PROCESS FOR THE ENCAPSULATION OF GRAPHITE INTO GLASS AND GLASS CERAMIC ARTICLES, AND ARTICLES SO PRODUCED

Abstract: A process for the manufacture of glass or glass ceramic articles with a graphite encapsulated insert. The encapsulation is made in the presence of one or more gas getter metals chosen from aluminium, chromium, niobium, tantalum, titanium and zirconium. The disclosure relates particularly to cookware articles.

Description: The present invention relates to a process for the encapsulation of graphite into glass or glass ceramic articles. In particular the present invention relates to an improved process for encapsulating at least one piece of graphite within a glass or glass ceramic article, in order, in particular, to improve heat spreading in glass ceramic cookware intended for direct heating on top of stoves, either gas or electric. With this improved process it is possible to make new products, not feasible with the encapsulation processes known until now, such as described, in particular, in the US Patents 4,248,925 and 4,260,405. The above patents describe manufacturing of glass ceramic articles containing a graphite insert, said articles being cerammed around 900°C and intended for cooking on top of stoves. At such ceramming temperatures, usually transparent glass ceramics are obtained, consisting of a stuffed β-quartz solid solution. Nevertheless, it may be desirable to ceram at a higher temperature, around 1100°C to obtain an opaque glass ceramic, with a β-spodumene structure, which is stronger and is a good substrate for decoration. It has now been found that if articles made according to the above patents are heated up to temperatures for opaque ceramming, they break or swell every time. The analysis of this phenomenon has indicated that it occurs when the stuffed β-quartz solid solution structure characteristic of the transparent glass ceramic turns into the β-spodumene structure typical of the opaque glass ceramic. It has been assumed that the stresses responsible for breakage or swelling, according to the viscosity of the material when this phenomenon occurs, are the consequence of a chemical reaction between graphite or impurities contained therein and the oxides constituting the glass, said reaction producing oxides of carbon which build up a pressure of gas in the cavity surrounding the graphite. There is therefore a demand for a new process to manufacture opaque glass ceramic articles with a graphite insert. In addition the graphite inserts described in the aforesaid patents were sliced from extruded or pressed synthetic graphite logs. For easier cutting and lower cost, it would be useful to be able to use graphite foil. But whenever a graphite foil thick enough to spread heat sufficiently is encapsulated according to aforesaid patents, it swells as soon as the pressing plunger is removed. Swelling of the graphite foil is assumed to be a consequence of desorption and thermal expansion of gases included between the parallel layers of flat crystals constituting the graphite foils; by contrast the graphite from logs is nearly isotropic, its pores are randomly oriented, so that the gases can evolve when the insert (usually a disc) is heated, lying on the first gob, before being encapsulated under the second gob. There is therefore also a demand for a new process to manufacture glass or glass ceramic articles with a graphite insert made from a graphite foil. The present invention provides such new improved process. We have discovered that some metals when used in particulate form which are, particularly capable to enter into combination with oxygen or to adsorb gases, resolve both above problems, i.e. opaque ceramming of glass ceramic articles containing a graphite insert, and encapsulation of graphite foils within glass articles, whether or not they are intended to become glass ceramic articles. The present invention provides a process making a glass or glass ceramic article having at least one graphite insert encapsulated therein which includes the step of encapsulating at least one graphite insert in a glass article and, optionally ceramming said glass characterized in that encapsulation takes place in the presence, with the graphite, of one or more gas getter metals in particulate form chosen from aluminum, chromium, niobium, tantalum, titanium, and zirconium. The six metals specified above are all usable according to the invention. Zirconium and tantalum are, however, the most effective, since they operate in the most difficult conditions, i.e. when opaque ceramming is combined with the use of graphite foil. Chromium, niobium, titanium and aluminum are less universally applicable and their use is indicated for less difficult conditions. The effectiveness of the metals in particulate form as gas getters depends on other various factors such as the purity of the graphite; the process employed for the encapsulation; the ceramming temperature, and the proportion of metal in relation to the amount of graphite. When the graphite contains higher amounts of oxidizable elements such as sulfur, iron or calcium, more metal is needed as gas getter. The gas getter metal can be provided in the cavity by various ways during the encapsulation. The metal can be, for example, dispersed, uniformly or not, inside the graphite; it can be spread onto the surface of the insert as a coating; it can be incorporated within a second piece of graphite lying on the main insert during encapsulation. A uniform incorporation of metal within the graphite (either the main insert or a secondary piece) can be made, for