Glass-ceramic materials have been used widely in various applications. For example, glass-ceramic cooktop plates and cooking utensils, such as bowls, dinner plates, and the like, are used widely in modern kitchens. Transparent glass-ceramic materials have been used in the production of stove and/or furnace windows, optical elements, mirror substrates and the like. Glass-ceramic materials are typically made from ceramming their precursor glass materials at elevated temperatures for specified periods of time. Two categories of glass-ceramic materials based on SiO2—Al2O3—Li2O glass system are those comprising β-quartz solid solution as the predominant crystalline phase and those comprising β-spodumene solid solution as the predominant crystalline phase. These two categories of glass-ceramic material can be produced from the same precursor glass material. Cooktop plates made of both types of glass-ceramic materials are available commercially.
One example of β-spodumene glass-ceramic material-based cooktop plate is Kerawhite® available from Eurokera. This plate has the advantages of relatively low coefficient of thermal expansion from about room temperature to about 700° C. and an appealing, clean, milky-white color. It has been accepted by a large volume of consumers in different markets.
After the commercial success of Kerawhite®, it was perceived that a more economical glass-ceramic material yet capable of being melted at a lower temperature would be desirable. The material may be translucent. For cooktops to be used with induction cooking, there is a special interest in opaque glass-ceramic material which can make the inductive heating elements invisible to the eyes.
It is known that when melting the precursor glass of any glass-ceramic material, fining agents are usually required in order to reduce the seed count in the glass. Common fining agents used include As2O3, Sb2O3, and the like. These oxides are batched as As2O5, Sb2O5 or oxidized into As2O5 and Sb2O5 before the glass melt is heated to the fining temperature, when they dissociate to release O2. The released O2 helps to reduce the bubble count in the glass melt. For environmental reasons, it is highly desired that the melting of the precursor glass does not require the use of such toxic fining agents.
It is not always straightforward in finding a replacement for the As2O3 and/or Sb2O3 fining agents when melting a particular glass. Different fining agents have differing fining capacity and usually require differing fining temperature ranges. Using a different fining agent can lead to the risks of requiring increased fining temperature and/or devitrification temperature of the glass melt. Increased devitrification temperature of glass usually means that the glass must be formed or processed at higher temperature in order to avoid devitrification thereof, which is highly undesirable.
Glass-ceramic material has been the subject of research and product development for decades. For example, a relative recent product development involving this material is the lamp reflector substrate used in modern projection display systems where high-power, high temperature, high-intensity discharge lamps are employed. Preferred lamps for projection displays comprise a high intensity arc discharge lamp positioned within a reflective structure to produce a high intensity light beam. Particularly for digital data projectors and digital projection large screen televisions, these lamps require a high temperature stable reflector. WO 2004/094327 discloses a glass-ceramic lamp reflector substrate. However, the glass-ceramic material for use in this product as disclosed in this reference requires the predominant crystalline phase of the material to be β-quartz in order to obtain high dimensional stability and low coefficient of thermal expansion. Moreover, there was no specific example in this reference where the precursor glass of the glass-ceramic material was fined without the use of As2O3 and/or Sb2O3.
It is generally known that if a precursor glass can be cerammed into glass-ceramic materials comprising β-quartz or β-spodumene solid solution as the predominant crystalline phases, the production of the latter usually entails a higher ceramming temperature. This means higher energy consumption, especially where prolonged ceramming cycle at such high temperature is required. Certain commercial products of β-spodumene-based glass-ceramic materials were produced by ceramming at temperatures higher than 1050° C. and for as long as over 100 minutes. Such prolonged, high temperature ceramming requires the use of high temperature-resistant, high-power ceramming kilns for their manufacture. Thus, it is highly desirable that the glass-ceramic material comprising β-spodumene solid solution as the predominant crystalline phase can be produced in a relatively short and/or relatively low temperature ceramming cycle.
There is a genuine need of a glass-ceramic material, advantageously opaque, comprising β-spodumene solid solution as the predominant crystalline phase, that is capable of being melted at a relatively lower melting temperature, such as below 1600° C., and be converted to glass-ceramics by a short ceramming thermal treatment at a relatively low temperature, such as below 1050° C.