Studies on the gas sensing behaviour of nanosized CuNb2O6 towards ammonia, hydrogen and liquefied petroleum gas

Appreciable changes in resistance of polycrystalline nanosized CuNb₂O₆ upon exposure to reducing gases like hydrogen, liquefied petroleum gas (LPG) and ammonia in ambient atmosphere recognize the material as a gas sensor. Nanosized CuNb₂O₆ synthesized by thermal decomposition of an aqueous precursor solution containing copper nitrate, niobium tartrate and tri-ethanol amine (TEA), followed by calcination at 700 °C for 2 h, has been characterized using X-ray diffraction (XRD) study, transmission electron microscopy (TEM), field-emission scanning electron microscope (FESEM), energy dispersive X-ray (EDX) analysis and Brunauer–Emmett–Teller (BET) surface area measurement. The synthesized CuNb₂O₆ exhibits monoclinic structure with crystallite size of 25 nm, average particle size of 25–40 nm and specific surface area of 55 m² g⁻¹.     

Zeolite cover layer for selectivity enhancement of p-type semiconducting hydrocarbon sensors

Screen-printed thick films of the p-type semiconducting materials family SrTi_1−xFe_xO_3−δ have been investigated for hydrocarbon sensing. Among the different compositions tested, the formulations containing 10 and 20% of iron are found to perform best for this purpose. A pronounced cross-interference of NO persisted at operating temperatures of about 400 °C. In order to eliminate this problem, the application of a zeolite cover layer was studied. The properties of this cover layer were optimized with respect to layer thickness and Pt content. Using initial results of a catalytic study on the zeolite powder in addition to a simple diffusion–reaction model, the effect of the zeolite layer with respect to NO cross-interference could be explained satisfactorily.     

Controlled synthesis and gas-sensing properties of hollow sea urchin-like α-Fe2O3 nanostructures and α-Fe2O3 nanocubes

Hollow sea urchin-like α-Fe₂O₃ nanostructures were successfully synthesized by a hydrothermal approach using FeCl₃ and Na₂SO₄ as raw materials, and subsequent annealing in air at 600 °C for 2 h. The hollow sea urchin-like α-Fe₂O₃ nanostructures with the diameters of 2–4.5 μm consist of well-aligned α-Fe₂O₃ nanorods with an average length of about 1 μm growing radially from the centers of the nanostructures, have a hollow interior with a diameter of about 2 μm. α-Fe₂O₃ nanocubes with a diameter of 700–900 nm were directly obtained by a hydrothermal reaction of FeCl₃ at 140 °C for 12 h. The response S_r (S_r = R_a/R_g) of the hollow sea urchin-like α-Fe₂O₃ nanostructures reached 2.4, 7.5, 5.9, 14.0 and 7.5 to 56 ppm ammonia, 32 ppm formaldehyde, 18 ppm triethylamine, 34 ppm acetone, and 42 ppm ethanol, respectively, which was excess twice that of the α-Fe₂O₃ nanocubes and the nanoparticle aggregations. Our results demonstrated that the hollow sea urchin-like α-Fe₂O₃ nanostructures were very promising for gas sensors for the detection of flammable and/or toxic gases with good-sensing characteristics.     

Fe2O3 modified thick films of nanostructured SnO2 powder consisting of hollow microspheres synthesized from pyrolysis of ultrasonically atomized aerosol for LPG sensing

Nanostructured hollow spheres of SnO₂ with fine nanoparticles were synthesized by ultrasonic atomization. Thick film gas sensors were fabricated by screen printing technique. Different surface modified films (Fe₂O₃ modified SnO₂) were obtained by dipping them into an aqueous solution (0.01 M) of ferric chloride for different intervals of time followed by firing at 500 °C. The structural and microstructural studies of the samples were carried out using XRD, SEM, and TEM. The sensing performance of pure and modified films was studied by exposing various gases at different operating temperatures. One of the modified sample exhibited high response (1990) to 1000 ppm of LPG at 350 °C. Optimum amount of Fe₂O₃ dispersed evenly on the surface_, adsorption and spillover of LPG on Fe₂O₃ misfits and high capacity of adsorption of oxygen on nanostructured hollow spheres may be the reasons of high response.     

