Alümina ilavesinin Li2O-ZnO-SiO2 cam ve cam seramiklerinin kristalizasyon davranışı, mekaniksel ve kimyasal özellikleri üzerine etkisi

Abstract

Bu çalışmada, yüksek mukavemet ve yüksek ısıl genleşme özellikleri ile karekterize edilen Lİ2O-ZnO-SiO2 cam sisteminde % 11 ağ. oranına kadar çinko oksit ile yer değiştiren alüminanın, camların kristalleşme davranışlarına, cam ve cam-seramiklerin mekanik ve kimyasal özelliklerine olan etkisi incelenmiştir. Bu amaçla, alüminanın çinko oksit ile % 4, 6, 8 ve 11 ağ. oranlarında yer değiştirmesi ile beş farklı bileşimde cam hazırlanmıştır. Camlara,» daha önceki çalışmalarda yapılmış diferansiyel termal analiz (DTA) 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-ışınlan difraktometresi (XRD) ile belirlenmiştir. Tavlanmış camların ve cam-seramiklerin eğme mukavemetleri üç noktadan eğme testi ile Instron Universal test cihazında îesbit edilmiştir. Cam ve cam-seramiklerin asidik ve bazik karekterli çözeltilere karşı dirençleri farklı konsantrasyona sahip çözeltiler kullanılarak zamana bağlı ağırlık kaybı olarak belirlenmiştir. Deneysel çalışma sonuçlan, çinko oksit ile yer değiştiren alüminanın, orijinal bileşimin camlaşma özelliğini bozmadan %8 AlaOs oranına kadar cam-seramiklerin eğme mukavemetini arttırdığı fakat camların mukavemetini değiştirmediğini, %8' den daha yüksek alümina içeriğinde cam-seramik mukavemetinde düşmeye neden olduğunu, alümina içeriğindeki artışın cam-seramiklerin asit dirençlerini arttırdığını göstermiştir. Alümina ilavesinin cam ve cam-seramiklerin bazlara karşı dirençleri üzerinde herhangi bir etkisinin bulunmadığı belirlenmiştir.

