Volkanik bazalt kayaçlarından cam-seramik malzeme üretim koşullarının araştırılması ve özelliklerinin incelenmesi

Abstract

Cam-serarnikier, kristallenmeye uygun camların çekirdeklerime ve kristal büyütme aşamalarından oluşan kontrollü kristalizasyonu ile üretilen malzemelerdir. Bileşim ve ısıl işlem koşullarının uygun olarak seçilmesi ile, amorf olarak üretilen camın mikroyapısında ince taneli ve düzenli dağılmış kristaller içeren porozitesiz malzemelerin üretimi mümkündür. Değişik mühendislik uygulamaları için geliştirilen birçok cam-seramik sistemi vardır. Bu çalışmada incelenen doğal volkanik bazalt kayaçlarından üretilen bazalt cam- seramikleri; yüksek mekanik mukavemeti, termal kararlılığı, iyi aşınma direnci ve özellikle alkali ortamlardaki kimyasal dayanıklılığı ile karakterize edilen malzemelerdir. Ülkemizin farklı iki bölgesinin bazalt kayaçlarından cam ve cam-seramik malzeme üretimi imkanlarının araştırıldığı deneysel çalışmalarda; feldispat, şeker ve amonyum nitrat katkılı olarak da hazırlanan harmanlara herhangi bir çekirdeklendirici ilavesi yapılmadan bazalt camlarının üretimi, camlaşma ve kontrollü kristalizasyon özellikleri incelenmiştir. 1450-!500°C sıcaklık aralığında dökülen cam örneklerine değişik ısıl işlemler uygulanarak, bu ısıl işlem koşullarında meydana gelen faz dönüşümleri ve mikroyapılar; diferansiyel termal analiz (DTA), x-ışınlan difraksiyonu (XRD) ve taramalı elektron mikroskobu (SEM) ile belirlenmiştir. Bazalt cam ve cam-seramiklerinin yoğunluk, sertlik, aşırıma, ısıl genleşme gibi fiziksel özelliklerinin yanısıra kimyasal dayanımları da tesbit edilerek, bazaltların kimyasal bileşimleri ile uygulanan ısıl işlem koşullarının malzeme özelliklerine olan etkileri araştırılmıştır. Ayrıca, farklı bileşimlerdeki refrakter tuğlalar üzerinde bazalt camlarının korozif etkisi incelenmiştir. Çalışmalar sonucunda, Ülkemiz bazaltlarından camlaşma özellikleri yüksek, homojen, siyah renkli bazalt camiamın elde edildiği görülmüş olup: uygun ısıl işlemler sonucunda bu camların kristallendirilmesi ile de diopsidik-ojit fazının kristallendiği belirlenmiştir. Kristallerime ile bazalt camlarının yoğunluk, sertlik, ısıl genleşme, aşınma ve kimyasal dayanımlarında önemli ölçülerde artışlar belirlenmiş; genel olarak ısıl işlem sıcaklığının arttırılması ile özelliklerde iyileşmeler görülmüş ve en iyi özelliklere sahip cam-seramiklerin katkısız ve amonyum nitrat katkılı bazaltlarda elde edilebildiği tesbit edilmiştir. Bazalt cam-seramiklerinin mikroyapıları açık olarak belirgin olmayan çok küçük kristallerden oluşmaktadır. Mikroyapı çalışmalarından çekirdeklenme mekanizmasının homojen çekirdeklerime olduğu ve bunun sonucunda da hacim kristallenmesinin gerçekleştiği tesbit edilmiştir. Ayrıca, bazalt camlan ile en az etkileşimin magnezit-krom refrakterlerde olduğu belirlenmiştir.