Preparation and gas sensing properties of Fe-doped yttrium manganate nanoparticles

Fe-doped yttrium manganate (YMn_(1−x)Fe_xO₃) nanoparticles were synthesized by the precursor method. X-ray diffractions showed that the structures of the as-prepared powders were crystallized in the normal yttrium manganate phase (space group: P6₃cm) when doping concentration was low, while they were crystallized into the high-temperature yttrium manganate phase (space group: P6₃/mmc) at a high doping concentration. Then, the gas sensing properties of YMnO₃ and YMn_(1−x)Fe_xO₃ nanoparticles were studied for the first time. Both YMnO₃ and YMn_(1−x)Fe_xO₃ exhibited sensor response to alcohols, organic amines, dichloromethane, acetone, acetonitrile, methylbenzene, THF and so on. Interestingly, high-concentration Fe-doped yttrium manganate showed much better sensor response than that of normal yttrium manganate phase. We conclude that the multiferroic material of YMn_(1−x)Fe_xO₃ is a promising potential new ABO₃ type gas sensing material.     

Preparation and gas sensing properties of pure and doped γ-Fe2O3 by an anhydrous solvent method

Nanocrystalline undoped and Cd-doped γ-Fe₂O₃ powders were synthesized by an anhydrous solvent method and characterized by thermogravimetric analysis (TGA), differential thermal analysis (DTA), X-ray diffraction (XRD) and transmission electron micrograph (TEM). The gas sensitivity measurements indicated that both the undoped (operated at 240 °C) and the 5 mol% Cd-doped γ-Fe₂O₃ sensors (operated at 270 °C) exhibited high response to acetone and ethanol, moderate response to petrol, poor response to liquefied petroleum gas (LPG), H₂ and CO. Furthermore, the 5 mol% Cd-doped γ-Fe₂O₃ sensor presented shorter response and recovery times, better long-time stability, larger response and better selectivity to acetone and ethanol than the undoped sensor. The present undoped and Cd-doped γ-Fe₂O₃ sensors obtained by an anhydrous solvent method were almost insensitive to LPG, while the reported γ-Fe₂O₃ sensors prepared by a hydrous solution method were generally sensitive to LPG, suggesting that the preparation method played a key role in determining the gas sensing properties.     

Investigation of spectroscopic properties of Sm-Eu codoped phosphate glasses

Phosphate glasses with chemical compositions of 74.5NaH₂PO₄–20ZnO–5Li₂O–0.5Sm₂O₃ and 74NaH₂PO₄–20ZnO–5Li₂O–0.5Sm₂O₃–0.5Eu₂O₃ were synthetized by melt quenching method. We investigated the influence of Sm³⁺/Eu³⁺ doping on the optical properties of phosphate glasses. X-ray Diffraction indicates that the samples have an amorphous structure. DSC measurements show a good thermal stability of phosphate glasses. Using the absorption spectra, Judd–Ofelt analysis was applied to absorption bands of Sm³⁺ (4f⁵) to carry out the three phenomenological parameters of Judd–Ofelt (JO). According to the obtained values of Ω₂, Ω₄ and Ω₆, some radiative properties were theoretically determined. We report both the photoluminescence (PL) and the PL lifetime measurements of a prominent emission transition ⁴G_5/2 → ⁶H_5/2 (604 nm) of Sm³⁺ both in absence and in presence of Eu³⁺. It is shown that Eu³⁺ ions act as sensitizers for Sm³⁺ ions and contribute largely to the improvement of the radiative properties of phosphate glasses. An improvement of the PL lifetime value after adding Eu³⁺ ions (4.58 ms) is reported. The predicted lifetime (τ_rad) calculated by Judd–Ofelt theory and the experimental lifetime (τ_meas) for the prepared phosphate glasses were compared with those of other works. Photoluminescence (PL) intensity of ⁴G_5/2 → ⁶H_5/2 (604 nm), ⁴G_5/2 → ⁶H_7/2 (567 nm), ⁴G_5/2 → ⁶H_9/2 (650 nm) and ⁴G_5/2 → ⁶H_11/2 (706 nm) and the quantum efficiency (η) for the excited ⁴G_5/2 level were enhanced after adding Eu³⁺. The radiative properties obtained for (Sm, Eu) codoped phosphate glasses suggest that the present material can be a potential candidate for the development of color display devices.     

