WO1989001921A1

WO1989001921A1 - Thermal ceramics and uses therefor

Info

Publication number: WO1989001921A1
Application number: PCT/AU1988/000336
Authority: WO
Inventors: Andrew Savva
Original assignee: Andrew Savva
Priority date: 1987-08-31
Filing date: 1988-08-31
Publication date: 1989-03-09
Also published as: ZA886466B

Abstract

A ceramic composition comprising clay, feldspar, grog, silica talc and whiting or alternatively a composition comprising ball clay, flint clay, kaolin, fine grog and carbon black for use as a thermal ceramic material in an oven. An oven for heating or cooking having a base (10), a top (12), two substantially parallel sides (14), a rear (16), a front (18), a heating means (26) inside the oven, and a door (36) in the front (18); the base (10), top (12), sides (14), and rear (16) have an outer skin (20) and an inner lining (24), the inner lining (24) being of a thermal ceramic material and being backed with a layer (22) of a ceramic insulating material, the thermal ceramic material (24) being capable of relatively rapidly absorbing and storing heat without damage to itself and being capable of radiating the stored heat.

Description

THERMAL CERAMICS AND USES THEREFOR

The present invention relates to ceramics having advantageous thermal properties and to uses of such ceramics.

A requirement exists for a ceramic material which is capable of withstanding sudden temperature changes without cracking, and which stores absorbed heat for radiation to the surrounding environment. Materials of this kind would be particularly useful in high temperature applications such as, for example, the interior of kilns or ovens.

We have surprisingly found that a combination of clay, silica, feldspar, talc, grog and whiting forms a ceramic

(hereinafter referred to as a "thermal ceramic") which satisfies the above requirements. In contrast, previously known ceramics are reasonable thermal conductors, fai] to absorb appreciable amounts of heat sufficiently quickly and/or fail to radiate stored heat effectively.

According to one aspect of the present invention, there is provided a thermal ceramic formed from clay, silica, feldspar, talc, grog and whiting. The thermal ceramic may additionally contain diatomite, olivine and aluminium. Preferably, the amounts of the various components comprising the thermal ceramic, expressed as a percentage of total weight of the solids content of the ceramic are as follows:

and Carbon Black 5% - 10% More preferably, the thermal ceramic comprises: Clay: 30% - 50%

Silica: 15% - 30%

Feldspar: 10% Talc: 5% - 15%

Grog: 9% - 11%

Whiting: 4% - 8%.

In a yet further preferred embodiment, the thermal ceramic additionally contains 0.1% - 8% diatomite; 0.1% - 8% olivine and 0.1% - 8% aluminium.

In a further preferred embodiment, the thermal ceramic additionally contains 4% - 8% diatomite; 4% - 8% olivine and 4% - 8% aluminium.

In another aspect of the present invention there is provided an oven for heating or cooking and which comprises a base, a top, two substantially parallel sides, a front and a rear, and a means for heating the interior of said oven, said f ont having at leasta portion thereof openable by a door; each of said base, top, sides and rear having an outer skin and an inner lining, said inner lining being of a thermal ceramic material and beingbacked with a layer of a ceramic insulating material; said thermal ceramic material being capable of relatively rapidly absorbing and storing heat and without damage to said thermal ceramic material, and being capable of radiating said stored heat.

A further aspect of the present invention relates to a method of heating or cooking in the oven comprising raising the temperature of the inner lining of said thermal ceramic material to a predetermined temperature by means of said heating means, removing the energy supply from said heating means, placing an item to be heated or cooked into said oven through said door and closing said door, and allowing said item to be heated or cooked by the radiated heat from said thermal ceramic material of said oven. In order that the invention may be readily understood and put into practical effect, there shall now be described by way of non-limitative example only preferred embodiments of the present invention, the description being with reference to the accompanying illustrative drawings. In the drawings:

Figure 1 is a front perspective view of an oven incorporating the principal features of the present invention;

