barde We

Patent attorney

D-8 Mönchen 26, PO Box 4 0811/292555

Munich, July 26, 1972

My reference: P 1475

Applicant:

Morgan Refractories Limited Liverpool Road, Neston Wirral, Cheshire England

Ceramic resistant to thermal shock

The invention relates to zirconium resistant to thermal shock. Alumina ceramics.

In the manufacture of such ceramics, the zirconium and the aluminum oxide react during firing to form zirconium dioxide and mullite according to the following equation:

2 ZrSiO, + 3 Al ₀ O ₀ ----- ^ 2 ZrO ₀ + AlcSi ₀ O ₁₀ .

H · Z O Z DZ IJ

It has now been found that excellent ceramics with a

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Versatility and good resistance to thermal shock can be obtained by mixing silicon carbide incorporated from zirconium and aluminum oxide.

The invention accordingly provides a ceramic which is resistant to thermal shock and which is characterized in that it Contains silicon carbide in a binding matrix of zirconium dioxide and mulite.

The new ceramic according to the invention has a high load-bearing strength or resistance to stress, a high level of resistance against thermal shock and high wear and corrosion resistance, especially under alkaline conditions. It is useful, for example, in the field of Ferrous and non-ferrous liquid metals for finishing: (das Accessories) of push ovens and for incinerators, in which a wear resistance be ε on: · (-; ι. ". erwüiaschi: is. The operating temperature 'hu, ". -fgt in the. usual atmospheres up to about 1600 C.

The band matrix protects the silicon carbide against oxidation and, for example, against attack by molten steel or slag, at the same time it allows, from the advantageous Properties of silicon carbide »to make use.

It is believed that the high resistance to thermal shock of the new ceramic according to the invention is primarily due to the good thermal conductivity of silicon carbide which prevents large temperature gradients from occurring. The matrix consists essentially of zirconium dioxide and mullite, the zirconium and the aluminum oxide do not have to be present in stoichiometric proportions and other phases can also be present, for example

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BATH ORIGINAL

unreacted zirconium or silicon dioxide that is present in the reaction arises:

ZrSiO ₄ -> ZrO ₂ + ₂

As is common in ceramics, clays, such as pottery or kaolin, can also be present in order to create a product to obtain stretchable (deformable) mixture. An alternative for using clay to achieve a stretchy (Deformable) mixture is, for example, the use of a resin that is burned out during firing, or the use of sufficiently finely divided alumina, for example one with a particle size of about 5 microns or below, which act as plasticizers themselves. When the silicon carbide content is high and not a sufficient amount of clay can be added to achieve a deformable mixture, a resin or other organic plasticizer is used in the cone used.

The matrix preferably represents 20 to 60 or 65% of the ceramic (all proportions are based on weight) and the silicon carbide is 80 to 40 or 35 percent, but it can other proportions are also present. In the mixture, the alumina preferably makes up 10 to 70% of the total of aluminum oxide and zirconium, but other proportions can also exist here.

The silicon carbide is retained in the matrix in the form of a relatively coarse aggregate and its particle size is preferably within such a range that there is a 4 mesh BSS screen (with a mesh size of about

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4 mm), but is retained by a 200 mesh BSS sieve (mesh size 0.075 mm), preferably at least 95% from a 120 mesh BSS sieve (clear mesh size

0.125 ram). With the zirconium and aluminum sieves Oxide types can be such as / for common zirconium-aluminum oxide ceramics be used. Usually the alumina has an order of magnitude finer in grain size than the zircon, for example a particle size of 15 or. 25 microns or below, preferably 10 microns or below, to ensure the desired reaction to form zirconium dioxide and mullite.

Examples of ranges for the composition of the new ceramics according to the invention are given in Table I below:

Table I. Material% of the mixture

Aluminum oxide, for example aluminum oxide ground in a jet mill or another Alumina having a particle size substantially below 10 microns 4-22

Zircon, e.g. wet milled zircon with a particle size of 325 BSS ⁺ 10 - 30

Clay, e.g. powdered kaolin or pottery clay or a mixture of both 6-12

Silicon carbide, e.g., those having a particle size of 8 to 120 B.S.S. mesh 36-80

⁺ [BSS = British Standard Sieve:

8 mesh corresponds to a mesh size of 2.05 mm. 120 mesh "" "" 0.125 mm

325 mesh "" "" about 0.05 mm]

It should be noted that the clay content can also be low, so that the formation of the zirconium dioxide-mullite

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Matrix is not disturbed.

Examples of special compositions are as follows:

example 1

Composition consisting of 9% of the above jet milled aluminum oxide, 26 % of zirconium with a particle size of 325 mesh, 3 % of kaolin and 4% of pottery clay with the remainder being silicon carbide, with a typical final weight analysis. as indicated in the following table II:

56.8 % Table II 0.2 % SiC 17.3 K ₂ O Oj 15 ZrO ₂ 13.3 CaO 0.1 SiO ₂ 11.8 Na ₂ O 0.1 Al ₂ O ₃ 0.2 TiO ₂ 0.05 Fe ₂ O ₃ 2 to 4 MgO Examples

Other examples of specific compositions made from these substances are as follows:

Al ₂ O ₃
Zircon
Sound mix SiC

The new ceramic according to the invention is produced in systems known per se. The starting materials can, for example, be mixed dry, in which case water is added.

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II III IV 9 7th 24 26th 20th 30th 12th 10 6th 53 63 40

ben and uniformly mixed in and the mixture can be pressed or otherwise shaped and fired at a temperature of, for example, 1300 to 1500 C until the desired density is reached. The amount of water used depends on the molding method, it is less when pressing or extruding, greater when casting, as is known to those skilled in the art of making ceramic articles. If the mixture is to be poured, it is preferably incorporated into it a dispersant, for example Calgon (trade name for sodium hexametaphosphate). For example, 8% water 0.1% Calgon, based on the total mixture, was added to the mixture of Example 1 above and subjected to vibro-casting to produce blocks which were then dried and fired at 1400.degree.

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Claims (7)

Claims / Iή Ceramic resistant to thermal shock «, characterized in that it contains silicon carbide in a binding matrix of zirconium dioxide and mullite» 2. Ceramic according to claim 1, characterized in that the silicon carbide 35 to 80% by weight. 3. A method for producing a ceramic according to claim 1 or 2, characterized in that one consists of a particulate mixture consisting of zircon, aluminum oxide and silicon carbide and optionally a clay or another inorganic or organic plasticizer burns, the proportions and particle sizes of the zirconium and the aluminum oxide are such that they are burned react with each other to form a binding and protective matrix for the silicon carbide, which consists of zirconium dioxide and Mullite is made up of. 4. The method according to claim 3, characterized in that the aluminum oxide in the mixture 10 to 70 wt .-% of the total amount of zirconium and aluminum oxide. 5. The method according to claim 3 or 4, characterized in that the mixture 4 to 22 wt .-% aluminum oxide, 10 to 30 wt .-% Zircon, 6 to 12 wt% clay and 36 to 80 wt% silicon carbide contains. 6. The method according to any one of claims 3 to 5, characterized in that that the particle size of the silicon carbide in the 209886/1205 - 8 - 223677A Mixture is such that it passes through a 4 mesh BSS sieve, however retained on a 200 mesh BSS screen. 7. The method according to claim 6, characterized in that the Particle size of the silicon carbide is such that at least 95% will be retained on a 120 mesh BSS screen. 209886/1206