**4. Acknowledgment**

The results shown arise from the research project «Activated sintering of magnesium oxide» which is financial supported by the Ministry of Science, Education and Sports of the Republic of Croatia.

#### **5. References**


wt.-%) binds almost all the CaO present (which has not reacted with B2O3). In the magnesium oxide (120 % precipitation) it takes 2 wt.-% TiO2 to bind all the CaO present (which has not reacted), so that only a greater quantity (5 wt.-%) TiO2 affects boron removal during sintering to a greater degree. The higher the CaO content, the more B2O3 is retained in the sintered samples. Two mutually dependent reactions of formation of Ca2B2O5 and CaTiO3 were analysed, and phenomenological coefficients calculated according to expresions used in the open system thermodynamics. Calculated phenomenological coefficients L11, L12 and L22 describe the mutual interdependence of two simultaneous irreversible processes, based on an important theorem due to Onsanger. It is thus possible to calculate the quantity of boron (B2O3) removed during the sintering process, i.e. the quantity of B2O3 which remains in the sample sintered, for the area examined. Analogous consideration can be carried out for all the other cases when similar laws are involved, i.e.

The results shown arise from the research project «Activated sintering of magnesium oxide» which is financial supported by the Ministry of Science, Education and Sports of the

Bocanegra-Bernal, M. H. (2008). Microstructural evolution during sintering in MgO Power

*Technology*, Vol.186, No.3, (September 2008), pp. (267-272), ISSN 032-5910 Bonney, O. V. (1982). Recovery of Magnesium as Magnesium Hydroxide from Seawater, US Pat. 43 149 85, 9 February 1982: *Chemical Abstract,* Vol.96, No.125549 Brown, E. et al. (1997). *Seawater: Its Composition, Properties and Behaviour*, 2nd Ed.,

Chaudhuri, M. N.; Kumar, A.; Bhadra, A. K. & Banerjee, G. (1990). Sintering and Grain

Chaudhuri, M. N.; Kumar, A.; Bhadra, A. K.; Banerjee, G. & Sarkar, S. L. (1992).

Chaudhuri, M. N.; Banerjee, G.; Kumar, A. & Sarkar, S. L. (1999). Secondary Phases in

Culkin, F. (1975). The Major Constituents of Seawater, In: *Chemical Oceanography,* Vol.1, J. P.

Čeh, M. & Kolar, D. (1994), Solubility of CaO in CaTiO3, *Journal of Materials Science*, Vol.29,

Vol.34, No.23, (December 1999), pp. (5821-5825),ISSN 0022-2461

Issue 23, (January 1994),pp. (6295-6300), ISSN 0022-2461

3715 3, Walton Hall, Milton Keynes, MK7 6AA, England

No.4/5, (April 1990), pp. (26-30), ISSN 0020-5214

Precipitated from Seawater under Induced Agglomeration conditions, *Powder* 

Butterworth Heinemann in association with The Open University, ISBN 0 7506

Growth in Indian Magnesites Doped with Titanium Dioxide, *Interceramics*, Vol.39,

Microstructure of Sintered Natural Indian Magnesites with Titania Addition, *American Ceramic Society Bulletin*, Vol.71, No.3, (March 1992), pp. (345-348), ISSN

Natural Magnesite Sintered with Addition of Titania, *Journal of Materials Science*,

Riley & G. Skirrow, (Eds.), pp. (136-151), Academic Press, ISBN: 0125887019/0-12-

when mass or energy flows occur in the system due to a motive force.

**4. Acknowledgment** 

Republic of Croatia.

0002-7812

588701-9, London

**5. References** 


**15** 

 *India* 

**The Role of Sintering in the Synthesis** 

The phenomena of calcination, roasting and sintering are closely related and often used intermittently. *Calcination* is the process of subjecting a substance to the action of heat, but without melting or fusion, for the purpose of causing some change in its physical or chemical constitution. The objects of calcination are usually: (1) to drive off water, present as absorbed moisture, as "water of crystallization," or as "water of constitution"; (2) to drive off carbon dioxide, sulphur dioxide, or other volatile constituent; (3) to oxidize a part or the whole of the substance. The process of calcination derives its name from the Latin *calcinare* (to burn lime) due to its most common application, the decomposition of calcium carbonate (limestone) to calcium oxide (lime) and carbon dioxide in order to produce cement. In *roasting*, the minerals impose heartburn, which is used to drive out volatile components whereas in *sintering*, small pieces of ore or powder are heated to make bonding. *Sintering* is a method for making objects from powder through agglomeration by heating the material in a furnace to 80-90% of its melting point until its particles adhere to each other. It is known as solid state sintering. The clay particles sinter even before they actually begin to melt into a glassy state (vitrification). The production of powder metal components can be summarized

Sintering is traditionally used for manufacturing ceramic objects, and has also found uses in such fields as powder metallurgy and synthesis of impurity doped luminescence phosphors. The source of power for solid-state processes is the change in free or chemical potential energy between the neck and the surface of the particle. This energy creates a transfer of material though the fastest means possible; if transfer were to take place from the particle volume or the grain boundary between particles then there would be particle reduction and pore destruction. The pore elimination occurs faster for a trial with many pores of uniform size and higher porosity where the boundary diffusion distance is smaller. Control of temperature is very important to the sintering the process, since grain-boundary diffusion and volume diffusion rely heavily upon temperature, the size and distribution of particles of the material, the materials composition, and often the sintering environment to be controlled. Through diffusion and other mass transport mechanisms, material from the particles is carried to the necks **(Fig.1**), allowing them to grow as the particle bonding enters the *intermediate stage*. The intermediate stage of bonding is characterized by the pores

in three steps; powder preparation, compaction and sintering.

**1. Introduction** 

 **of Luminescence Phosphors** 

Arunachalam Lakshmanan *Saveetha Engineering College,* 

*Thandalam, Chennai,* 