example, by carefully mixing metal powder and graphite powder, then pressing the mixture into the desired shape. A non-uniform incorporation can be made, for example, by drilling many holes through the graphite insert, then filling up the holes with metal powder. Coating the graphite insert with metal in particulate form can also be done, for example, by vacuum deposition, with a plasma spray gun, by spreading a metal suspension (by silk screening or pad printing) or other suitable means. The following processes are the most effective, probably because they prevent premature oxidation of the metals: uniform incorporation of metal within graphite (insert or secondary piece) and coating of the insert, either by vacuum deposition or by plasma spraying. The ceramming temperature is also a key factor for the selection of the metal and the amount of metal required. As a consequence of all above, it is not possible to indicate the amount of metal required in each case; the person skilled in the art will have to do some experiments, taking into account the nature of the metal, the way it is used, the nature of the graphite insert, the ceramming schedule (if ceramming is considered), in order to be able to determine the right amount of metal required to obtain a satisfactory encapsulation. For guidance it can be stated that the minimum amount of metal required is approximately the quantity which would coat the whole surface of the insert with 0.5 µm thick layer, for the articles intended only for a transparent ceramming, and a 1 µm thick layer for an opaque ceramming, assuming that the metal layer would be uniform at the whole interface between graphite and glass (even if practically the metal is incorporated within the insert or within a secondary graphite piece). It should be understood that the use of an amount of metal equal to or slightly higher than the amount indicated above for guidance, will not necessarily give a satisfactory result, because of the multiplicity of factors to be considered. The above amounts are only minimums under which no satisfactory or reproducible result can be expected. It has been observed also that chromium is not sufficient for opaque ceramming. For the fuller understanding of the invention, the following Examples are given by way of illustration only:- The accompanying drawings, to which reference is made in the Examples, are as follows:- Fig. 1 represents the section of an article broken during opaque ceramming. The origin of the fracture is located at the periphery of the cavity, where the tensile stress is at a maximum in the case of excessive pressure in the cavity Fig. 2 represents, also in section, an article swollen during opaque ceramming. Swelling can be either non symmetrical, as represented, or symmetrical Fig. 3 represents the section of an article whose graphite foil has swollen just after encapsulation Fig. 4 represents the section of an article whose graphite insert, properly coated or impregnated with a metal as gas getter, stays undamaged, as well as the glass ceramic encapsulating it, and with which it has remained in intimate contact until after ceramming, either transparent or opaque. In the drawing, 1 indicates the graphite insert, and 2 indicates the glass ceramic of the article in which the graphite insert is encapsulated. The examples have been chosen among the most typical experiments, to illustrate the need and the effectiveness of some metals as gas getters. EXAMPLE 1 The graphite insert to be encapsulated is a disc, diameter 184 mm and 1.4 mm thick, cut from an extruded graphite log supplied by Union Carbide France as code SF 18, characterized in particular by a mean thermal expansion coefficient between 25 and 300°C of 3,6 MK⁻¹. This disc is encapsulated in the green glass of the glass ceramic Corning code 9608, according to the process described in the US Patent 4,260,405. The glass article is then heat treated according to the following schedule to be turned into a glass ceramic : heating in 60 min up to 820°C, nucleating for 30 min, heating up to 1100°C at 4 k/min, crystallization hold of 15 min and uncontrolled cooling. The article obtained at the end is broken as represented by Fig. 1. Certain variations of the ceramming temperature schedule would give swollen articles as represented by Fig. 2. EXAMPLE 2 The graphite insert to be encapsulated is a disc, diameter 180 mm, cut from a graphite foil, Papyex R N brand, 1 mm thick, density 1.1 g/cm³ supplied by Le Carbone Lorraine. This disc is encapsulated in the same glass and utilizing the same process as in Example 1. As soon as the pressing plunger is removed, the bottom on the article becomes wavy, because of swelling of the graphite foil, as depicted by Fig. 3. EXAMPLE 3 A graphite disc identical to the disc of Example 1 is first coated, on one face, with a plasma spray gun, with a titanium layer, 9 µm thick. The disc is then encapsulated, and the article is cerammed, as in Example 1. The article obtained at the end is undamaged and its section looks exactly like Fig. 4. This section indicates that the graphite has remained in intimate contact with the glass ceramic, and that there has been no gas release in the cavity. This good contact is proved also by a ball drop test on the bottom of the article. While the article lies on three hemispherical supports, a steel ball weighing 56 g is dropped on the center of the bottom from increasing altitudes, until the article is damaged or broken. When the graphite is bound with the glass ceramic, the bottom behaves mechanically like a continuous composite