Glass-ceramics produced by the controlled crystallization of special glasses are micro-crystalline solids. Crystallization is accomplished by subjecting suitable glasses to a carefully regulated heat treatment schedule which results in the nucleation and growth of crystal phases within the glass. In many cases, the crystallization process can be taken almost to completion but a small proportion of residual glass phase is often present. In the glass-ceramics, the crystalline phases are entirely produced by crystal growth from a homogeneous glass phase and this distinguishes these materials from traditional ceramics where most of the crystalline materials are introduced when the ceramic composition is prepared although some recrystallization may occur or new crystal types may arise due to solid state reactions. Glass-ceramics are distinguished from glasses by the presence of major amounts of crystals since glasses are amorphous or non-crystalline. When these materials are fabricated, bodies of desired shapes are formed with conventional glass-forming techniques such as casting, drawing, rolling, blowing and pressing. Conventional crystallization of glasses is almost invariably observed to initiate at the external surfaces, followed by the crystals growing into the amorphous phase and producing a non uniform body of large grain size. For a variety of reasons, it is desirable that the crystals be small (less than 1 micron) and uniform in size. To obtain such small crystals occupying a large volume fraction of the material, a uniform density of nuclei of the order of 1012 to 10i5 per cubic centimetre is required. It is well known that the addition of nucleating agents is the key for achieving controlled crystallization. The nucleating agents is soluble in the molten glass but during either controlled cooling or reheating of the initially quenched glass it takes part in or promotes structural changes in the glass. The nucleating agents, in some cases, may precipitate out by homogeneous nucleation. The nuclei thus formed can then promote heterogeneous nucleation of major crystal phases. The most commonly used nucleating agents are TİO2, ZrCb, P2O5, also the Pt group and noble metals, and fluorides are used. TİO2 is often used in concentrations of 4 to 12 wt %; Zr02 is used in concentrations near its solubility limit (4 to 5 wt % in most silicate melts). In some cases, Zr02 and TİO2 are used in combination to obtain desired properties in the final crystallised bodies. xix The giass-cetamic process comprises the preparation of a homogeneous glass, the shaping of the glass to produce the required articles and, finally, the application of a controlled heat treatment process to convert the glass into a micro crystalline glass- ceramics. In selecting raw materials for the glass-ceramic production, the most important aspect to be taken into account is purity. Since some types of impurity, even in quite small concentrations, could affect the crystallization characteristics of the glass. The glass can be melted in crucibles or pots, but for large scale production melting is carried out in continuous furnaces in which a bath of a molten glass is continually replenished by charcing batch into one end of the furnace while molten glass is removed for shaping at the other end of the furnace. After melting and refining stages glass is shaped by various shaping processes. Perhaps the simplest shaping operation available to glass maker is that of casting. Casting processes are useful for glasses having short working ranges, such as alkali free glasses. Flat plates of glass can be produced by rolling or continuous drawing processes. Pressing of glass is used for the production of lens or dish shaped articles. Many types of hollow ware are made by blowing and, with full automatic machines. A suitable heat treatment process must be applied for conversion of the process involves heating the glass from room temperature to the nucleation temperature. Normally heating rates between 2°C and 5°C per minute will be employed, although for thin glass ware rates as high as 10°C per minute can safely be used. The optimum nucleation temperature generally seems to lie within the range of temperature corresponding with viscosity's of 10n to I0'2 poises. The temperature within this range which gives optimum nucleation is determined by experimentation. As first approximation, the optimum nucleation temperature lies between glass transition (Tg) point and a temperature 50°C higher than this. 