Figure 2 is a vertical cross-section along the lines and in the direction of arrows 2-2 of Figure 1;

Figure 3 is a view corresponding to Figure 2 but of a different embodiment; and

Figure 4 is a graph of radiant heat vs time for the ovens of Figures 1 to 3. The oven of Figures 1 and 2 comprises a base 10, top 12, two parallel sides 14, rear 16, and a front 18. Each of the rear 16, side walls 14, base 10 and top 12 comprises an outer skin 20 of metal, plastics or any other suitable material, an intermediate insulating layer 22 preferably of a stable insulator such as, for example, ceramic fibre, and an inner layer 24 of a thermal ceramic. A heating element 26 is provided which, in this instance, is an electric element. Racks 28 are provided on the inner surface of the inner layer 24 of the two sides 14 so as to hold any required shelves or the like (not shown), the rear 16 has a second skin 21 to cover the connections to the element 26.

The front 18 comprises a panel portion 30 the same as the remainder of the oven and in which is located an ON/OFF switch 32 which may incorporate an indicator light, and a temperature control34. The remainder of front 18 is a door 36. The door 36 is hinged to one of the side walls 14. A catch of any suitable type may be used to releasably retain the door in the closed position. A handle 38 is provided. The door 36 may be made of two spaced apart layers of a transparent or semi- transparent material, and one which preferably has relatively good insulating characteristics; or of the same construction as the remainder of the oven.

In Figure 3 there is shown a variation where the electric element 26 of Figures 1 and 2 is replaced by a gas burner 42 located in the rear wall 16.

The thermal ceramic of which the panels 24 are comprised may be formed by mixing the various components with water to form a suitable blend which is cast into a desired form and fired under normal conditions for ceramic formation. Suitable firing conditions generally employ temperatures in excess of 1000°C, preferably between 1050 to 1100°C for approximately 1 to 4 hrs. These conditions for ceramic formation are not limiting however, as the skilled person will readily appreciate.

The properties of the thermal ceramic are believed to arise from a synergistic interaction between its various constituents. The advantageous properties of heat storage (i.e. a large heat capacity) and release could not have been predicted from the properties of the various constituents which form the ceramic.

Studies have indicated that the thermal ceramic is extremely porous. It is believed that the advantageous properties of the thermal ceramic stem from its extreme porosity. In particular, large amounts of air trapped within the pores of the ceramic may be gradually heated, this allowing large amounts of heat to be stored within the ceramic. This trapped heat may be slowly released from the ceramic in the form of infra-red (heat) radiation. The thermal ceramic is non-brittle and is relatively strong when compared with known porous ceramics such as earthenware. These features are most desirable, as robust, durable ceramics are in considerable demand. The thermal ceramic may be formed into bricks, sheets, or other articles.

The thermal ceramic is preferably used in conjunction with an insulating layer. For example, where a thermal ceramic forms the interior of an oven, an insulating layer,which in a preferred form comprises a ceramic fibre, is applied to the external surface of the ceramic to assst in heat retention.

The thermal ceramic may also be used in low temperature applications, such as interior of refrigerators and refrigerated rooms. The unique heat exchange properties of the thermal ceramic make.it ideally suitable for such applications.

The thermal ceramic in one form is formed from the follow¬ ing components expressed as a percentage of total solids content:

For each 1000 kg. of the above material, there is added 360 1 of water and 4 1 of sodium silicate. The material is mixed together, until well combined, and then allowed to set for a period of approximately 24 hrs. whereafter the mixture is again mixed. After a second mixing, the material is deposited into a suitably shaped mould and fired at 1075°C for 1 hr.

Alternatively, the components of the thermal ceramic may be blended together, and then mixed with water to form a plastic dough. This dough may be extruded, pressed into moulds, or shaped by other appropriate means to form suitably shaped components. The material may then be fired at 1250°C for 1 hr. to form the thermal ceramic. A ceramic formed in this manner is highly porous, and is capable of absorbing large 5 amounts of heat, and of radiating such heat in the form of infra-red (heat) radiation over a substantial time period, which may be as long as 12 hrs. after heating of the thermal ceramic.

