GB2106142A - Sintering refractory articles using direct-heated gases - Google Patents

Abstract

A shaped green body of refractory material is fired by feeding an oxygen free gas preheated to at least 1500 DEG C into direct contact with the bodies in the furnace. SiC may be fired with preheated argon or nitrogen or SiC+Si is fired in nitrogen to form Si3Nu bonded SiC. The gas may be preheated by an electric arc in the furnace wall.

Description

SPECIFICATION A sintering process for refractory articles using direct-heated gases The present invention relates to a sintering process for making shaped refractory articles by using preheated oxygen-free, noble or inert gases (in some instances nitrogen); and in particular, it relates to making silicon carbide (SiC) and silicon nitride (Si3N4) bonded refractory articles. If a silicon carbide sintered product is desired and all the silicon in the green body is in the form of silicon carbide, then any oxygen-free gas (inert), including nitrogen, can be used to heat the green body to the sintering or firing temperature.

If silicon nitride or silicon nitride bonded products are desired then elemental silicon must be included in the green body and heated nitrogen must be used to convert the silicon to silicon nitride.

Silicon carbide has several physical and chemical properties which make it an excellent material for high temperature structural uses.

Because of its high thermal conductivity, silicon carbide can reduce fuel costs and is an excellent material for muffle type furnaces, gas-turbine engines and retorts in the carbothermic production and distillation of zinc. Silicon carbide is also used in electrical resistance elements, ceramic tiles, boilers, around tapping holes, in heat treating, annealing and forging furnaces, in gas producers, and in other places where strength at high temperatures, shock resistance and slag resistance are required. Other properties associated with silicon carbide are supperior strength, refractoriness, corrosion resistance, abrasion resistance, thermal shock resistance, and high specific gravity.

Silicon nitride has some advantages over silicon carbide, such as a lower thermal expansion and higher fracture toughness. Other properties associated with silicon nitride are high thermalshock resistance, high thermal conductivity, strength at high temperatures, and corrosion resistance.

Most ceramic or refractory articles are formed by combining fine powders of a refractory material with binders at low temperatures, then sintering this formed green body at high temperatures.

Refractory articles are usually formed by conventional procedures such as dry pressing, airhammering, or vibrating (jolting). These formed "green" (unsintered) bodies are then sintered at high temperatures (over 1 0000C) to develop desirable physical and chemical properties such as high strength, low porosity, or low chemical reactivity.

In practice, many ceramic or refractory materials such as those consisting of alumina and silica are heated in kilns which are fired by fossil fuels and air or oxygen. If the ceramic material can be exposed to air and/or the products of combustion, then the kiln may be directly fired, in which case the heating and utilisation of energy may be reasonably efficient. However, for certain ceramic materials, including the carbides, the firing must be done in the absence of oxygen or oxygenbearing gases, such as water and carbon dioxide, to prevent formation of oxides, which may have inferior physical or chemical properties. Under such conditions, fossil fuel-fired furnaces may be used but the ceramic parts must be kept in a controlled environment isolated from the combustion products of the fuel.Because the shaped green bodies must be heated indirectly, the heating is inefficient and slow. On a commercial scale such a process, using a tunnel kiln, for example, requires about 84 hours (including the cooling cycle).

Electric kilns are also used to sinter ceramic or refractory materials, but also tend to be energy inefficient and slow. In the case of a kiln equipped with graphite electrodes, the voltage can be controlled and the kiln can be heated to fairly high temperatures, yet there are several disadvantages: 1) The graphite electrodes have a limited size and must be kept under a strictly controlled atmosphere to maintain a long life, and 2) Furnace size is limited and it is difficult to achieve a uniform temperature in this type of kiln because only the graphite electrodes are the source of the radiant heat. Because of this radiant heat transfer as well as a size limit for graphite electrodes, the kiln has a limited productivity and poor energy efficiency.

Several patents reveal slow bonding times for silicon carbide or silicon nitride-bonded refractories. U.S. Patent 3,206,318 teaches a process which is representative of the prior art.

