Li2O - ZnO - Al2O3 camlarının kontrollü kristalizasyonu ve cam seramiklerin karakterizasyonu
The Controlled crystallization of Li2O - ZnO - Al2O3 - SiO2 glasses and the characterization of glass ceramics
Yazar: ENGİN MAYTALMAN
Danışman: DOÇ. DR. ERDEM DEMİRKESEN
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 1999
Konu: Metalurji Mühendisliği = Metallurgical Engineering
Dizin: CAM =   ; Cam = Glass ; Kimyasal özellikler = Chemical properties ; Kristalleşme = Crystallization ; Mekanik özellikler = Mechanical properties ; Seramikler = Ceramics

Özet:
Bu çalışmada, yüksek mukavemet ve yüksek ısıl genleşme özellikleri ile karakterize edilen Lİ20-ZnO-Si02 cam sisteminde % 11 ağırlık oranına kadar ZnO ile yer değiştiren AI2O3' nın, camların kristalleşme davranışlarına, cam ve cam-seramiklerin mekanik ve kimyasal özelliklerine olan etkisi incelenmiştir. Ayrıca %1 1 AI2O3 içeren camlarda, çekirdeklerime katalisti olarak P2O5' in yerine TİO2' nin etkisi araştırılmıştır. Bu amaçla, çekirdeklerime katalisti olarak P2O5' in kullanıldığı, AI2O3' nın ZnO ile % 4, 6, 8 ve 1 1 ağırlık oranlarında yer değiştirmesi ile beş; %1 1 AI2O3 içeren bileşimde çekirdeklerime katalisti kullanılmayan ve katalist olarak TİO2' nin kullanıldığı üç bileşim olmak üzere, sekiz ayrı bileşimde cam hazırlanmıştır. Camlara, yapılan diferansiyel termal analiz (DTA) deney verilerine göre planlanan değişik ısıl işlemler uygulanmış, bu ısıl işlemlerle gelişen mikroyapılar, taramalı elektron mikroskobunda (SEM) incelenmiştir. Isıl işlemlerin değişik aşamalarında cam fazından çökelen kristaller X-ışınları difraktometresi (XRD) ile belirlenmiştir. Tavlanmış camların ve cam-seramiklerin eğme mukavemetleri üç noktadan eğme testi ile Instron Universal test cihazında belirlenmiştir. Cam ve cam-seramiklerin asidik karekterli çözeltilere karşı dirençleri farklı konsantrasyona sahip asidik çözeltiler kullanılarak, zamana bağlı birim yüzey alanından ağırlık kaybı olarak belirlenmiştir. Bu incelemeler sonucunda, ZnO ile yer değiştiren AI2O3' nın orijinal bileşimin camlaşma özelliğini bozmadan mukavemetini ve asit direncini arttırdığı; ağırlıkça %11 AI2O3 içeren cam bileşiminde çekirdeklendirme katalisti olarak P20s'in yerine TİO2' nm kullanılması dırumunda viskozitenin P2O5 içeren bileşime göre daha ileri derecede düştüğü, bu sayede camın döküm ve rafınasyon özellikleri geliştiği, bu nedenle yüksek AI2O3' lı bileşimlerde çekirdeklerime katalisti olarak TİO2' nın kullanılmasının bu yönden çok daha uygun olduğu belirlenmiştir.

Abstract:
Glass-ceramics are a special group of materials produced by controlled crystallization of suitable glasses. The process of manufacturing a glass-ceramic involves the preparetion first of a glass which is shaped in its molten or plastic state to produce articles of the required form. The glass-ware is next subjected to a controlled heat treatment cycle which brings about nucleation and crystallization of various phases so that the final product is a polycrystalline ceramic. This metod of making a ceramic material represents a radical departure from conventional ceramic manufacturing processes and it offers a number of important advantages. The homogenity of the parent glass together with the controlled manner in which the crystals are developed results in ceramic materials having a very fine grained uniform structure free from porosity. The use of glass- working processes such as pressing, casting, blowing or drawing offers certain advantages over the techniques available for shaping conventional ceramics. The advantages of the glass-ceramic process are particularly apparent in the production of thin-walled hollow-ware and other shapes where the section of the material is small since unfired conventional ceramic articles of this type are fragile while the parent glass articles in the glass- ceramic process are relatively strong. Another important feature of glass-ceramic process is that it is applicable to a wide range of compositions. This means that various crystal types can be developed in controlled proportions. As a result, the physical characteristics of glass-ceramics can be varried in a controlled manner and this fact has an important bearing upon the practical applications of glass-ceramics. For example, the thermal expansion coefficients of glass-ceramics can be varried over a wide range so that at one extreme materials possesing low expansion coefficients and having very good resistance to thermal shock are possible while at the other extreme materials possesing very high thermal expansion coefficients closely matched to those of common metals can be obtained. The investigation and development of glass-ceramics are closely related to studies of nucleation and crystallization of supercooled liquids and are therefore of general interest in this field. Glass is a very convenient medium for fundamental studies of this type because glass-like liquids have high viscosities so that the diffusion processes and atomic rearrangements which control nucleation and crystal growth occur relatively slowly. Because of the rapid increase of viscosity which occurs when the temperature falls, it is possible to arrest the crystallization process by rapid cooling. Thus various stages in crystal growth and development can be "frozenin" permitting the use of convenient metods of examination. xvClosely related to crystal nucleation and growth studies are investigations of amorphous phase separation. This subject is of interest both from the viewpoint of the basic phenomena involved and with regard to modifications of glass properties that accompany the structural change. Furthermore, the