Pulsed laser deposited Y-doped BaZrO3 thin films for high temperature humidity sensors

Pulsed laser deposited (PLD) Y-doped BaZrO₃ thin films (BaZr_1-xY_xO_3-y/2, x = 0.2, y > 0), were investigated as to their viability for reliable humidity microsensors with long-term stability at high operating temperatures (T > 500 °C) as required for in situ point of source emissions control as used in power plant combustion processes. Defect chemistry based models and initial experimental results in recent humidity sensor literature [1] and [2]. indicate that bulk Y-doped BaZrO₃ could be suitable for use in highly selective, high temperature compatible humidity sensors. In order to accomplish faster response and leverage low cost batch microfabrication technologies we have developed thin film deposition processes, characterized layer properties, fabricated and tested high temperature humidity micro sensors using these thin films. Previously published results on sputtering Y-doped BaZrO₃ thin films have confirmed the principle validity of our approach [3]. However, the difficulty in controlling the stoichiometry of the films and their electrical properties as well as mud flat cracking of the films occurring either at films thicker than 400 nm or at annealing temperature above 800 °C have rendered sputtering a difficult process for the fabrication of reproducible and reliable thin film high temperature humidity microsensors, leading to the evaluation of PLD as alternative deposition method for these films.X-ray Photoelectron Spectroscopy (XPS) data was collected from as deposited samples at the sample surface as well as after 4 min of Ar⁺ etching. PLD samples were close to the desired stoichiometry. X-ray diffraction (XRD) spectra from all as deposited BaZrO₃:Y films show that the material is polycrystalline when deposited at substrate temperatures of 800 °C. AFM results revealed that PLD samples have a particle size between 32 nm and 72 nm and root mean square (RMS) roughness between 0.2 nm and 1.2 nm. The film conductivity increases as a function of temperature (from 200 °C to 650 °C) and upon exposure to a humid atmosphere, supporting our hypothesis of a proton conduction based conduction and sensing mechanism. Humidity measurements are presented for 200–500 nm thick films from 500 °C to 650 °C at vapor pressures of between 0.05 and 0.5 atm, with 0.03–2% error in repeatability and 1.2–15.7% error in hysteresis during cycling for over 2 h. Sensitivities of up to 7.5 atm⁻¹ for 200 nm thick PLD samples at 0.058 atm partial pressure of water were measured.     

In2O3 + xBaO (x = 0.5-5 at.%) - A novel material for trace level detection of NOx in the ambient

Indium oxide (In₂O₃) doped with 0.5-5 at.% of Ba was examined for their response towards trace levels of NO_x in the ambient. Crystallographic phase studies, electrical conductivity and sensor studies for NO_x with cross interference for hydrogen, petroleum gas (PG) and ammonia were carried out. Bulk compositions with x ≤ 1 at.% of Ba exhibited high response towards NO_x with extremely low cross interference for hydrogen, PG and ammonia, offering high selectivity. Thin films of 0.5 at.% Ba doped In₂O₃ were deposited using pulsed laser deposition technique using an excimer laser (KrF) operating at a wavelength of (λ) 248 nm with a fluence of ∼3 J/cm² and pulsed at 10 Hz. Thin film sensors exhibited better response towards 3 ppm NO_x quite reliably and reproducibly and offer the potential to develop NO_x sensors (Threshold limit value of NO₂ and NO is 3 and 25 ppm, respectively).     