Specifically, it teaches the bonding or nitriding of particulate silicon and silicon carbide refractory materials, by placing the green ceramic body in a nitrogen atmosphere within a muffle furnace and then heating the furnace contents to 1300-1 4200C, whereby the silicon reacts with the nitrogen to form silicon nitride bonds. The examples reveal that the entire process (including the cooling cycle) requires about 1 6 hours.

U.S. Patent 4,127,630 discloses a process for nitriding a refractory article formed from an elemental silicon powder. Example 1 describes the use of a double-walled, gas tight silicon carbide box, into which the green body is placed. The box is then flooded with nitrogen and placed within an electric furnace whereby the contents of the box are incrementally heated to 14500 C. The example reveals that the total heating cycle is 63 hours.

U.S. Patent 3,222,438 relates to a 1 9-20 hour process for nitriding a formed silicon refractory article, and U.S. Patent 3,926,857 reveals a 20 hour bonding process for silicon, carbon, and nitrogen reacting to form silicon carbide and silicon nitride. U.S. Patent 2,618,538 teaches the use of a fluoride catalyst to speed up the nitriding reaction between silicon and nitrogen to form silicon nitride.

Thus, prior art processes for forming oxygenfree bonded refractory articles, in general, require tedious techniques for providing an oxygen-free atmosphere, have a low productivity, are time consuming and energy inefficient. Another problem associated specifically with nitriding silicon arises when the silicon nitride forms a layer on the silicon material: this layer is fairly impervious to nitrogen. Thus, a longer reaction time is required for conversion of the silicon to silicon nitride bonds.

The present invention provides a solution to several of the aforementioned problems associated with the prior processes for producing oxygen-free bonded refractory articles.

Specifically, in accordance with the present invention, a shaped green body of refractory material is sintered or fired comprising the steps of: a) forming a shaped green body of a particulate refractory material by conventional means; b) placing the shaped green body in a furnace that can be flushed free of oxygen or oxygen bearing gases by introducing an oxygen-free, inert, or noble gas (including nitrogen); c) preheating the oxygen-free or noble gas to at least 1 5000C and preferably higher, and d) introducing the preheated gas directly into the furnace containing the shaped green body, causing direct heat transfer from the preheated gas to the shaped green body, for a minimum time necessary to complete a bonding reaction.

More specifically, in accordance with the present invention, a green refractory body is formed from silicon carbide particles, placed in a furnace which can be flushed free of oxygenbearing gases, and subjected to argon or inert plasma gases preheated to greater than 40000 C, resulting in a faster bonding time than that required by prior art processes. The resulting temperature of the green refractory body caused by direct heat transfer from the preheated gases is around 1900--22000C, which is below the melting point of silicon carbide.

Also in accordance with the present invention, a shaped green body of admixed particulate refractory and elemental silicon is formed, placed in an oxygen-free atmosphere furnace, and exposed to preheated nitrogen gas (at 1 5000C and preferably higher and up to 20,0000 C). This effects a faster nitriding reaction to form silicon nitride bonds than that required by previous prior art processes because of the nitrogen gases increased reactivity and direct heat transfer. The resulting temperature of the green refractory body during nitriding is 1000-1 9000C, which is below the melting point of silicon nitride.

Preheating the oxygen-free gas to 1 5000C or higher is preferably achieved by using an electric arc and more preferably, a plasma arc fired directly into the kiln. Electric arc or plasma arc fired gases differ greatly from ordinary furnace heated gases in that they contain electrically charged particles capable of transferring electricity and heat, and become ionized; or as in the case of nitrogen become dissociated and highly reactive. These phenomena greatly increase the reaction rates for bonding refractory materials. Nitrogen, for example, which dissociates at around 57000C and 1 atmosphere pressure, would no dissociate under the normal furnace heating conditions of around 1 4500C required for a silicon nitriding reaction.

Even if a furnace could reach the high dissociation temperature of nitrogen, it would be undesirable for the refractory green body to be at this high temperature because the silicon nitride bonds would decompose at 1 9000C. Thus, the "plasma" gas can be superheated to effect ionization or dissociation, while the refractory green body can be directly heated by the preheated gas to a much lower temperature. Nitrogen gas dissociates into a highly reactive mixture of N2-molecules, N-atoms, N±ions and electrons. Argon ionizes rather than dissociates when used with a plasma arc.

