The influence of processing parameters on luminescent oxides produced by combustion synthesis

Rare-earth activated oxide phosphors have application in high energy photoluminescent (plasma panels) and cathodoluminescent (field emission devices) flat panel displays. These phosphors are composed of a highly insulating host lattice with fluorescence arising from the 3d→3d, 5d→4f or 4f→4f transitions in transition metal or rare earth ions. Fabrication of complex host compositions Y₂SiO₅, Y₃Al₅O₁₂, Y₂O₃, and BaMgAl₁₀O₂₇ along with controlled amounts of the activators (Cr³⁺, Mn²⁺, Ce³⁺, Eu²⁺, Eu³⁺, Tb³⁺, Tm³⁺) represent a challenge to the materials synthesis community. High purity, compositionally uniform, single phase, small and uniform particle size powders are required for high resolution and high luminous efficiency in the new flat panel display developments. This paper will review the synthesis techniques and present physical and luminescent data on the resulting materials.

1. Introduction

The visible-light-generating components of emissive, full color, flat panel displays are called phosphors. Phosphors are composed of an inert host lattice and an optically excited activator, typically a 3d or 4f electron metal. For application in the emerging full color, flat panel display industry, thermally stable, high luminous efficiency, radiation resistant, fine particle size powders are required. The demands of these newer technologies have produced a search for new materials and synthesis techniques to improve the performance of phosphors.Oxide phosphors were found to be optimal for field emission display (FED) and plasma panel display (PDP) devices. Compared with a cathode ray tube, an FED operates with lower energy (3–10 keV) but higher current density (1 mA/cm²) beams impinging on the phosphors. This requires more luminous efficient and thermally stable materials. Luminous efficiency is defined as the ratio of the energy out (lumens) to the input energy. Outgassing from the highly efficient sulfide based phosphors has been shown to degrade the cathode tips of the field emitter array and cause irreversible damage 1]. For PDPs, high energy photons (147 nm, 8.5 eV) impinge on the phosphor powders and cause a reduction in luminous efficiency of the display over time because of radiation damage induced in the material 2].Another requirement is on the particle size distribution: there is a maximum and minimum particle size limitation to the powders. For FED applications, about five particle layers are required to achieve optimal light output 3]. Large particles (>8 μm) require thicker layers, increasing the phosphor cost and also producing more light scattering. Additionally the pixel pitch (250 μm) places a maximum on particle size 4]. Alternatively, it was found that small particles (<0.2 μm) do not have high luminous efficiency arising from grain boundary effects 5]. The activator ion in the crystal is most efficient when located in the bulk material in a regular crystal field. Activators located on the surface or on the grain boundaries are thought to be non-luminescent or even luminescence quenching regions.For full color displays, three phosphor compositions are necessary to emit in the red (611–650 nm), green (530–580 nm) and blue (420–450 nm) regions of the visible spectrum. Some oxide based phosphors used in FEDs are the red-emitting (Y_1−xEu_x)₂O₃, the green-emitting (Y_1−xTb_x)₃Al₅O₁₂ and the blue-emitting (Y_1−xCe_x)₂SiO₅ 6]. For some PDPs the red-emitting component is (Y_1−xEu_x)₂O₃, the green-emitting is Zn_1−xMn_xSi₂O₅ and the blue-emitting is (Ba_1−xEu_x)MgAl₁₀O₁₇ 7]. Sulfide phosphors are also used in FEDs but suffer from the aforementioned degradation problems.

2. Phosphor synthesis techniques

Synthesis of oxide phosphors has been achieved by a variety of routes: solid-state reactions 8 and 9], sol–gel techniques 10], hydroxide precipitation 11], hydrothermal synthesis 12 and 13] and combustion synthesis 14, 15, 16 and 17]. Solid-state reactions are performed at high temperatures, typically around 1600°C, because of the refractory nature of the oxide precursors. For multielement compositions, an incomplete reaction is often obtained with undesirable precursor products present in the final product. This technique requires several heating and grinding steps in order to achieve well-reacted, small particle size phosphors. For sol–gel and hydroxide precipitation methods, dilute solutions of metallorganics or metal salts are reacted and condensed into an amorphous or weakly crystalline mass. The advantage of these methods is that atomically mixed powders are obtained in the as-synthesized condition and problems associated with incomplete reactions are avoided. However, these as-synthesized materials must also be heat treated to high temperatures to crystallize the desired phase and to achieve particle sizes greater than 0.2 μm. Hydrothermal synthesis is a low temperature and high pressure decomposition technique that produces fine, well-crystallized powders 13]. These powders must also be heat treated to high temperature to extract the maximum luminous efficiency. Combustion synthesis is a novel technique that has been applied to phosphor synthesis in the past few years. This technique produces highly crystalline powders in the as-synthesized state and will be described in more detail in Section 3.