Electrical conductivity dependence of Ni doped Sm0.95Ce0.05FeO3−δ on surface morphology and composition

Sm_0.95Ce_0.05Fe_1−xNi_xO_3−δ materials are considered as candidates for sensing reducing gases. The total electrical conductivity of Ni doped Sm_0.95Ce_0.05FeO_3−δ perovskite materials is discussed in terms of Ni concentration, surface morphology and relative surface atomic ratios. Powders of formula Sm_0.95Ce_0.05Fe_1−xNi_xO_3−δ (x = 0-0.10) were prepared from citrate precursors by using a sol gel method and were then pressed uniaxially and sintered at 1350 °C for 4 h to form pellets. In fresh pellets the relative surface atomic ratios of Sm and Ni increased while that of Fe and Ce decreased as a function of nickel concentration, showing the segregation of samarium species. In contrast, the chemically reduced pellets show Fe rich surfaces. The electrical conductivity of fresh, partially reduced (700 °C under 5% (v/v) H₂/N₂ for 1 h) and fully reduced (1000 °C under 5% (v/v) H₂/N₂ for 1 h) pellets was measured by the four probe DC method.Under air, x = 0.07 and x = 0.10 showed the highest electrical conductivity in the series. Interestingly the x = 0.01-0.05 materials were n-type conductors while x = 0.07-0.10 exhibited p-type behaviour. The reduction treatment at 1000 °C enhanced electrical conductivities up to ∼5000 fold due to changes associated with surface morphology and surface elemental composition. While phase separations are usually detrimental, in this case the reduced sensors are more sensitive without sacrificing reproducibility.     

Synthesis and photoluminescent properties of orange-emitting Sm3+-activated KPb4(PO4)3 phosphor for LEDs

A series of Sm³⁺-doped KPb₄(PO₄)₃ phosphors have been successfully synthesized by high-temperature solid-state reaction method, and the structure, morphology and luminescent properties were investigated. The SEM images suggest that the prepared phosphor has an irregular morphology with a diameter of about 10 ~ 20 μm. Under near-ultraviolet (NUV) light (404 nm) excitation, all prepared phosphors KPb_4-xSm_xP₃O₁₂ (x = 0, 0.02, 0.04, 0.06, 0.08, 0.1, 0.15, 0.2, 0.3, 0.4 and 0.5) show the characteristic f → f emission bands of the Sm³⁺ activator. And the emission intensities have an upward trend with increasing the Sm³⁺ concentration when x is lower than 0.1. By monitoring 598 nm emission, the excitation spectrum of KPb_3.9Sm_0.1(PO₄)₃ contains a series of sharp bands in the range of 250 ~ 500 nm, which matches well with the NUV LED chip. The CIE coordinates of KPb_3.9Sm_0.1(PO₄)₃ phosphor was evaluated to be (0.5714, 0.4253), corresponding to orange color with a high color purity of about 90%.     

A technique for monitoring SO2 in combustion exhausts: Use of a non-Nernstian sensing element in combination with an upstream catalytic filter

Detection of sulfur dioxide (SO₂) at high temperature (600–750 °C) in the presence of some interferents found in combustion exhausts (NO₂, NO, CO₂, CO, and hydrocarbon (C₃H₆)) is described. The detection scheme involves use of a catalytic filter in front of a non-Nernstian (mixed-potential) sensing element. The catalytic filter was a Ni:Cr powder bed operating at 850 °C, and the sensing elements were pairs of platinum (Pt) and oxide (Ba-promoted copper chromite ((Ba,Cu)_xCr_yO_z) or Sr-modified lanthanum ferrite (LSF)) electrodes on yttria-stabilized zirconia. The Ni:Cr powder bed was capable of reducing the sensing element response to NO₂, NO, CO, and C₃H₆, but the presence of NO₂ or NO (“NO_x”, at 100 ppm by volume) still interfered with the SO₂ response of the Pt–(Ba,Cu)_xCr_yO_z sensing element at 600 °C, causing approximately a 7 mV (20%) reduction in the response to 120 ppm SO₂ and a response equivalent to about 20 ppm SO₂ in the absence of SO₂. The Pt–LSF sensing element, operated at 750 °C, did not suffer from this NO_x interference but at the cost of a reduced SO₂ response magnitude (120 ppm SO₂ yielded 10 mV, in contrast to 30 mV for the Pt-(Ba,Cu)_xCr_yO_z sensing element). The powder bed and Pt–LSF sensing element were operated continuously over approximately 350 h, and the response to SO₂ drifted downward by about 7%, with most of this change occurring during the initial 100 h of operation.     