Glass-ceramics are a special group of materials produced by controlied crystallization of suitable glasses. The process of manufecturing a glass-ceramic involves the preparetion fîrst of a glass which is shaped in its molten ör 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 method of making a ceramic material represents a radical departure from conventional ceramic manufacturing processes and it oflfers 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 ör drawing oflfers certain advantages över the techniques avaikble 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 gîass-ceramic process are rektiveîy 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 vanied in a controlled manner and this fact has an important bearing upon the practical applications of glass-ceramics. For example, the thermal expansion coefiîcients of glass-ceramics can be varried över a wide range so that at öne extrenıe 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 crysîallization of supercooled liquids and are therefore of general interest in this field. Gîass is a very convenient medium for fundamental studies of this type because glass-like liquids have high viscosities so that the diflûsion processes and atomic rearrangements which control nucleation and crystal growth occur rektiveîy slowly. Because of îhe rapid increase of viscosity which occurs when the temperature VII fâlîs, it is possible to arrest the crystallization process by rapid cooling. Thus various stages in crystai growth and development can be "frezenin" permitting the use of convenient methods of examination. Closely related to crystai nucleation and growth studies are investigations of amorphous phase separation. This subject is of interest boîh from the viewpoint of the basic phenomena involved and with regard to modifications of glass properîies that accompany the sîractural 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 widw range of compositions that can be produced in the vitreous state is particularly valuable since it allows phase transformations to be investigated in widely diflering chemical environments. The development of many crystai types, including metastabîe and stable phases and the fonnation of solid sohıtions, can be investigated under controlîed conditions. Because molten glass is a good solvent for most oxides, for certain metals and for some halides and other compounds, the effect of îhese, present as minör 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 fiındamental investigations of certain physical properties. Öne important fîeld 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 fiırther valuable possibility is that for identical chemical compositions, the degree of crystallinity can be varied from the amorphous gkss at öne extreme to the almost completely crystalîine glass-ceramic at the other. This latter possibility is of interest not only in studies of mechanical feilure but also in the investigation of properties which are dependent on diffiısion process, such as ionic conductivity. Basic studies on glass-ceramic systems are of interest in connection with other areas of Materials Scıence. in the general fîeld 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 inîo 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 manufâcturing process. in the field of conventional ceramics it is of interest to study the reîationship crystaîlographic constitution and physical properties. investigations of gîass-ceramics may be particuîarîy valuable because the crystal phases present can be varied in a controlîed manner and materials having identical chemical compositions but different crystaîlographic compositions can be prepared. The possibility of investigating the eflFects of variations in the proportion and chemical composition of the vitreous phase vın in glass-ceramics is aîso of vahıe since in some conventionai ceramics îhe vitreous phase pîays 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 addition there is the possîbility of developing entirely new cıystal phases which are not formed except by the devitrification of unusual glass compositions. «A Öne of the notable characteristics of glass-ceramics is the exîremely fine grain size, and it is likely that this feaîureis 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 300 °A are known. in addition to crystalline phases, there is usualîy 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. Attaimnent of high tensile strength requires a fine grained microstructure but such a sîructure is not the best for optimising impact resistance. The mismatch in the thermal exp'ansion coefBcient 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 180 îo 300 kg/mm2, ör annealed stainless steel for which the corresponding Knoop hardness is 150-200 kg/mm2. Some crystaî 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 stabiîity. 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 earry stages of attack involve ion exchange between hydrogen and mobüe cations (usualîy 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 thaî the chemical composition of this phase itself must have a good stabilîty. The avoidance of high concentrations of alkali metal oxides in the glass phase will assisî in attaining improved chemical durabiîities. Certain types of glass-ceramics have good resistance to attack by corrosive chemical reagents. Low expansion glass-ceramics derived ironi lithium aluminosilicate glasses are only slightly inferior to borosilicate chemically resistant gîass with regard to attack by strong acids and are somewhat more resistant to attack by alkaline solutions. Gîass-ceramics derived from the Lİ2O-ZnO-SiC>2 system possess high mechanical strengths and other desirable properties. Metallic phosphates ör metals such as copper, siîver ör gold can be used as nucleating agent. The weight percentages of the DC major glass constituents lie in the range: Si02 = 34-81, ZnO = 10-59, Li20 = 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 43x10 "7 to 174x10 "7 °C"', 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 exploitatioa 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 in a Lİ20-ZnO-SiQ2 system on the crystallization behavior, strength and chemical stability of glasses and corresponding glass-ceramics. For this purpose, a relatively hüıg-zinc-content Lİ20-ZnO-Si02 glass having the follwing composition was selected for investigations; Si02 = 55, ZnO = 32, Li20 = 10, P205 = 3 % wt. This main composition was coded as Aq. The replacement of 4, 6, 8, 1 1 % wt. ZnO by AI2O3 in this material gave another four compositions coded as A», Ae, Ag and Ai 1 respectively. High purity (Merck quality) Lİ2CO3, Si02, ZnO, P205 and AI2O3 were used as row materials. The weighed mixtures to give 100 g batch were melted at 1300-1350 °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 moulds. The glass transition and crystallization temperatures were measured by differential thermal analyses (DTA) at a heating rate of 10 °C miri1. DTA tests were performed on the as-cast glass samples. Various isothermal and non isothermal heat-treatments planned according to the DTA in formations, were applied 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 a scanning electron microscopy (SEM). The bending strengths of the annealed glasses and glass-ceramics were measured by three-point bending test. The resistance of glasses and glass-ceramics to chemical attack by HC1, H2SO4 and NaOH solutions were determined at room temperature by measuring the 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 nonisothermaily heat treated samples, the first crystallization product was Lİ2Z1İSİO4 in all compositions. In the composition Aq, cristobalite was found as second product while the composition A», fj-quartz solid solution was detected after crystallization of Lİ2ZÎÎSİO4 compound. For other compositions, A6, Ag and An 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 used 3 % wt. as nucleating agent is effective for all compositions studied. The addition of AI2O3 was found to cause the bending strength of the glass-ceramics to increase up to 8 % wt. AI2O3 content beyond which the strength began to fall. The strength of the annealed glasses were not affected by the presence of AI2O3. 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 Lİ2ZnSi04 phase. Alkaline resistance of the glass-ceramics is higher than acid resistance. No detectable weight loss in the acid and alkaline solutions was found for glasses indicating that they have higher stability than glass- ceramics.