Development of Low Temperature Glass Ceramic From Local Raw Materials

Development of Low Temperature Glass Ceramic From Local Raw Materials

CHAPTER ONE

OBJECTIVES OF THE STUDY

The precise objectives are to:

Identify a source of raw material suitable for glass ceramic production in
Carry out chemical analysis on samples of the raw material to ascertain the chemical constituents.
Identify the appropriate glass formula with the least energy point
Explore the most appropriate route for conversion of the raw material into glass ceramic with respect to the energy demand of the product
Test-melting the batch
Characterize the product of the experiment as the basis for further development

Chapter Two

LITERATURE REVIEW

DEVELOPMENT OF GLASS CERAMICS

The development of glass ceramics has a conflicting beginning but there is a convergence of views centering on the fact that it was discovered accidentally. While Moody (1971) stated that it came about as a result of a furnace fault, Macmillan (1964) in his own account gave credit to S. D. Stookey of the Corning Glass Laboratories in the United States, who discovered it accidentally in an attempt to heat a photosensitive glass to a temperature beyond its normal melting temperature. Stookey found out that the glass, instead of melting, turned into an opaque, polycrystalline material with derivative properties superior to those of the original glass. Unlike traditional ceramics, where most of the crystalline materials are introduced at the stage of preparation, the crystalline phase in glass ceramics is produced by the growth of the crystals in a homogenous glass melt.

Crystallization itself is not a new phenomenon in glass production and when encountered in standard glass production it is viewed as a defect called devitrification. However, the roots of glass ceramics can be linked to the works of Rémur, the 16^th French chemist who earned the reputation of preparing opal glasses from photosensitive glass melts. The problem with Rémur’s opal glasses was his inability to control the degree of opacity. The opacity of glass ceramics, unlike that of Rémur’s opal glasses, can be adjusted by controlling the size and concentration of the crystals, and this can be done precisely by controlling the firing cycle. Tiny crystals scattered throughout the glass body produce opacity in the same way as tiny water droplets suspended in the air produce fog.

GLASS FORMATION VERSUS CRYSTALLIZATION

Most elements and compounds exhibit Arrhenuis type dependency in the sense that an increase in temperature or decrease in activation energy increases their rates of reaction. On cooling these materials crystallize at or a little below their freezing points. There are however a few other materials which rather than follow the path of crystallization, transform into glasses on cooling. The uniqueness of the latter category, chief among them, silica (SiO₂), lies in their ability to form highly viscous melts. The viscosity of SiO₂ at the melting point of cristabolite is 10⁷ Pa s implying that silica melts could be 10 billion times as “thick” as water when measured on the same scale! Rawson (1980), argued that the high viscosity of glassformers at their melting points constituted a barrier in the process of crystallization as “the atoms or molecules are not easily moved, relative to one another by applied forces”. The process of crystallization itself involves structural changes in the form of rearrangement of atoms relative to one another in a long-range pattern. Figure 2.1, which is perhaps the most used diagram in glass technology best, illustrates the difference between glassy materials and crystalline solids at their formation stages.

From the volume-temperature diagram in Figure 2.1, the melting temperature or the freezing point (Tf) in a reversed order is marked by a sudden decrease in volume. No such discontinuous change is experienced if the melt follows the path of supercooling along be. The decease in volume on cooling can be attributed partly to the decreasing amplitude of atomic vibrations, and partly to changes in the structure of the melt making it more compact as the temperature drops. At temperatures near Tf, these structural changes occur at such a speed that they will appear to occur instantaneously with any slight change in the temperature of the material.

As the viscosity increases with falling temperature, the tempo of these changes slows down until the viscosity becomes so high that no such changes are possible any longer. The consequence is a decrease in slope at the point e along the path of supercooling.

Chapter Three

METHODOLOGY

THE SILICA SOURCE

Based on information obtained from published monographs of the Raw Materials Research and Development Council reinforced by data obtained from the Nigerian Mining Corporation, the statutory bodies responsible for mining activities in documentation and practical involvement respectively, the Zangon Daji Quartzite was used as the silica source. There is no data available with regards to its chemical composition. It therefore became imperative to first find out the chemical composition to ascertain its usefulness for glass production.

FIELD SAMPLING

Since sampling, according to Potts (1992), is not an exact science the formats of Scholes (1974) and Fraňce (1980) had to be adopted in a selective sampling procedure that involved first chipping off a boulder weighing about 1 kg and then crushing to fist sizes before subjecting a sub sample to chemical analysis.

CHEMICAL ANALYSIS

The chemical analysis conducted at the Geochemical Laboratories, Earth & Planetary Sciences, McGill University Montreal, Canada used the X-Ray Fluorescence Analytical Procedure to determine the elemental content of the silica source. The procedure is as contained in Appendix I

RAW MATERIAL PROCESSING AND PARTICLE CHARACTERIZATION

The pebbles now reduced to fist sizes were calcined for 7 hours in an electric furnace at a temperature of 500^oC following the Fraňce (1980) prescription. They were next reduced to the required particle size (between 0.6 and 0.06mm) (BS 2975) by ball milling. The grains were washed to remove discrete particles of iron, organic materials and other adhering fine particles. The sand was dried for final screening by magnetic separation and stored in a covered specimen bottle. The ground sample was leached in H₂SO₄ acid to get rid of Fe₂O₃. The leaching acid was then neutralized by Ca(OH)₂ and the sample washed again with water to remove all traces of acid. It was then washed manually five times by attrition with distilled water.