Electrical response of Sm2O3-doped SnO2 to C2H2 and effect of humidity interference

Pure and Sm₂O₃-doped SnO₂ are prepared through a sol–gel method and characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The sensor based on 6 wt% Sm₂O₃-doped SnO₂ displays superior response at an operating temperature of 180 °C, and the response magnitude to 1000 ppm C₂H₂ can reach 63.8, which is 16.8 times larger than that of pure SnO₂. This sensor also shows high sensitivity under various humidity conditions. These results make our product be a good candidate in fabricating C₂H₂ sensors.     

Sprayed CdIn2O4 thin films for liquefied petroleum gas (LPG) detection

Nanocrystalline cadmium indium oxide (CdIn₂O₄) thin films of different thicknesses were deposited by chemical spray pyrolysis technique and utilized as a liquefied petroleum gas (LPG) sensors. These CdIn₂O₄ films were characterized for their structural and morphological properties by means of X-ray diffraction (XRD) and scanning electron microscope (SEM), respectively. The dependence of the LPG response on the operating temperature, LPG concentration and CdIn₂O₄ film thickness were investigated. The results showed that the phase structure and the LPG sensing properties changes with the different thicknesses. The maximum LPG response of 46% at the operation temperature of 673 K was achieved for the CdIn₂O₄ film of thickness of 695 nm. The CdIn₂O₄ thin films exhibited good response and rapid response/recovery characteristics to LPG.     

Complexity of Computing the Local Dimension of a Semialgebraic Set

The paper describes several algorithms related to a problem of computing the local dimension of a semialgebraic set. Let a semialgebraic set V be defined by a system of k inequalities of the formf  ≥  0 with f  R [ X₁, ,X_n ], deg(f)  < d , andx   V . An algorithm is constructed for computing the dimension of the Zariski tangent space to V at x in time (kd)^O(n). Let x belong to a stratum of codimension l_xin V with respect to a smooth stratification ofV . Another algorithm computes the local dimension dim_x(V) with the complexity (k(l_x +  1)d)^O(l_x2n). Ifl  = max_x  Vl_x, and for every connected component the local dimension is the same at each point, then the algorithm computes the dimension of every connected component with complexity (k(l +  1)d)^O(l2n). If V is a real algebraic variety defined by a system of equations, then the complexity of the algorithm is less thankd^O(l2n) , and the algorithm also finds the dimension of the tangent space to V at x in time kd^O(n). Whenl is fixed, like in the case of a smooth V , the complexity bounds for computing the local dimension are (kd)^O(n)andkd^O(n) respectively. A third algorithm finds the singular locus ofV in time (kd)^O(n2).     

Nanostructured spinel ZnCo2O4 for the detection of LPG

Nanostrucutred spinel ZnCo₂O₄ (∼26-30 nm) was synthesized by calcining the mixed precursor (consisting of cobalt hydroxyl carbonate and zinc hydroxyl carbonate) in air at 600 °C for 5 h. The mixed precursor was prepared through a low cost and simple co-precipitation/digestion method. The transformation of the mixed precursor into nanostructured spinel ZnCo₂O₄ upon calcinations was confirmed by X-ray diffraction (XRD) measurement, thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS) and high resolution transmission electron microscopy (HRTEM). To demonstrate the potential applicability of ZnCo₂O₄ spinel in the fabrication of gas sensors, its LPG sensing characteristics were systematically investigated. The ZnCo₂O₄ spinel exhibited outstanding gas sensing characteristics such as, higher gas response (∼72-50 ppm LPG gas at 350 °C), response time (∼85-90 s), recovery time (∼75-80 s), excellent repeatability, good selectivity and relatively lower operating temperature (∼350 °C). The experimental results demonstrated that the nanostructured spinel ZnCo₂O₄ is a very promising material for the fabrication of LPG sensors with good sensing characteristics. Plausible LPG sensing mechanism is also discussed.     