influence of prior phase seperation upon glass crystallization processes is of prime importance both with regard to glass-ceramics formation and in relation to the stability of glasses. The wide range of compositions that can be produced in the vitreous state is particularly valuable since it allows phase transformations to be investigated in widely differing chemical environments. The development of many crystal types, including metastable and stable phases and the formation of solid solutions, can be investigated under controlled conditions. Because molten glass is a good solvent for most oxides, for certain metals and for some halides and other compounds, the effect of these, present as minor constituents, upon crystal nucleation and growth process can be investigated. Such studies, in addition to their basic importance, are of considerable interest in relation to the development of glass-ceramic microstructures. In addition to their value for the study of physico-chemical effects, glass-ceramics are also valuable for fundamental investigations of certain physical properties. One important field concerns the investigation of mechanical strength and fracture processes for brittle solids. Glass-ceramics are especially valuable in such studies because they can be produced to have very fine microstructure and in addition can contain a wide variety of crystal types. A further valuable possibility is that for identical chemical compositions, the degree of crystallinity can be varied from the amorphous glass at one extreme to the almost completely crystalline glass-ceramic at the other. This latter possibility is of interest not only in studies of mechanical failure but also in the investigation of properties which are dependent on diffusion process, such as ionic conductivity. Basic studies on glass-ceramic systems are of interest in connection with other areas of Materials Science. In the general field they are of importance because they offer combinations of physical properties not available with other classes of materials. To the glass technologist, the development of glass-ceramics is of great interest not only because they extend the possible applications of glass-making techniques but also because the search for new glass-ceramics stimulates research into glass compositions and the relative stabilities of various types of glass. Many of these data can be of value in the development of conventional glasses and manufacturing process. In the field of conventional ceramics it is of interest to study the relationship crystallographic constitution and physical properties. Investigations of glass- ceramics may be particularly valuable because the crystal phases present can be varied in a controlled manner and materials having identical chemical compositions but different crystallographic compositions can be prepared. The possibility of investigating the effects of variations in the proportion and chemical composition of the vitreous phase in glass-ceramics is also of value since in some conventional ceramics the vitreous phase plays an important part in determining certain properties. Finally, the investigation of glass-ceramics is of interest to the mineralogist since materials containing unusual combinations of known crystals are possible and in xviaddition there is the possibility of developing entirely new crystal phases which are not formed except by the devitrification of unusual glass compositions. One of the notable characteristics of glass-ceramics is the extremely fine grain size, and it is likely that this featureis responsible in a large measure for valuable properties of the materials. In general the average crystal size in useful glass- ceramics is not greater than a few microns and materials with mean crystal sizes as small as 200 to 300Â are known. In addition to crystalline phases, there is usually present a residual glass phase. At room temperature glass-ceramics, like ordinary glasses and ceramics, are brittle materials. As a general rule the strengths of glass- ceramics are high compared with ordinary glasses and with other types of ceramics. The strengthsof glass-ceramics can vary widely depending upon the glass-ceramic system and also on the heat treatment cycle employed. Attainment of high tensile strength requires a fine grained microstructure but such a structure is not the best for optimising impact resistance. The mismatch in the thermal expansion coefficient between the crystalline phases and the residual glass phase can have a strong effect on the strength since the mismatch leads the generation of internal stresses. Glass-ceramics are apparently harder than gray cast-irons, for which the Knoop hardness (500 g load) ranges from 1 80 to 300 kg/mm2, or annealed stainless steel for which the corresponding Knoop hardness is 150-200 kg/mm2. Some crystal phases, even when present in only a small volume fraction seem to result in marked enhancement of hardness. Phases of spinel type appear to be particularly effective in this respect. In general, glass-ceramics possess good chemical stability. In many cases, it is likely that when a glass-ceramic is chemically attacked the initial effect is upon the glass phase present. This occurs because the early stages of attack involve ion exchange between hydrogen and mobile cations (usually alkali metal ions) in the glass. Subsequently the silica network structure can be attacked by a process of hydration. The achievement of high chemical durability in glass-ceramics therefore requires the volume of residual glass phase to be small and also that the chemical composition of this phase