3. Combustion synthesis of oxide phosphors

Combustion synthesis involves the exothermic reaction between metal nitrates and a fuel. Combustion synthesis is an important powder processing technique generally used to produce complex oxide ceramics such as aluminates 18, 19, 20 and 21], ferrites 22, 23, 24 and 25], and chromites 26 and 27]. The process involves the exothermic reaction of an oxidizer such as metal nitrates, ammonium nitrate, and ammonium perchlorate 28], and an organic fuel, typically urea (CH₄N₂O), carbohydrazide (CH₆N₄O), or glycine (C₂H₅NO₂).The combustion reaction is initiated in a muffle furnace or on a hot plate at temperatures of 500°C or less; much lower than the phase transition of the target material. In a typical reaction, the precursor mixture of water, metal nitrates, and fuel decomposes, dehydrates, and ruptures into a flame after about 3–5 min. The resultant product is a voluminous, foamy powder which occupies the entire volume of the reaction vessel. The chemical energy released from the exothermic reaction between the metal nitrates and fuel can rapidly heat the system to high temperatures (>1600°C) without an external heat source. Combustion synthesized powders are generally more homogeneous, have fewer impurities, and have higher surface areas than powders prepared by conventional solid-state methods 28].The mechanism of the combustion reaction is quite complex. The parameters that influence the reaction include: type of fuel, fuel to oxidizer ratio, use of excess oxidizer, ignition temperature, and water content of the precursor mixture. In general, a good fuel should react non-violently, produce non-toxic gases, and act as a complexant for metal cations 28]. Complexes increase the solubility of metal cations, thereby preventing preferential crystallization as the water in the precursor solution evaporates 29]. The adiabatic flame temperature, T_f, of the reaction is influenced by the type of fuel, fuel to oxidizer ratio, and the amount of water remaining in the precursor solution at the ignition temperature 27]. The flame temperature can be increased with the addition of excess oxidizer such as ammonium nitrate 28], or by increasing the fuel/oxidizer molar ratio. The following equation can be used to approximate the adiabatic flame temperature for a combustion reaction:

(1)

where ΔH_r and ΔH_p are the enthalpies of formation of the reactants and products, respectively, c_p is the heat capacity of products at constant pressure, and T₀ is 298 K. Measured flame temperatures are typically lower than calculated values of flame temperature as a result of heat loss. Table 1 lists the various phosphor compositions that were synthesized by combustion synthesis.

Table 1. Phosphor compositions obtained by combustion synthesis

An example of a stochiometric combustion reaction of yttrium, aluminum and terbium nitrate with carbohydrazide to form (Y_1−xTb_x)₃Al₅O₁₂ is:

(2)

(1−x)Y(NO₃)₃+5Al(NO₃)₃+3xTb(NO₃)₃+15CH₆N₄O→ (Y_1−xTb_x)₃Al₅O₁₂+42N₂+15CO₂+45H₂O.

When complete combustion occurs, the only gaseous products obtained are N₂, CO₂, and H₂O, making this an environmentally clean processing technique. The generation of gaseous products increases the surface area of the powders by creating micro- and nanoporous regions. For the earlier reaction, for every mole of solid produced, 102 mol of gas are produced.The difference in particle size with the use of different fuels depends upon the number of moles of gaseous products released during combustion. As more gases are liberated, the agglomerates are disintegrated and more heat is carried from the system thereby hindering particle growth. A greater number of moles of gas are produced in combustion reactions with carbohydrazide. If complete combustion is assumed, the gaseous product amounts liberated in combustion reactions with glycine, urea and carbohydrazide, are shown in Table 2. The reactions shown are for 2 mol of nitrate producing 1 mol of metal sesquioxide. If Y₃Al₅O₁₂ (YAG) is produced, the reactions must be multiplied by four, as 8 mol of nitrate are used in the reaction.

Table 2. Number of moles of gas produced for different fuels per mole of metal sesquioxide formed

The BET surface area for the as-synthesized YAG phosphors made with glycine, urea and carbohydrazide was measured to be 19, 22 and 25 m²/g, respectively 16], which is consistent with the increase in number of moles of gas produced.Fig. 1 shows the efficiency in lumens per watt (lm/W) as a function of electron accelerating voltage for (Y_1−xTb_x)Al₅O₁₂ made by solid-state, hydrothermal synthesis and combustion synthesis. The combustion synthesized YAG produced in this work has low-voltage cathodoluminescence efficiencies that are comparable to powders produced by other techniques. The efficiencies for all three phosphors were essentially the same at voltages below 600 V. At these voltages, the penetration depth of the incident electron beam is low, (0.004 nm at 600 V) exciting the surface layer of the phosphor particles. In the higher voltage regime, (>600 V), the penetration depth of the electron beam is greater (0.03 nm at 1 kV). The efficiencies of solid-state and hydrothermal synthesized (Y_1−xTb_x)Al₅O₁₂ at these voltages were approximately 1.0 lm/W greater than combustion synthesized (Y_1−xTb_x)Al₅O₁₂. This is because of the smaller crystallite size of these powders (60 nm) compared with hydrothermal and solid-state synthesized powders (100 nm).

Fig. 1. Effect of synthetic route on the low-voltage cathodoluminescence efficiency of (Y_1−xTb_x)3Al₅O₁₂.