Düşük çinkolu Li2O-ZnO-SiO2 camlarının kristalizasyon davranışı ve cam-seramiklerinin eğme mukavemetlerine P2O5'in etkisi

Abstract

Bu çalışmanın amacı, P205' in ve uygulanan ısıl işlemlerin Li20-ZnO-Sı'02 camlarının kristalizasyon, yeniden kristalizasyon ve eğme mukavemetine etkisini belirlemektir. Bu amaçla, saf başlangıç malzemeleri kullanılarak birbirine yakın üç ayrı cam bileşimi hazırlanmıştır. Bu bileşimler P205 içeriklerine göre, P0,P1,P2 (% 0, 1 ve 2 mol P205) simgeleriyle gösterilmiştir. Hazırlanan camlara çeşitli ısıl işlemler uygulanarak gösterdikleri kristalizasyon davranıştan taramalı elektron mikroskobunda (SEM) incelenmiştir. Diferansiyel Termal Analiz (DTA) çalışmaları temel alınarak herbir grup cam için kristalizasyon sırası ve nihai kristalizasyon ürünleri X-ışınları difraktometresi (XRD) ile tesbit edilmiştir. Aynca, kristalize olmuş camların eğme mukavemeti Instron Universal test cihazı ile belirlenmiş ve sonuçlar P205 içeriğine göre yorumlanmıştır. Çalışmalar sonucunda herbir bileşimde ilk kristalizasyon ürününün U2Sİ2O5 (LS2) fazı olduğu, diğer kristalizasyon ürünlerinin ise, P0 bileşiminde Li2ZnSi04 (LZS) ve Lİ2SİO3 (LS), P1 bileşiminde LZS, P2 bileşiminde ise LZS ve a-kuvars olduğu tesbit edilmiştir. P0 ve P1 cam bileşimindeki cam-seramiklerde sferülitik morfolojinin bozulma sıcaklığının P205 miktarının artmasıyla düştüğü, P2 bileşiminde ise sferülitik morfolojinin oluşmadığı tesbit edilmiştir. Ayrıca P1 ve P2 bileşimlerinin eğme mukavemetlerinin birbirine yakın olması nedeniyle çekirdeklenme katalisti olarak kullanılan P205' in optimum miktarının % 1 mol olması gerektiği sonucuna vanlmıştır.

Glass-ceramics are inorganic materials, generally but not necessarily silicate-based materials, which are initially prepared as glasses and which, in bulk form, are shaped by glass-forming techniqes. They are then processsed furter by suitable heat-treatment to develop, firstly, nuclei in the glass and subsequently crystal phases. In many cases a small proportion of residual glass phase is often present after crystallisation heat-treatment. Investigation into their use in a wide range of technical and engineering aplications is proceeding throughout the world. In certain areas, for example microwave radomes, vacuum envelopes, telescope mirrors and domestic cooker tops and cooking ware, their use is now well established. The extensive range of properties which can be realised with glass-ceramics make them suitable for consideration as substrate materials in applications covering a wide range of frequencies and where either thick or thin film circuitry is required. In many cases, they provide attractive alternatives to other substrate materials, e.g. glasses and ceramics. The good mechanical properties of glass-ceramics combined with their ability to take a very smooth surface finish have enabled them to be used for special purpose bearings. Glass-ceramics are superior to conventional ceramics with regard to the surface finish that can be attained since the best surface finish achievable for a 95 per cent alumina ceramic is 200 to 250 nm whereas glass-ceramics can be polished to surface finishes of 12.5 nm. The high hardness and excellent abrasion resistance of glass-ceramics suggest their use for the construction of pumps, valves and pipes for handling abrasive slurries. In addition, the good chemical durabilities of many glass-ceramicsenable them to be used in contact with corrosive liquids under conditions where many metals would undergo unacceptable deterioration. The high dielectric breakdown strengths and mechanical strengths of glass-ceramics offer advantage for insulators as compared with conventional electrical porcelains because thinner sections can be used resulting in weight savings and greater freedom of design. Glass-ceramics can also be used for sealing to metals. The versatility of glass-ceramics for metal sealing applications derives mainly from the fact that the thermal expansion coefficients can be varied over an extremely wide range enabling matching of the coefficient to practically every metal. It is not possiple to achieve this flexibility with either glasses or conventional ceramics. The glass-ceramic process comprises the preparation of a homogeneaus glass, the shaping of the glass to produce the required article and, finally, the application of a controlled heat-treatment process to convert the glass into a microcrystalline glass- ceramic. Glasses are made by heating together a mixture of raw materials (known as "batch") at a sufficiently high temperature to permit the materials to react with VI one another and to encourage the escape of gas bubbles from the melt; this latter process is referred to as refining the glass. In selecting raw materials the most important aspect to be taken into account is the purity. Economic considerations will also play an important part in the corse of materials. Upon completion of the refining process, the glass is cooled from the melting temperature to the working temperature where the glass has a higer viscosity. Various shaping methods can be applied to the glass to produce articles of the required form. Perhaps the simplest shaping operation avaiable to the glass maker is that of casting. Casting processes are useful for glasses having short working ranges, such as alkali-free glasses. Alternatively the glass-ceramic can be processed via a powder route (e.g. die or isostaticaily pressed, slip cast, tape cast or as a powder coating) and sintered to achieve full density. Further heat-treatment may be necessary to convert the sintered body to glass-ceramic, or the required crystallisation may take place in the one firing schedule. In selecting compositions for glass-ceramic production