Another important difference resulting from the direct heating of a refractory green body by preheated gases is the quality of the product.

Refractory products which are fired by a rapid heating rate and a short soaking time have good mechanical strength, density and alkali-resistance.

However, a short-reaction time tends to produce a matrix of soft crystals, whereas a long-reaction time tends to produce a hard-crystal structure.

Densities tend to be the same using either slow or fast heating rates. Quality of the product must be considered when selecting a fast or slow heating rate for a refractory bonding process.

Accordingly, it is an object of the present invention to provide a sintering process for producing a bonded refractory article by preheating an oxygen-free, noble, or inert gas and contacting a formed particulate refractory body with it.

It is another object of the present invention to reduce the reaction time for the refractory green body to form bonds because of the higher reactivity of the preheated gas and direct heat transfer to the refractory green body; thereby providing higher furnace productivity, and minimizing capital and operating costs.

Still another object of the present invention is to provide a precision method for controlling the atmosphere surrounding the refractory green body during the bonding reaction, whereby each refractory article may be exposed to identical, reproducible conditions including heating rate, dwell time within the furnace, composition of gaseous environment and heating temperatures.

Other objects and further scope of applicability of the present invention will become apparent from the detailed description to follow, taken in conjunction with the accompanying drawings in which: Fig. 1 is a schematic illustration of a prior art indirect heating process for producing bonded refractory articles in a conventional fuel-fired ceramic kiln, and Fig. 2 is a schematic illustration of an embodiment of the present invention wherein the gaseous atmosphere surrounding and in intimate contact with the refractory green body is directly heated by an electric arc.

At the outset, the process of the present invention is described in its broadest overall aspects with a more detailed description following. The present invention is a process for sintering or firing ceramic or refractory articles, particularly non-oxide materials such as silicon and silicon carbide, using gases electrically preheated to at least 1 5000 C, and preferably higher, for efficient and rapid sintering. Particular applications are in the manufacture of self-bonded silicon carbide, silicon nitride-bonded silicon carbide and silicon-bonded refractory articles.

When the gas is preheated to at least 1 5000 C, and preferably higher, the gas causes direct heat transfer to the refractory green body, thus effecting bond formation. In the case of elemental silicon exposed to preheated nitrogen gas, a silicon nitride bond is formed.

It has been found that by using an electric arc, and more preferably, a plasma arc, gases become ionized or dissociated, making them highly reactive, thus increasing the bonding reaction rate.

Plasma arc systems which can fire directly into a kiln can be fitted to conventional periodic (batch) kilns as shown in Fig. 2, or continuous kilns. Thus, the process of the present invention may be operated as either a batch process or as a continuous process.

The shaped green refractory body, which is treated in accordance with the present invention, is formed from powders of refractory materials in a conventional manner. The furnace used in accordance with the present invention may be specifically constructed for the purpose of the present invention, or as noted above, may be any conventional furnace including batch and continuous type furnaces, modified by using electric-arc or plasma-arc devices instead of fuel burners or electrodes.

The gases employed in the process of the present invention should be completely free of oxygen, water, carbon dioxide, or other oxygenbearing gases to prevent oxidation of the refractory product. To effect the bonding reaction, the oxygen-free gas is electrically heated to a high temperature which may vary anywhere from 1500--20,0000C.

The green refractory bodies are directly contacted with the preheated oxygen-free gases, for a sufficient time to heat the green articles to bonding temperatures (usually within the range of 1000--20000C) and to complete the bonding reaction. It has been found that the use of nitrogen gas heated to about 3000"C will bring green bricks of silicon and silicon carbide powders up to nitriding temperatures (1000-1 6000 C) in two to eight hours, depending on the design of the furnace; and the use of argon or nitrogen gas heated to above 40000C will bring green bricks of silicon carbide powders up to bonding temperatures (1 900-22000C) in the same time period.

EXAMPLE 1 Powdered silicon carbide is admixed with particulate elemental silicon and a carbonaceous binder and pressformed into green bricks in a conventional manner. One hundred pounds of such green bricks are placed within an insulated retort (batch kiln) such as is illustrated in Fig. 2.

