Spray-Pyrolysis Technique

The tendency to seek technological miniaturization of devices is associated with a reduction in manufacturing costs (smaller amounts of reagents, lower operating times, lower power consumption). The use of materials in the form of thin films, therefore, has emerged as an alternative to fulfill these requirements. Thin films can be produced by a variety of processes, but in any of them there are two basic steps of nucleation and growth. 

Smdies on techniques for thin film deposition divide these into two groups, depending on the nature of the deposition process. Physical methods include physical vapor deposition (PVD), laser ablation, molecular beam epitaxy and sputtering. Chemical methods comprise of gas phase and solution deposition. The gas phase techniques include chanical vapor deposition (CVD) and atomic layer epitaxy (ALE). Spray-pyrolysis deposition, sol-gel, spin-coating and dip-coating are techniques based on solution deposition. 

Spray pyrolysis is a technique through which dense or porous films and powders can be obtained by controlling the deposition parameters. This technique involves the atomization of a precursor solution that is thrown directly over the heated substrate where the film will be formed. 

Compared with other techniques for obtaining films, this technique presents a simple experimental arrangement and is a cost effective method, mainly due to its simple equipment. It does not require the use of high quality reagents and substrates. The composition of the films can be easily controlled with the reagents used in the precursor solution. Several types of films have been deposited by spray pyrolysis, and this technique has been used for decades in various industries, such as in the production of glass, solar cells and electrical conductive electrodes [1]. 

The equipment used in the spray-pyrolysis technique consists basically of an atomizer and a temperature controller. There are several types of atomizers. The most common ones are compressed air atomizers [2] (when the spray of the precursor solution is produced by a jet of air), electric field atomizers [3] (when the spray of the precursor solution is produced by an electric field) and ultrasound atomizers [4] (when the spray of the precursor solution is produced through high frequencies). 

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Thermal treatment synthesis technique of mists formed from ultrasonic atomizer (Ultrasonic spray pyrolysis technique) Y203-Zr02, NiO, ZnS, BaTi03-SrTi03, MoS2, BiV04 [11-14]... 

Correa-Lozano, B., Comninellis, C. and De Battisri, A. (1996), Physicochemical properties of Sn02-Sb205 films prepared by the spray pyrolysis technique. J. Electrochem. Soc., 143(1) 203-209. 

Rather than produce an atomic vapor by evaporation from a solid surface, an aerosol may be generated from an aqueous salt solution by an atomization procedure. The aerosol can then be evaporated so that the salt condenses into a particle. This is known as the spray-pyrolysis technique. The flame decomposition method is a modification of this technique, in which the aerosol is introduced into a high-temperature flame (1200-3000 K). The precursor is vaporized and oxidized to form metal-oxide particles. 

Aizawa M, Itatani K, Howell FS, Kishioka A (1996) Effects of starting materials on properties of hydroxylapatite powders prepared by spray-pyrolysis technique. J Ceram Soc Japan 104 126-132... 

Ikeuchi M, Yamamoto H, Shibata T, Otani M (2001) Mechanical augmentation of the vertebral body by calcium phosphate cement injection. J Orthop Sci 6 39-45 Inoue S, Ono A (1987) Preparation of hydroxylapatite by spray-pyrolysis technique. J Ceram Soc Japan 95 759-763... 

Figure 4. Microphotograph of a Ti/Sn02 electrode prepared by the spray pyrolysis technique.

Mglu204 thin films were prepared by the spray pyrolysis technique and H+, Li+ ions were implanted at room temperature. The acceleration energy was 1.5 MeV and the fluence range studied spanned from 10 to 10 ions cm . Some of the implanted films were annealed at 450°C for 5 h. Electrical conductivity measurements were carried out using a four-probe method and Hall measurements were performed by the Van der Pauw method under the magnetic field of 5000 Gauss (0.5 T). 

Diagram of the spray pyrolysis technique for depositing thin films. 

Diagne, E.H. A. and Lumbreras, M. (2001), Elaboration and characterization of tin oxide-lanthanum oxide mixed layers prepared by the electrostatic spray pyrolysis technique . Sensors and Actuators B Chemical, 78,1-3,98-105. 