Under the standard specification, glass sands as the major raw material are put in three categories on the basis of contents of SiO₂ and the deleterious oxides. For sand meant for the highest grade glasses (optical category), grade A, the silica content should not be less than 99.5% while the tolerant level of the following impurities, Fe₂O₃, TiO₂ and Cr₂O₃ are limited to 80, 300, and 2ppm respectively (Doyle, 1994). Sand used in glass manufacture consists essentially of quartz grains restricted to a given size range. Under the British Standard Specification (BS 2975) the particle size requirement is such that 80% should fall between 420 microns (36 BS mesh) and 150 microns (16 BS mesh). There should be no grain coarser than 1000 microns (16 BS mesh) and those coarser than 600 microns (25 BS mesh) should not be more than 2%. In the smaller particle range, not more than 5% should be finer than 125 microns (120 BS mesh)

Chapter Four

RESULTS AND ANALYSIS

CHEMICAL ANALYSIS

Tables 4.1 (a and b) show the contents in percentage of oxides, which must follow a specified standard against the actual contents as revealed by the chemical analysis. The figures determine the extent of beneficiation the material must be subjected to bring it to acceptable standard. The results are expressed as weight percent for the major elements and the trace elements (Ba to Zn) as mg/kg. The total iron present has been recalculated as Fe₂O₃

CHOICE OF GLASS COMPOSITION

The basis for glass number 28 having the lowest shrinkage value. The shrinkage is a stage that precedes melting so if a composition attains shrinkage before others it can as well be used as an index of lower melting composition. At 45% SiO₂ content (the lowest among the five candidates) the corresponding influence on the glass structure is a reduced melting temperature given that the higher the silica content, the higher the melting point. The complementary role played by B₂O₃ as a low melting glass former increases the range of glass formation of SiO₂ which under normal conditions loses its glass forming ability at less than 50% content (Rawson, 1980). PbO and BaO play interchangeable roles in the glass structure in promoting low viscous melts at relatively low temperatures and their combined content at 32% is in line with experimental observations that the glass would have the lowest melting temperature.

Chapter Five

SUMMARY, CONCLUSION AND RECOMMENDATION

SUMMARY

This project demonstrated that a glass-ceramic could be produced at a temperature of less than 1000°C via a novel route that involves a sintering crystallization of a composite of a solid crystalline phase and an amorphous phase. The outcome is a tremendous saving in cost of energy. This method, which has no regard to the composition of the starting materials, has a promising future especially with regards to glass waste reuse with the attendant advantage of promotion of a safe environment.

The preference given to ophthalmic glass composition as the composition of choice notwithstanding the fact that any other composition would have been adequate as a starting material was to maintain the experimental conditions within the operating environment at a maximum temperature of 1200^oC. The presence of crystalline phases was confirmed by XRD and the amorphous phase was shown by optical microscopy. The coexistence of both crystalline and amorphous phases as seen in the micrograph confirms the material’s identity as that of a glass ceramic.

CONCLUSION

Notwithstanding the fact that success was recorded in fabricating a low temperature glass ceramic product as witnessed in this experiment, any hope of an immediate market launch of the product by way of patenting is too ambitious for two reasons. In the first place reliability which is the most fundamental concern encountered in any application of materials property data would require some tests for validity. What constitutes reliable data, however, depends on the purpose of the data set and how it will be used. To establish a degree of reliability in the context of “purpose” and “use” requires both quantitative and non-quantitative criteria. Details of material composition, chemical phase, microstructure, and methodology, in particular, all play critical roles in assessing reliability. Consequently, the American based National Institute of Standards and Technology (NIST) recommends the use of a data quality indicator called the “data evaluation level” as a protocol for determining its value.

In essence, a data evaluation level summarizes the status of the evaluation process that has been applied to the set of data at the base of which are two counterbalancing mandates: (1) ensure the reliability of the data; and (2) do not reject data without cause (Munro, 2003). In the NIST protocol, there are seven classes of accepted data listed as follows:

Certified data are standard reference values, usually established by recognized national standards laboratories
Validated data values confirmed or supported by means of correlations and model calculations.
Qualified data, the broadest category of data which must satisfy at least the minimum acceptance criteria. Such data are generally compatible with other related values in the database
Commercial data is the manufacturer’s data
Typical data usually values or ranges of values derived from surveys or broad reviews
Research indicates the preliminary values obtained from work in
Unevaluated are all other accepted data commonly, the measurement method is unknown or described insufficiently but, useful supplementary

These set criteria explain why the design of a glass composition of technological significance follows an arduous routine with a gestation period stretching several years (Rawson, 1980). In the same vain, this research effort as successful as it is marks the beginning of a new effort geared towards transforming the product of this experiment into a material of technological significance.

RECOMMENDATIONS

In the light of the conclusion arrived reached at the end of this research, the following recommendation are made with regards to the direction of efforts to enable the product of the experiment assume technological significance.

The characterization should be expanded to include such properties as the dialectic and mechanical strength of the material under study. Such properties which could not be determined under the limitations of this research are vital for determining the area of product
Studies should be carried out determine chemical parameters like level of volatility of constituent materials as well as effect of variation of composition on the general properties of the material for ease of product reproducibility and improvement of its vital properties.
Government through agencies like the Engineering Materials Institute and RMRDC should pick interest in the development of this material in view its potential as a material that is capable of bringing in high return on

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