**The Role of Sintering in the Synthesis of Luminescence Phosphors**

Arunachalam Lakshmanan

*Saveetha Engineering College, Thandalam, Chennai, India* 

### **1. Introduction**

322 Sintering of Ceramics – New Emerging Techniques

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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 in three steps; powder preparation, compaction and sintering.

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

The Role of Sintering in the Synthesis of Luminescence Phosphors 325

temperature, repetitive heat treatment, milling, and sieving), (2) inhomogeneous mixing and contamination by impurities, (3) product with irregularly shaped and aggregated particles unsuitable for screen brightness and high resolution. Deagglomeration of sintered phosphor chunks is quite cumbersome involving pulverizing, milling, sieving etc. As a result, many attempts have been carried out to find alternative methods for the preparation of phosphors. Superior display performance requires improvement in phosphors particle characteristics such as grain morphology and particle size on the luminescent intensity, efficiency, and resolution. Powders with optimal properties are obtained by different methods such as *chemical precipitation, the sol-gel, solution combustion, plasma chemical, hydrothermal, spray pyrolysis, microwave* etc. However, in most cases, high temperature (although lower than those used in SSD) sintering of samples prepared by these methods was often found to be essential as it increased their luminescence efficiencies due to improved crystallization and

BaMgAl10O17:Eu2+ (BAM:Eu) phosphor is an important blue-emitting phosphor and has found widespread applications in plasma display panels (PDPS) and fluorescent lamps. The BAM phosphor powders synthesized with individual flux materials, such as AlF3, NH4F, LiF, and so on, have been found to exhibit different morphologies. Flux materials are usually compounds of alkali- or alkaline earth metals with lower melting temperatures than that of the host. In this study, BaMgAl10O17:Eu2+ phosphor was prepared with fluxes by spray drying and post-treatment processes. The phosphor prepared with combination of KF and H3BO3 resulted in fairly uniform hexagonal plate-like morphology **(Fig. 2),** and the morphology as well as the plate size is actually in between those obtained by each of these fluxes. However, the phosphor prepared with the combination of KF and NaCl gives particles showing two distinct morphologies, including thin hexagonal plates and rounded particles **(Fig. 3).** These morphologies had appeared to be a mixture of the products

> Fig. 3. SEM photographs of the BAM : Eu phosphor prepared with KF and NaCl **[1].**

optimal incorporation of dopants in the host crystals.

**2. Effects of sintering fluxes on morphology** 

Fig. 2. SEM photographs of the BAM : Eu phosphor prepared with KF and H3BO3 **[1].**

Fig. 1. (a) Process of sintering (b) The initial stage of the bonding occurs as small "necks" form between the particles.

beginning to round. As the mass transport continues, the pores will become even more rounded and some will appear to be isolated away from the grain boundaries of the particles. This is referred to as the *final stage* of bonding. The final step of the sintering process is to cool the bonded compact to a temperature at which it can be handled. This cooling is performed in an atmosphere that is no longer required to chemically react with the compact. The atmosphere in this stage of the process aids in the transport of the heat away from the compact and minimizes the re-oxidation of the compact during cooling. There are two types of sintering: with pressure (also known as hot pressing), and without pressure. Pressureless sintering is possible with graded metal-ceramic composites, with a nanoparticle sintering aid and bulk molding technology.

Luminescence phosphors owe their practical importance to their property of absorbing incident energy and converting it into visible radiations. This phenomenon, known as luminescence, is driven by electronic processes in the material due to the presence of trapping levels created by the presence of impurity atoms or lattice defects. *Solid-state diffusion* (SSD) reaction is the most popular method used in the synthesis of commercial luminescence phosphors as it is easily reproducible and amenable to large scale production. The products obtained yield a high luminescence efficiency. However, SSD has some disadvantages, such as (1) process complexity and energy-consuming (firing at high

(a)

(b) Fig. 1. (a) Process of sintering (b) The initial stage of the bonding occurs as small "necks"

beginning to round. As the mass transport continues, the pores will become even more rounded and some will appear to be isolated away from the grain boundaries of the particles. This is referred to as the *final stage* of bonding. The final step of the sintering process is to cool the bonded compact to a temperature at which it can be handled. This cooling is performed in an atmosphere that is no longer required to chemically react with the compact. The atmosphere in this stage of the process aids in the transport of the heat away from the compact and minimizes the re-oxidation of the compact during cooling. There are two types of sintering: with pressure (also known as hot pressing), and without pressure. Pressureless sintering is possible with graded metal-ceramic composites, with a

Luminescence phosphors owe their practical importance to their property of absorbing incident energy and converting it into visible radiations. This phenomenon, known as luminescence, is driven by electronic processes in the material due to the presence of trapping levels created by the presence of impurity atoms or lattice defects. *Solid-state diffusion* (SSD) reaction is the most popular method used in the synthesis of commercial luminescence phosphors as it is easily reproducible and amenable to large scale production. The products obtained yield a high luminescence efficiency. However, SSD has some disadvantages, such as (1) process complexity and energy-consuming (firing at high

form between the particles.

nanoparticle sintering aid and bulk molding technology.

temperature, repetitive heat treatment, milling, and sieving), (2) inhomogeneous mixing and contamination by impurities, (3) product with irregularly shaped and aggregated particles unsuitable for screen brightness and high resolution. Deagglomeration of sintered phosphor chunks is quite cumbersome involving pulverizing, milling, sieving etc. As a result, many attempts have been carried out to find alternative methods for the preparation of phosphors. Superior display performance requires improvement in phosphors particle characteristics such as grain morphology and particle size on the luminescent intensity, efficiency, and resolution. Powders with optimal properties are obtained by different methods such as *chemical precipitation, the sol-gel, solution combustion, plasma chemical, hydrothermal, spray pyrolysis, microwave* etc. However, in most cases, high temperature (although lower than those used in SSD) sintering of samples prepared by these methods was often found to be essential as it increased their luminescence efficiencies due to improved crystallization and optimal incorporation of dopants in the host crystals.