New gas sensitive MIS structures Pt/Al2O3(M = Pt, Rh)/Si with a granular dielectric layer

New gas sensitive MIS structures Pt/Al₂O₃(M)/p-Si, where M = Pt, Rh, with granular dielectric Al₂O₃ layers doped with noble metals were obtained by an aerosol pyrolysis method. Surface morphology and composition of the structures were studied by TEM, AFM and EPMA. Sensor properties of the MIS structures were studied towards reducing gases (1000 ppm H₂, 300 ppm CO, 1000 ppm CH₄ in air) at 100 and 200 °C. The Pt/Al₂O₃(M = Pt, Rh)/Si structures showed a very high sensor response to reducing gases. A shift of C–V characteristics was up to 2.5 V under CO, 2.2 V under hydrogen and 0.7 V under methane. High values of shift of C–V curves can be related with cooperative influence of a change of surface state density in dielectric layer, reduction of platinum electrode and dipole layer formation.     

Microstructural,dielectric and piezoelectric properties of low-temperature sintering Pb(Co1/2W1/2)O3–Pb(Mn1/2Nb2/3)O3–Pb(Zr,Ti)O3 ceramics with the addition of Li2CO3 and Bi2O3

In this study, in order to develop the low-temperature sintering ceramics for multilayer piezoelectric devices, Pb(Co_1/2W_1/2)O₃–Pb(Mn_1/2Nb_2/3)O₃–Pb(Zr,Ti)O₃ (PCW–PMN–PZT) ceramics doped with Li₂CO₃, Bi₂O₃ and CuO as sintering aids were manufactured, and their microstructural, dielectric and piezoelectric properties were investigated. When the only CuO was added, specimens could not be sintered below 980 °C. However, when Li₂CO₃ and Bi₂O₃ with CuO were simultaneously added to the basic composition ceramics, specimens could be sintered below 980 °C. The addition of Li₂CO₃ and Bi₂O₃ were proved to lower sintering temperature of piezoelectric ceramics due to the effect of Li₂CO₃–Bi₂O₃ liquid phase. Piezoelectric properties of Li₂CO₃ and Bi₂O₃ added specimens showed higher values than those of the only CuO added specimens. At 0.2 wt% Li₂CO₃ and 0.3 wt% Bi₂O₃ added specimen sintered at 920 °C, the dielectric constant (ɛ_r) of 1457, electromechanical coupling factor (k_p) of 0.56 and mechanical quality factor (Q_m) of 1000 were shown, respectively. It is considered that these values are suitable for piezoelectric device application such as multilayer piezoelectric transformer and ultrasonic vibrator with pure Ag internal electrode.     

Structural and electrical properties of mixed oxides of manganese and vanadium: A new semiconductor oxide thermistor material

Binary oxides of manganese and vanadium have been synthesized by solid state sintering, in which the mass ratio of the individual components Mn₂O₃ and VO₂ have been varied from 90:10 to 5:95. The bulk ceramic samples were characterized by X-ray diffraction and scanning electron microscopy with energy dispersive X-ray analysis. The initial compositions either rich in Mn₂O₃ or in equi-proportion by mass with VO₂ yield β-Mn₂V₂O₇ or a new crystalline form of Mn₂V₂O₇, with unit cell parameters: a = 7.73091 Å, b = 6.640788 Å, c = 6.70779 Å α = γ = 90° and β = 98.7086° which is designated as γ-Mn₂V₂O₇. The compositions, richer in VO₂ produce MnV₂O₆ co-existing with V₂O₅ the proportion of which increases with increase in VO₂. The surface microanalysis shows a spherical-granular morphology in Mn₂V₂O₇ structure and plate/rod-like structures co-existing with granular morphology in case of MnV₂O₆ together with V₂O₅. The electrical parameters of the negative temperature coefficient thermistors were determined. Depending on the constituent oxide composition, the NTC thermistors showed room temperature resistivity in the range of 6.52 × 10² to 6.1 × 10⁶ Ω-cm. The thermistor constant and activation energy are in the range of 0.12–0.458 eV and 1393–4801 K, respectively.     

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.