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. The upper crystallization temperature for a glass-ceramics is chosen so that maximumum crystallization can be achieved without leading to excessive deformation of the material. One of the notable characteristics of glass-ceramics is the extremely fine grain size and it is likely that this feature is responsible in a large measure for the 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 Â are known. In addition to the crystalline phases, there is usually a residual glass phase. This phase does not normally have the same chemical composition as the parent glass since it will be deficient in those oxides which have taken part in crystal formation. The mechanical strengths of glass-ceramics are generally high compared with ordinary glasses and with other type of ceramics. Composition, applied heat treatments, type and amount of crystalline phases, residual, glassy phase, size and morphology of the crystals present are all have an effect on the mechanical behaviour of glass-ceramics. xx Basalt is a grey to black, fine grained volcanic rock which is the major constituent of oceanic islands and a common component of the continental masses as well. Chemically it is composed of major oxides: silica, alumina, iron oxide, calcia, magnesia, and of lesser importance, soda, potassia, titania and manganese and phosphorus oxides, as well as trace amounts of other species. Plagioclase feldspar and monoclinic pyroxene, normally augite, are two major minerals, with magnetite, olivine and certain other accessory minerals often present. Basalt glass and glass-ceramics find wide application in industry as abrasion and corrosion resistant tiles (and other shapes) and mineral wool for heat, noise and fire insulation. In mineral wool application, the crystallization of amorphous fibres impairs their mechanical properties. On the other hand, the literature cited the superior abrasion and chemical resistance of molten basalt. They can be used wherever the transport of material causes mechanical or chemical abrasion. The aim of the present work is to study the possibilities of glass and glass-ceramic materials production by melting the natural volcanic basalt rocks from Thrace and West Black Sea Regions of Türkiye. Basalts were obtained as a chunk and crushing was carried out in a jaw and conic crushers. The batches of basalt glass were prepared from the Thrace basalt powders without and with the additions of feldspar that called as "Minareci feldspar", sugar, ammonium nitrate. The amounts of additives were up to 30 % wt, 2 % wt and 4 % wt for feldspar, ammonium nitrate and sugar, respectively. All batches were ground in an agate mortar in order to break the agglomerates, to get homogeneity and to decrease particle size for increased oxidation and reducing state of the melt. The chemical compositions of volcanic basalt rocks and feldspar are given in Table 1. Table 1. Chemical composition of volcanic basalts rocks and feldspar (wt %). Glass samples were prepared by melting the ground homogeneous basalt batches in a platinum crucible, in a laboratory electrical furnace between 1450-1 500°C for 1 h. The molten material was cast in graphite or stainless steel moulds (approximately 5 mm high, 10 mm wide and 10 mm long) and annealed at 600°C for 1 h and subsequently cooled to room temperature in the furnace by cooling rate of furnace. xxi The crystallization of the glasses were carried out by two different heat treatment, in the first one, glasses were directly heated up to 800°C to 1 100°C for Î h. In the second one, the two step heat treatment was applied. First, the nucleation heat tratment was applied at 700°C for 1 h and then the crystal growth was realized at 900 and 1000°C for 1-24 h. The crystallization kinetics were studied by differential thermal analysis (DTA) to determine the activation energies for the crystallization and the -viscous flow. Crystalline phases and microstractural changes were studied using X-ray diffraction analysis (XRD) and scanning electron microscope (SEM). Density, microhardness, wear, thermal expansion and chemical durability studies of basalt glasses and glass-ceramics were done to see the effect of basalt glass compositions and the heat treatment conditions. Furthermore, the corrosive effect of basalt glass on refractory bricks were studied. The results of the study can be summarised as follows; 1) Black coloured homogenous basalt glasses were easily obtained by melting at 1450-1500°Cfor 1 h. 2) The activation energies of the crystallization and the viscous flow were calculated as 238 kJ mol"1 and 413 kJ mol"1 for basalt glasses (A) from the Thrace Region and 265 kJ mol"1 and 442 kJ mol"1 for basalt glasses (B) from the West Black Sea Region. The dimensionless parameter (n) related to the reaction mechanism by using DTA measurements at different heating rates were found to vary between 4.1 and 9.0, which indicates bulk nucleation in basalt glass by three dimensional growth in all heating rates. 