itself must have a good stability. The avoidance of high concentrations of alkali metal oxides in the glass phase will assist in attaining improved chemical durabilities. Certain types of glass-ceramics have good resistance to attack by corrosive chemical reagents. Low expansion glass-ceramics derived from lithium aluminosilicate glasses are only slightly inferior to borosilicate chemically resistant glass with regard to attack by strong acids and are somewhat more resistant to attack by alkaline solutions. Glass-ceramics derived from the Lİ20-ZnO-Si02 system possess high mechanical strengths and other desirable properties. Metallic phosphates or metals such as copper, silver or gold can be used as nucleating agent. The weight percentages of the major glass constituents lie in the range: S1O2 34-81, ZnO 10-59, Lİ2O 2-27, and these constituents should total at least 90 percent of the glass composition. In addition to the essential constituents, alkali metal oxides, alkaline earth oxides, aluminium oxide, boric oxide and lead oxide can be present in the glasses in minor proportions. Glass-ceramics having thermal expansion coefficients within the range xvn43.1 0"7 to 174.10"7oC ', and modulus of rupture within the range 176 to 340 MPa can be produced from this system. Glass-ceramics have achieved wide usage in a number of fields but there are many potential applications still awaiting industrial exploitation. The high mechanical strengths, good dimensional stability and abrasion resistance of glass-ceramics render them suitable for a number of applications in mechanical engineering, such as bearing, pumps, valves, pipes, heat exchangers furnace construction. The use of glass-ceramics for metal sealing applications derives from the fact that the thermal expansion coefficients can be varried over an extremely wide range as mentioned above. In recent years, important advantages have taken place in the controlled of glass-ceramic microstructures resulting, for example, in the development of machinable glass-ceramics and fibrous glass-ceramics having orientated microstructures. High temperature electrical insulations, preformed circuitry for electronics, substrates for microelectronics, and capasitors are the other applications of glass-ceramics. The aim of this work present, is to investigate the effect of AI2O3 substitution for ZnO and TİO2 for P2O5 as nucleating agent, in a Li20-ZnO-Si02 system on the crystallization behavior, mechanical strength and chemical durability of glasses and corresponding glass-ceramics. The compositions of the glasses studied are given below; A relatively high zinc content L^O-ZnO-SiO^ glass as main composition was coded as Ao. The replacement of 4, 6, 8% wt. ZnO by AI2O3 in this material gave another tree compositions coded as A4, A& and As respectively. The other tree compositions, containing 11% AI2O3 and 3% P2O5 as nucleating agent was coded as A11P3, containing 3 and 5% TİO2 as nucleating agent were coded as A11T3 and A11T5. High purity (Merck quality) Lİ2CO3, SİO2, ZnO, AI2O3 and P2O5 or Ti02 were used as row materials. The weighed mixtures to give 80 g batch were melted at 1 300-1 350°C in a Pt crucible. To insure a good homogenity, the melts were than poured into water, after drying and crushing were remelted at the same temperature level for a time long enough to obtain bubble-free melts. After this procedure, the refined melts were poured into preheated graphite and steel moulds. The glass transition and crystallization temperatures were measured by differential thermal analyses (DTA) at a heating rate of 5, 10, 15 and 20°C/min. DTA tests were performed on the as-cast and heat treated glass samples. The activation energies of each cristalization step were calculated by the results of these DTA tests. Various isothermal and non isothermal heat-treatments planned according to the DTA in formations, were xvmapplied to the glass samples. The crystallising phases were identified by X-ray diffaction analysis using Co Ka radiation. Microstructures developed during various heat treatments were examined by the scanning electron microscopy (SEM). The bending strengths of the glass-ceramics were measured by three-point bending test. The resistance of glasses and glass-ceramics to chemical attack by HC1 and H2SO4 solutions were determined at room temperature by measuring the percent weight loss. Experimental results have showed that, the substitution of ZnO by AI2O3 up to 11% wt. in this system had no detrimental effect on the glass formation. According to the XRD results for nonisothermally heat treated samples, the first crystallization product was Lİ2ZnSiC>4 in all compositions. In the composition Ao, cristobalite was found as second product while the composition A4, p-quartz solid solution was detected after crystallization of Lİ2ZnSiC»4 compound. For other Ao, Ag and all An compositions, similar results have been obtained except that p-quartz solid solution was replaced by p-spodumene solid solution. The presence of fine microstructures suggest that P2O5 and TİO2 used as nucleating agent are effective for the compositions studied. The addition of AI2O3 was found to cause the bending strength of the glass-ceramics to increase. On the other hand, addition of AI2O3 was found to have a marked effect on the acid resistance of glass-ceramics. This behavior can be attributed to the formation of P-spodumene in the alumina containing glass-ceramics which has higher chemical stability than Li2ZnSiC>4 phase. No detectable weight loss in the acid solutions was found for glasses indicating that they have higher stability than glass-ceramics.