Electric-arc heaters, operating at 3.75 kilowatts, are provided in each of the inlet ports located at the bottom of the insulated retort. Nitrogen gas is introduced through the retort inlet ports, passing through the electric arcs and thereby heated to about 2000"C. The preheated, reactive nitrogen gas is allowed to circulate through the green bricks for eight hours to bring the green bricks up to nitriding temperature at 12000 C, which is below the melting point of silicon (14500C), and for an additional period of time to complete the transformation of the elemental silicon into silicon nitride (thereby forming the desired silicon nitride bonds). The overall thermal efficiency is calculated to be 67 percent.

EXAMPLE 2 Powdered silicon carbide admixed with particulate elemental silicon and a phenolic resin binder is formed into green refractory bricks in a conventional manner. Plasma-fired nitrogen gas at about 30000C is introduced into the furnace, heating the refractory bricks to a temperature of 1400--1600"C, which is above the melting point of silicon (1 4500C). A phenolic resin instead of a conventional carbonaceous binder is used to effect formation of beta silicon carbide by reaction of the silicon carbide powder with carbon, present as graphite inclusions and in the phenolic resin. It is believed that beta silicon carbide increases the refractory's alkali resistance, without impairing the refractory's mechanical or physical properties.

Nitriding is determined to be completed within one hour. The total cycle time, including cooling, is about eight hours.

Although the invention has been described with reference to these preferred embodiments, other embodiments can achieve the same results.

Variations and modifications of the present invention will be obvious to those skilled in the art and it is intended to cover in the appended claims all such modifications and equivalents.

Claims (14)

1. A process for firing a shaped green body of refractory material comprising the steps of: a. forming a shaped green body of refractory material, b. placing the shaped green body in a furnace that is flushed free of oxygen or oxygen-bearing gases, c. preheating an oxygen-free gas to at least 1 5000C, and d. introducing the preheated gas directly into the furnace containing the shaped green body, causing direct heat transfer from the preheated gas to the shaped green body, for a minimum time necessary to complete a bonding reaction.

2. A process in accordance with claim 1 wherein the gas in step d is nitrogen.

3. A process in accordance with claim 1 wherein the gas in step d is argon.

4. A process in accordance with claim 1 wherein the particulate refractory material is alpha silicon carbide.

5. A process in accordance with claim 4 wherein the temperature of the preheated gas is greater than 40000C and the resulting temperature of the green body caused by direct heat transfer from the preheated gas is 1900--22000C.

6. A process for making a silicon-nitride bonded shaped refractory material, comprising the steps of: a. forming a shaped green body of an admixture of a particulate refractory material and elemental silicon, b. placing the shaped green body in a furnace flushed free of oxygen or oxygen-bearing gases, c. preheating nitrogen gas to at least 1 5000 C, and d. introducing the preheated nitrogen gas into the furnace whereby the preheated nitrogen gas causes direct heat transfer and, additionally, a reaction with the silicon in the shaped green body to form silicon nitride.

7. A process in accordance with claim 6 wherein said particulate refractory material is silicon carbide.

8. A process in accordance with claim 7 wherein said silicon carbide is alpha silicon carbide.

9. A process in accordance with claim 9 wherein said silicon carbide is beta silicon carbide.

1 0. A process in accordance with claim 6 wherein the temperatures of the preheated nitrogen gas is 1500--20,0000C and the resulting temperature of the green body caused by direct heat transfer from the preheated nitrogen gas is 1000-1 9000C.

11. A process in accordance with claim 6 wherein said particulate refractory material contains residual carbon in the form of a binder, which reacts with the heated elemental silicon to form beta silicon carbide.

12. A process in accordance with claim 6 wherein the gas in step d is electrically preheated by an electric arc.

13. A process in accordance with claim 12 wherein the electric arc is a plasma arc.

14. A process for firing a shaped green body of refractory material substantially as herein described with reference to Example 1 or Example 2.

1 5. A fired shaped body of refractory material produced by the process according to any one of the preceding claims.