there are a number of important factors which have to be taken into account. The glass compositions must be capable of being melted and shaped by economic methods so that in the formulation of glasses for glass-ceramic production the influence of composition upon these factors must be born in mind. It is important that the melting temparature of the glass shall not be excessively high and, generally speaking, 1600 °C would be regarded as an upper limit for practical operations. Excessively high temperatures result in problems concerning the glass furnace refractories and for this reason glass compositions which have sufficient fludity to permit them to be melted and refined at temperatures not higer than 1400 to 1500 °C will be chosen wherever possible. Certain glass constituents lower the viscosity of the glass melt and are therefore valuable in speeding up melting and refining (removal of gas bubbles). The alkali metal oxides have this effect and are to be regarded as useful fluxes in the melting process. Lithium oxide has a greater effect in reducing the viscosity of the melt than sodium oxide which, in turn, has a greather effect than potassium oxide, the comparisions being made for equimolecular proportions of the oxides. The alkaline earth oxides also have useful fluxing effects as do zinc oxide and lead oxide. In adition to oxides which have a beneficial effect on melting characteristics, there are those which increase the difficulty of melting by increasing the viscosity of the glass. Alumina is an example of such an oxide but since the presence of this oxide is necessary for certain important types of glass-ceramics, the lower melting and refining rates of glasses containing alumina have to be accepted. The working characteristics of a glass are of great importance since they determine which shaping process other than a simple gravity technique can be used. Many glass shaping process depend on the fact that the glasses are fluid or plastic over a fairly wide temperature range. For many glass-shaping operations a long working range is desirable to give adequate time for the flow or deformation of the hot glass as it cools after removal from the furnace. The presence of alkali metal oxides in the glass in desirable to enhance the working range and glasses which are alkali free, especially those which contain high proportions of calcium or magnesium oxides, tend to have undesirable short working ranges; this is because such glasses have high viscosites with the result that the temparatures at which shaping is carried out are high. Another important aspect of the working characteristics concerns the possibility of devitrification during the cooling of the glass in the shaping operation. Altough the primary object in the production of a glass-ceramic is to devitrify the glass, this process must be carried out in a controlled manner. Uncontrolled crystallisation is not likely to enable high strengeth glass-ceramics of controlled properties to be produced because of the formation of large crystals due to homogenous nucleation during cooling. In VII addition, crystal growth during the shaping operations will adversely effect the working properties since it will be accompained by sharp changes of the viscosity. Also the large crystalls produced may lead to the generation of high stresses which could result in fracture of the glass articles. Glasses containg high proportions of alkali metal oxides tend to devitrify rather readily during working of the glass, and both lithium and magnesium oxides are especially likely to accentuate the tendency of a glass to devitrify. Fortunately, quite small additions of certain oxides tend to suppress devitrificationduring cooling of the glass and these can be used to improve the working characteristics in this respect. Aluminium oxide has a marked effect end even afew per cent will suppress devitrification during shaping of the glass. Zinc oxide has a similar but less marked effect in certain glasses and small additions of boric oxide can also be useful. A particularly useful and interesting observation is that small addition of phosphours pentoxide suppress crystallisation of the glass-ceramic compositions derived from lithia-silica system at temperatures within the glass-working range. This results from the marked effect of P205 in reducing crystal growth rates in this temperature range. The object of the heat-treatment process is to convert the glass into a microcrystaline ceramic having properties superior to these of the orginal glass. Controlled heat treatment consists of two steps; nucleation and crystal growth. The first stage of the process involves heating the glass form room temperature to the nucleation temperature. The optimum nucleation temperature generally seems to lie within the range of temperature corresponding with viscoties of 1011 to 1012 poises. The period of time for which the glass is maintained at the nucleation temperature will usually be form 0.5 to 2 hours, altough longer periods may not have a detrimental effect. Following the nucleation stage, the temperature of the glass is increased at a controlled rate sufficiently slowly to permit crystal growth to occur so that deformation of the glass article will not take place. An obvious change brought about by the heat-treatment is the conversion of the transparent glass to an opaque polycrystaline material. Also, the thermal expansion coefficients of glass-ceramics are generally different from those of the parent glasses. Perhaps the most striking and important change in characteristics which is brought about by the crystallisation heat treatment is the increase of mechanical strength. Generally speaking, the electrical properties of glass-ceramics are