APOORV NEGI in partial fulfillment of the requirements of the award of Bachelor of Technology Degree in Ceramic Engineering at National Institute of Technology, Rourkela is an authentic work done by her under my supervision and guidance. Densification of aluminum is studied in this work in the presence of zinc oxide. Aluminum oxide (Al2O3) or alumina is one of the most versatile refractory ceramic oxides and finds use in a wide range of applications.
It is the close packing of the aluminum and oxygen atoms in this structure that leads to the good mechanical and thermal properties. 10 The most common process for the extraction and purification of alumina is the 'Bayer' process. After the contents of the tank have passed through other vessels where the pressure and temperature are reduced and impurities are removed, the sodium aluminate solution is placed in a special tank where the alumina is precipitated.
The oxygen ions form almost a hexagonal close-packed (HCP) structure, with aluminum ions filling two-thirds of the octahedral interstices. The high hardness of alumina provides abrasion and abrasion resistance and is therefore used in various applications such as wear-resistant coatings for pipes and vessels, pump and tap seals, wire and wire guides, etc. In materials science, ZnO is often called a II-VI semiconductor because zinc and oxygen belong to the 2nd and 6th groups of the periodic table respectively.
In this work, we use zinc oxide as an additive for sintering aluminum oxide at various elevated temperatures.
LITERATURE REVIEW
What is sintering?
Different types of sintering
There are different stages of sintering in both types of sintering are described below
Intermediate stage of sintering
Final stage of sintering
18 pathways that define the sintering mechanism, matter is transported from regions of higher chemical potential to regions of lower chemical potential, there are six different sintering mechanisms in polycrystalline materials. a) Surface diffusion (b) Lattice diffusion (c) Vapor transport. Surface diffusion, lattice diffusion, and vapor diffusion are the only mechanisms leading to actual densification, but all cause the neck to grow and thus affect the rate of e. While the viscous flow of amorphous materials leads to neck growth as well as densification.
MgO is added to Al2O3 with a very small amount of 0.25 wt% of the Al2O3 content, which allows obtaining fine-grained material at full density. Microstructure studies revealed that MgO eliminates the discontinuous grain growth of Al2O3 grains. The grain boundary is not broken. away from the pores, preventing the inclusion of pores trapped inside new large grains, with slow/long diffusion path densification. The mechanism by which MgO slows grain boundary movement in alumina could be as follows. a) The majority of MgO doped to Al2O3 is at the grain boundaries because the dissolution of MgO in Al2O3 is small or 300 ppm. This is due to the relatively large difference in ionic radius, 0.72 À for Mg2+ and 0.53 À for Al3+.
Any rapid grain boundary migration should incorporate Mg2+ ions into the Al2O3 lattice due to the resulting increase in internal energy unless a new composite spinel forms. In our experiment, different amount of ZnO is used as sintering aid, namely 0.5%, 1% and 2 wt%, to find the sintering effect with respect to a batch of 0% pure reactive alumina at different levels. The chemical reaction between ZnO and alumina took place prior to the densification of the powder compact and was accompanied by a fairly large expansion.
Mixing also plays an important role.. the degree of densification during reaction sintering and the uniformity of the microstructure of the initial powder compacts. It is made of ZnO which helps to identify the difference between the above series after sintering at different temperatures, 15500C, 16000C and 1650°C. The soaking time at the picking temperature in each process is 2 hours.
EXPERIMENTAL WORK
- The raw materials used are
- Compaction into pellets
- Drying of pellets
- Sintering of pellets
- Bulk Density and Porosity of sintered pellets
- Phases in sintered powder
- Microstructural analysis by SEM
The bulk density and apparent porosity of the sintered grains were determined by Archimedes principle using water. Dry weight is measured and then the pills are kept in distilled water and then vacuuming is done instead of boiling for about 45 min-1 hour. After that, suspended weight is measured using an apparatus in which grain is suspended in water and weight is measured.
24 Sintered pellets were pulverized with 2% composition at 1550 °C and 1650 °C together with 0% at 1550 °C and subjected to X-ray diffraction phase analysis (PW1830 diffractometer, Phillips, The Netherlands). This is done to know the different phases present in the calcined powder. The angle range was 10o-50o. From the XRD results, we find that all the peaks correspond to aluminum oxide or zinc aluminate.
30 A favorable effect of ZnO on densification was found at all temperatures at 2 wt. % additive, and at a lower amount there is a negative effect on densification for 15500C and 16000C. However, all of these beneficial effects were found to be slightly better than the additive-free formulations. 31 From the graph, we can see that the porosity decreases when the temperature is increased, as well as with an increase in the dopant concentration.
32 From the graph, we found that the shrinkage increases with both the increase in temperature and the increase in dopant concentration. SEM research on pure aluminum oxide showed that there is variation in the grain size and that the grains are also not very compacted. Bulk density, apparent porosity, and shrinkage values indicate that zinc oxide is only effective as a dopant after 1650°C.