Analysis of the thermal ceramic produced above indicates 10 that it has the following formula:

0.4 K₂0 3.6 A1₂0₃

0.3 CaO 10 SiO

0.3 MgO.

■_jc To cook foods using the oven of Figures 1 and 2, the switch 32 is turned to the ON position and temperature control 34 rotated to select the desired temperature. This supplies electricity to the element 26 which in turn heats up. This heats up the interior of the oven. The thermal ceramic 0 panels 24 then absorb this heat. Once the desired temperature is reached an indicator light is switched ON. The door 36 is then opened and the food placed inside the oven. The door 36 is then closed. The panels 24 radiate heat, with the rate of radiation being demonstrated by the graph of Figure 4. This 5 cooks the food.

As the surface area of the panels 24 is great, the heat cooks the food from all sides at once, thus providing a far more even heat and far more even cooking. Also, the temperature differential from the panels 24 to the foods is 0 not as great as from the heating element in a normal oven to the food therein and thus there is a far less chance of the food being burnt. Also, as the heat output reduces the food is merely kept warm. In this way, a meal may be cooked well in advance and kept warm. As the oven is sealed, there is very little heat loss and no loss of moisture, thus allowing the food to stay relatively fresh for a considerable time.

Claims

CLAIMS:The claims defining the invention are as follows:

1. An oven for heating or cooking and which comprises a base, a top, two substantially parallel sides, a front, a rear, and a heating means located inside said oven; said front having at least a part thereof operable by a door; each of said base, top, sides and rear having an outer skin and an inner lining, said inner lining being of a thermal ceramic material and being backed with a layer of ceramic insulating material; said thermal ceramic material being capable of relatively rapidly absorbingand storing heat without damage to said thermal ceramic material, and being capable of radiating said stored heat.

2. An oven as claimed in claim 1, wherein said front has a panel portion also has said outer skin and said inner lining of said thermal ceramic material backed with said layer of ceramic insulating material.

3. An oven as claimed in claim 1 or claim 2, wherein said door has said outer skin and said inner lining of said thermal ceramic material backed with said layer of ceramic insulating material.

4. A thermal ceramic material for use in the oven of any one of claims 1 to 3, wherein said thermal ceramic material, expressed as a percentage of total weight of the solids content, comprises:

Clay: 15% - 60%

Silica: 10% - 50%

Feldspar: 5% - 15%

Talc: 2% - 20%

Grog: 5% - 15% and Whiting: 2% - 10%

5. A thermal ceramic material for use in the oven of any one of claims 1 to 3, wherein said thermal ceramic material, expressed as a percentage of total weight of the solids content, comprises:

Ball Clay 40%

Kaoun 10%

Flint Clay 10% Fine Grog 30% and Carbon Black 5% - 10%.

6. A thermal ceramic material as claimed in claim 4 or claim 5_. wherein there is in addition:

0.1% - 8% Diatomite 0.1% - 8% Olivine and 0.1% - 8% Aluminium.

7. A thermal ceramic material as claimed in claim 4 or claim 5 , wherein thre is in addition:

4% - 8% Diatomite 4% - 8% Olivine and 4% - 8% Aluminium.

8. A .method of heating or cooking using said oven as defined in claims 1 to 3, comprising supplying energy to said heating means so as to raise the temperature of said inner lining of said thermal ceramic material to a predetermined temperature, discontinuing said energy supply, opening said door and placing the item or items to be heated or cooked inside said oven, closing said door, and allowing said item or items to be heated orcooked by the radiated heat f om said thermal ceramic material.

9. An oven substantially as hereinbefore described with reference to the accompanying drawings.

10. A thermal ceramic material substantially as hereinbefore described.

11. A method of heating or cooking substantially as hereinbefore described with reference to the accompanying drawings.