superior to those of the parent glasses and in particular the electrical resistivities are higer and the electric losses are lower. Glass-ceramics possesing high mecanical strengths and other desirable properties can be produced from glasses of the lithium zinc silicate type. Suitable nucleation catalysts include metallic phosphates or metals such as cooper, silver or gold. These glasses do not require irradiation in order to sensitise the metallic nucleation catalysts. The weight percentages of the major glass constituents lie in the range: Si02 = 34-41; ZnO=10-59; Li20=2-27 and these constituents should total at least 90 per cent of the glass composition. The nucleation catalysts include phosphorous pentoxide 0.5 to 6 per cent, gold 0.02 to 0.03 per cent, silver computed as AgCI 0.02 to 0.03 per cent or copper computed as Cu20 0.5 to 1 per cent. The main crystallising phases in this system are LS2, LS, LZS and silica depending on the glass composition. Lithium disilicate is an interesting phase since it grows with a spherulitic morphology at relatively low temperatures and recrystallises at higer temperatures giving a more desirable microstructure. Nucleation, crystallisation and vra addition, crystal growth during the shaping operations will adversely effect the working properties since it will be accompained by sharp changes of the viscosity. Also the large crystalls produced may lead to the generation of high stresses which could result in fracture of the glass articles. Glasses containg high proportions of alkali metal oxides tend to devitrify rather readily during working of the glass, and both lithium and magnesium oxides are especially likely to accentuate the tendency of a glass to devitrify. Fortunately, quite small additions of certain oxides tend to suppress devitrificationduring cooling of the glass and these can be used to improve the working characteristics in this respect. Aluminium oxide has a marked effect end even afew per cent will suppress devitrification during shaping of the glass. Zinc oxide has a similar but less marked effect in certain glasses and small additions of boric oxide can also be useful. A particularly useful and interesting observation is that small addition of phosphours pentoxide suppress crystallisation of the glass-ceramic compositions derived from lithia-silica system at temperatures within the glass-working range. This results from the marked effect of P205 in reducing crystal growth rates in this temperature range. The object of the heat-treatment process is to convert the glass into a microcrystaline ceramic having properties superior to these of the orginal glass. Controlled heat treatment consists of two steps; nucleation and crystal growth. The first stage of the process involves heating the glass form room temperature to the nucleation temperature. The optimum nucleation temperature generally seems to lie within the range of temperature corresponding with viscoties of 1011 to 1012 poises. The period of time for which the glass is maintained at the nucleation temperature will usually be form 0.5 to 2 hours, altough longer periods may not have a detrimental effect. Following the nucleation stage, the temperature of the glass is increased at a controlled rate sufficiently slowly to permit crystal growth to occur so that deformation of the glass article will not take place. An obvious change brought about by the heat-treatment is the conversion of the transparent glass to an opaque polycrystaline material. Also, the thermal expansion coefficients of glass-ceramics are generally different from those of the parent glasses. Perhaps the most striking and important change in characteristics which is brought about by the crystallisation heat treatment is the increase of mechanical strength. Generally speaking, the electrical properties of glass-ceramics are superior to those of the parent glasses and in particular the electrical resistivities are higer and the electric losses are lower. Glass-ceramics possesing high mecanical strengths and other desirable properties can be produced from glasses of the lithium zinc silicate type. Suitable nucleation catalysts include metallic phosphates or metals such as cooper, silver or gold. These glasses do not require irradiation in order to sensitise the metallic nucleation catalysts. The weight percentages of the major glass constituents lie in the range: Si02 = 34-41; ZnO=10-59; Li20=2-27 and these constituents should total at least 90 per cent of the glass composition. The nucleation catalysts include phosphorous pentoxide 0.5 to 6 per cent, gold 0.02 to 0.03 per cent, silver computed as AgCI 0.02 to 0.03 per cent or copper computed as Cu20 0.5 to 1 per cent. The main crystallising phases in this system are LS2, LS, LZS and silica depending on the glass composition. Lithium disilicate is an interesting phase since it grows with a spherulitic morphology at relatively low temperatures and recrystallises at higer temperatures giving a more desirable microstructure. Nucleation, crystallisation and vra and also on their size. The increase of the number of phase-separated particles with increasing time suggested that non-steady state homogenous nucleation occured in composition PO. The decrease of spherulitic crystal size in the samples hold at nucleation temperature for 16 hours supported this idea. X-ray studies have showed that the equilibrium phases formed in composition PO are LS2, LS and LZS; in P1 LS2 and LZS and in P2 LS2, LZS and a-Cristobalite. The bending strengths of the glass-ceramics P1 and P2 were determined to be equal approximately and higer than that of PO composition. This result could be attributed to the finer microstructures developed in P1 and P2 glass-ceramics.