solid, and fracture is initiated by bending on the lower face, on the face opposite to the impact, when the drop height reaches a certain value, 55 cm in our example. On the contrary, when the graphite was not bound with the glass ceramic, as was the case of Example 1, which looked like Fig. 2. the glass ceramic layer over the graphite broke first, and for a drop height much smaller, 31 cm in average in Example 1, where there was no metal as gas getter. EXAMPLE 4 The graphite insert to be encapsulated is a disc identical to the disc of Example 1. A quantity of 2.7 g of niobium powder (that quantity which is necessary to coat both faces with a 5 µm thick layer) is mixed with 2.5 g of natural graphite powder, and pressed into a pellet diameter 42 mm and 1 mm thick. During encapsulation of the disc, in the same glass and by the same process as in Example 1, the above pellet is set on the graphite disc, before the second gob arrives on the insert. The article is then cerammed like in Example 1 and an undamaged article is obtained, like depicted by Fig. 4. EXAMPLE 5 The graphite insert, to be encapsulated, is a graphite foil impregnated with chromium, made as follows from expanded graphite. Natural graphite powder, code S 18590 from Le Carbone Lorraine, is mixed with a sulfuric solution of potassium dichromate, at the concentration of 10 g of graphite in 90 g of a 2% solution of K₂Cr₂O₇ in concentrated SO₄H₂. After soaking 1 h, the powder is filtered, rinsed with water, and treated at 900°C in a kiln, where it expands up to a density of only 30 g/l. This expanded graphite is mixed with 3.2 g of chromium powder, then pressed at 14 MPa, to be molded as a disc diameter 42 mm, 1 mm thick, with density 1.4 g/cm³. This graphite foil is encapsulated in the green glass of the Corning Glass ceramic code 9607, according to the process described in the US Patent 4,260,405. The glass article which looks like Fig. 4, remains unaltered even after the final heat treatment which turns it into a transparent glass ceramic : heating in 30 min up to 780°C. 30 min nucleation hold, heating up to 900°C at 4 K/min, 15 min crystallization hold and uncontrolled cooling. EXAMPLE 6 The insert to be encapsulated is a disc cut from a graphite foil identical to Example 2. A face of the disc is coated by silk screening with an aluminum powder previously mixed with a squeegee oil supplied as code 63/2 by Blythe Colors Ltd, of Stoke on Trent, U.K. The coating is 20 µm thick in average, expressed as pure metal. The coated disc is dried at 100°C, then encapsulated in the same glass and according to the same process as for Example 5. After the same heat treatment, the final article has also the good appearance depicted by Fig. 4.

Claims: A process of making a glass or glass ceramic article having at least one graphite insert encapsulated therein which includes the step of encapsulating at least one graphite insert in a glass article and, optionally ceramming said glass characterized in that encapsulation takes place in the presence, with the graphite, of one or more gas getter metals in particulate form chosen from aluminum, chromium, niobium, tantalum, titanium and zirconium . A process as claimed in claim 1, characterized in that zirconium or tantalum or a mixture thereof are used as metal added to graphite. A process as claimed in claim 1 or claim 2 characterized in that the gas getter metals in particulate form are dispersed within the graphite insert. A process as claimed in claim 1 or claim 2 characterized in that the gas getter metals in particulate form are applied as coatings at the surface of the graphite insert. A process as claimed in claim 1 or claim 2 characterized in that the gas getter metals in particulate form are associated with a second piece of graphite, added to the main graphite insert. A process as claimed in any of claims 1 to 3 characterized in that the graphite insert is a piece of graphite foil made by rolling or pressing of expanded graphite within which one or several of the aforesaid gas getter metals have been previously mixed. A glass ceramic product containing a fully encapsulated graphite insert, said insert being made either by slicing of a pressed or extruded graphite log or by trimming (or shearing) of graphite foil, said product characterized on the other hand by the fact that the graphite insert is in intimate contact, all over its surface, with the surface of the cavity surrounding it, and on the other hand by the presence of one or several gas getter metals in particulate form selected from aluminum, chromium, niobium, tantalum, titanium and zirconium, either in their elemental state or as oxides or other compounds resulting from reaction with the glass ceramic or the graphite. A product as claimed in claim 7 characterized in that the amount of at least one of the aforesaid gas getter metals in particulate form is at least sufficient to coat all the interface between the graphite insert and the glass or glass ceramic with an uniform layer 0.5 µm thick. A product as claimed in claim 8 characterized in that the amount of at least one of the aforesaid gas getter metals in particulate form is at least sufficient to coat all the interface between the graphite insert and the glass or glass ceramic with an uniform layer 1µm thick. A product as claimed in any of claims 7-9 which is opaque.

Inventors: ANDRIEU, ANDRE, DAVOT, JEAN-PIERRE PAUL, THIBIEROZ, JEAN-PIERRE ALPHONSE MARIE, Andrieu, André
Applicants: CORNING GLASS WORKS
Application year: 1988
Main classification: C04B37
IPC codes: A47J36, B32B17, C03B11, C04B37