3) XRD patterns of all basalt glasses indicate no crystalline peak. XRD patterns of heat treated basalt glass-ceramics with and without the additions of feldspar, sugar and ammonium nitrate at different heat treatment schedules are similar. The crystalline phases in A-series basalt glass determined by XRD are diopside [Ca, Mg (Si03)2], augite [(Ca, Fe, Mg) Si03] and Al-augite [Ca (Mg, Al, Fe) Si206]. XRD patterns of these phases are very similar and they are usually referred to as one phase named "diopsidic-augite" in the literature. An extra crystalline phase (larnite [(P-Ca2Si04)]) was observed in A-series basalt glasses with no additives heat treated atl000andll00°Cfor 1 h. The crystalline phases in B-series basalt glasses determined by XRD are; wollastonite [(CaSiQj)], Al-diopside [Ca (Mg,Al) (Si,Al)206] and anorthite [(CaAh^Og)]. Generally, intensities of crystalline peaks in XRD patterns increased slightly when the crystallization temperature and the crystal growth time increased. The increase in the intensities is due to the higher degree of crystallization. 4) The densities of basalt glasses and glass-ceramics are in the range of 2.71 to 2.80 g cm"3 and 2.87 to 3.09 g cm"3, respectively. The increase in densities of basalt xxu giasses after heat treatment is due to the presence of higher density crystalline phases in basalt glass-ceramics. 5) The Knoop microhardness of basalt glasses and glass-ceramics are in the range of 370.2 to 470.4 kg mm"2 and 526.2 to 1097.9 kg mm"2, respectively. The Knoop microhardness values increased when the crystallization temperature and the crystal growth time increased until the temperature exceeds 1000°C, where the glass- ceramic material deforms. In glass-ceramic materials, B-series have the minimum microhardness values. With the increased addition of feldspar in A series glass-ceramics resulted with a decrease in microhardness values. Glass-ceramics with ammonium nitrate additives have the highest microhardness values in all heat treatment schedules. 6) The abrasive wear rate of basalt glasses and glass-ceramics are in the range of 767.20 X 10"s to 293.82 X 10"s mm3 m"! and 287.26 X 10"5 to 3.35 X 10"5 mm3 m"1, respectively. Lower wear rate were observed with increasing transformation of glasses to glass-ceramics. The abrasive wear rate of A-series glass-ceramics with no additives is 3.35 X 10"5 mm3 m"! which is the minimum rate and the abrasive wear rate in basalt glass is 495. 10X 10"5 mm3 m"!. There is approximately 148 times a decrease in wear rate with glass to glass-ceramic transformation. The abrasive wear rate values decreased slightly with increasing the crystallization temperature. The crystal growth time in A-series basalt giasses have no important effect on abrasive wear rate. 7) The minimum and maximum erosive wear rate (as the weight loss) A and B-series glass-ceramics without additives are 2.10 % and 3.16 %, respectively. Furthermore, the erosive wear rate is increased in A-series glass-ceramics with feldspar addition. 8) The mean values of thermal expansion coefficients of A series glass with no additives and its glass-ceramic are 70.0X10"7 (1/°C) and 81.0X10"7 (1/°C), respectively. There is an increase in the thermal expansion coefficient by the glass- ceramic transformation. 9) The chemical durability of basalt glass-ceramics are higher than that of the basalt glasses. For example, in 5 % HCİ the weight losses are in the range of 0,957-1,151 mg cm-2 and 0,335-0,427 mg cm-2 for the glasses and glass-ceramics, respectively. Î0) The SEM micrographs of basalt glass-ceramics showed a fine crystallized microstructure. This type of microstracture is typical characteristic of the basalt glass-ceramic which is also observed and reported in the literature. In glass-ceramics, the fine and homogeneous distribution of crystalline phases are desirable for good mechanical and physical properties. To obtain this desired micro structure, it is common to use nucleating agents such as TİO2, Zr02 and P2O5. In basalt glass-ceramics such nucleating agent is not needed. The mechanism of volume nucleation and crystallization in basalt glasses are attributed to the presence XXill of FeO and Fe203. It was reported in the literature that during melting of basalt rocks, FeO or FeaOs oxidises to Fe304 which act as nucleating agent and crystals growth site. This behaviour of basalt glass-ceramics gives advantages over other glass-ceramics where nucleation agents are necessary to obtain similar microstructure. When the crystallization temperature and the crystal growth time increased, there was no important changes in the microstructure of basalt glass-ceramics. As a result of this study, it is seen that glass and glass-ceramic materials from Turkish natural volcanic basalt rocks can be produced. Glass-ceramics showed good abrasion resistance, microhardness and chemical durability.