Some advantages associated with each of these spray pyrolysis techniques for producing ceramic powders are given by the SAD method published by Xia et al., which focuses on a strategy of separation of nanocrystals contained in powder particles using some compounds (salts) which melt under maximal temperature of sohd-state particle consoUdation. The melted salts resemble eutectic mixture chlorides and nitrates of Li, Na, and K distributed on the nanocrystallite surfaces and they are prevented from agglomerating. After their removal, by washing the obtained powders, it is possible to get, finally, nanosized powders in different systems [19-26]. 

Spray pyrolysis technique has been used to deposit polycrystalline thin films comprising of CuInSa nanocrystals onto glass substrates. p-XRD studies demonstrate that the films have a chalcopyrite structure with preferred orientation along (112) lattice plane. Average diameter of the nanocrystals, as determined by SEM and TEM images, was found to be about 40-60 nm while band gap calculated through optical absorption studies was found to be 1.55 eV. A solar device fabricated by using these films demonstrated a power conversion efficiency of 7.60%. 

Patil, PS. (1999) Versatility of chemical spray pyrolysis technique. Materials Chemistry and Physics, 59 (3), 185-198. 

The spray pyrolysis technique was applied by Lux et al. (1993) for the preparation of spherical lanthanum aluminate powders approximately 1 pm in diameter. Intermediate products converted completely to lanthanum aluminate after annealing in air for 1 h at 1373 K. Putnam et al. (2000) reported the synthesis of CeA103 by spray drying stoichiometric nitrate solution. The voluminous precursors were decomposed in air at 873 K and then reacted at 1373 K in hydrogen atmosphere for 8 h. 

Some review articles pertaining to spray pyrolysis processing and the range of thin films deposited by this technique for various appKcations have appeared in the literature. Viguie and Spitz [74] have classified chemical spray deposition process according to the type of reaction. A most recent survey on thin-film materials prepared by spray pyrolysis and advances in the field, especially different atomization techniques, was presented [12j. The critical operations for the spray pyrolysis technique are (1) preparation of uniform and fine droplets and (2) the controlled thermal decomposition of these droplets in terms of environment, location, and time. 

Hass [109] reported that Ti02 films formed by the vapor reaction process on substrates at temperatures below 280 °C are amorphous, and above this temperature have a structure of anatase. The anatase thin films of Ti02 with good crystallinity and large area were obtained from the spray pyrolysis technique [110-113]. 

J.H. Kim, et al, Preparation of Ni powders through ultrasonic spray pyrolysis technique . Journal of Materials Research, 18(7), (2003), 1614-1622. 

Recently, a novel method for the preparation of highly dispersed solids using a spray-pyrolysis technique has been reported 14], This technique has been applied to the synthesis of iron-titanium(fV) oxide photocataiysts in the present study, in which are provided details of the method of spray pyrolysis used to prepare solids with a range of nominal iron contents from 0.1 -10 atom %. Structural characterisation has... 

Fig. 20.18 Grain size for different concentration of precursors in solution for metal oxide synthesis vs. growth and calcinations temperature, (a) Sn02 was synthesized using a method based on the pyrolytic reaction of SnCl -SH O dissolved in methanol. In contrast to the conventional spray pyrolysis technique, pyrolytic reaction does not take place during deposition on the surface of nanocrystals. The treatment in the range of 400-900 °C was carried out after drop deposition on the substrate (Adapted with permission from Cirera et al. 1999, Copyright 1999 Elsevier), (b) Sn02 powders were synthesized by hydrothermal method (Adapted with permission from Baik et al. 2000a, b, c. Copyright 2000 John Wiley Sons)...

The gas-phase spray pyrolysis technique is not limited to the production of oxide ceramic powders. Lindquist et al. used this method to generate boron nitride powders by the decomposition of poly(borazinylamine) solutions. The process was... 

Correa-lozano B, Comninellis C, Battisti AD (1996) Preparation of Sn02-Sb205 films by the spray pyrolysis technique. J Appl Electrochem 26 83-89... 

Fig. 1.3 Articles publishing survey regarding the Spray-Pyrolysis technique since 2003 (www.sciencedirect.com, accessed on 27th of July 2013)...

The spray-pyrolysis technique has been used for the deposition of YSZ films on top of cerium oxide substrates doped with gadolinium [10]. The authors obtained... 

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