Low temperature green synthesis of LaAlO₃ using microcrystalline LaOCl and amorphous Al₂O₃ precursors derived from spray pyrolysis

1 Introduction

LaAlO₃ has attracted attention in recent years due to its varied application. LaAlO₃ has a rhombohedral structure at room temperature and is usually defined as hexagonal with a=5.364 Å and c=13.11 Å [1]. It has good dielectric characteristics, including high relative permittivity (ε_r=23), a high quality factor (Q×f≈68,000; Q=1/tanδ; f, measuring frequency; and tanδ, dissipation factor), and a very small temperature coefficient of resonant frequency (τ_f=−44×10⁻⁶ K) [2], which is compatible with applications in dielectric resonators. LaAlO₃ has been widely used as a substrate for depositing superconducting thin films for microwave devices because it provides a high quality factor, as well as excellent lattice and thermal expansion equal to that of Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O superconductors [3], [4]. Due to its high surface area and catalytic activity, LaAlO₃ has also been used to catalyze the oxidative coupling of methane. It is necessary to reduce the sintering temperatures of LaAlO₃ ceramics, which are usually in the range of 1673–1873 K, to produce multilayered miniaturized devices because the LaAlO₃ ceramics must be co-sintered with low-melting conductors. Lowering the sintering temperature can be achieved by using fine particle-sized homogenous powders, as well as the addition of melting glass and ceramic pigments [5], [6].

LaAlO₃ is generally synthesized at temperatures >1823 K via a solid-solid reaction route (conventional mixed oxide synthesis) [7]. This method presents a number of drawbacks, such as high reaction temperature, long soaking time, large particle size, limited chemical homogeneity, and low sinterability. Various low temperature chemical routes have been tried, including the following: co-precipitation [8], [9], emulsion combustion [10], a mechanochemical route [11], citrate combustion [12], and a PVA evaporation route. The major drawback of most of these chemical routes is that they require a variety of inorganic and organic precursors, which adds significant complexity to the process. Spray pyrolysis is a green method for the decomposition of single inorganic precursors in the air. In this method, hydrogen chloride produced by exhaust gas is absorbed directly, which provides advantages including short residence time, high production efficiency, low operating cost, and minimized energy consumption. During the preparation process, atomized droplets of a precursor solution undergo evaporation and shrinkage while flowing through a high-temperature reactor. Eventually, these drop into solid particles. Because evaporation, precipitation, drying, and decomposition all occur in a dispersed phase within a single step, it is possible to control the particle properties (size, morphology, chemical composition, etc.) by controlling the process parameters (residence time and decomposition temperature) [13], [14], [15], [16], [17]. Therefore, this method provides a continuous flow process that is greener and more economical than other methods. Lux et al. accomplished the synthesis of aluminate lanthanum by spray pyrolysis with metal nitrate as the raw material [18]. The method presented here is superior to that described by Byron et al. because metal nitrates are not products from an industrial production line, and Byron et al.’s method requires higher pyrolysis and roasting temperatures of 1773 K and 1373 K, respectively.

In this study, we report an easy, inexpensive, and reliable method to synthesize LaAlO₃ powders with a high surface area that are crystallized at low temperatures and sintered to near full density. The synthesized precursor and LaAlO₃ powders have been characterized using powder X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM/EDS). The synthetic mechanism is discussed herein.

2 Materials and methods

2.1 Microcrystalline LaOCl and amorphous Al₂O₃ precursor synthesis

Solutions of AlCl₃·6H₂O and LaCl₃·7H₂O (AR, >99.0%, Sinopharm Chemical Reagent Co. Ltd.) were used as precursors for the LaAlO₃ powder. The La-containing solution was added to the Al-containing solution in a molar ratio of 1:1. The concentration of the precursors was 20.0 wt%. The schematic representation of the spray pyrolysis equipment (Northeastern University in Shenyang, Liaoning Province, China) is shown in Figure 1. The spray pyrolysis system consisted of a home-made atomizer, a corundum tube located inside a tubular furnace, and three cyclones as the collectors; hydrogen chloride produced by exhaust gas is absorbed directly. Droplet diffused through the high temperature zone, quickly evaporated, and underwent dehydration, and the resultant pyrolysate was collected by a filter sampler. The temperature of the tubular furnace was controlled and adjusted in the temperature range of 873–1073 K. Particle residence time in the hot zone is controlled by the flow controllers, which corresponds to a residence time of 1.0–2.0 s.

Figure 1:

Schematic diagram of automated spray pyrolysis experimental device. (1) Corundum tube, (2) tubular furnace, (3) the first-stage cyclone, (4) the first stage collector, (5) the second level cyclone separator, (6) the second stage collector, (7) the third cyclone, (8) the third stage collector, (9) flow controllers, (10) tail gas absorber, (11) lifting device, (12) air compressor, (13) tank, (14) solution, (15) hose, (16) injection pipe, (17–19) water bath temperature control.

3 Results and discussion

The LaOCl precursor was generated from LaCl₃ via pyrolysis. The Al₂O₃ precursor was generated from AlCl₃ via pyrolysis. A mixture of LaOCl and Al₂O₃ was obtained via spray pyrolysis of a mixed solution of LaCl₃ and AlCl₃. This pyrolysis reaction can be expressed as follows:

Figure 2 shows the XRD patterns of samples obtained by spray pyrolysis. Figure 2A shows three broad diffuse peaks between diffraction angles 20° and 70°; the presence of small-shaped peaks indicates that there is a crystalline phase mixed with the primary amorphous structures. Figure 2B presents the XRD patterns of LaOCl prepared by spray pyrolysis. The LaOCl peaks are observed at 12.87°, 25.20°, 30.70°, 33.95°, 43.96°, and 56.68°. Peak broadening observed in the XRD pattern for LaOCl indicates the presence of LaOCl crystallites with an average diameter (D) of 26.6 nm. Due to the amorphous nature of aluminum oxide, the XRD pattern (Figure 2C) obtained from a mixture of Al₂O₃ and LaOCl precursors primarily displays LaOCl peaks. Figure 3 shows the XRD pattern for the LaAlO₃ product obtained after roasting at 1073 K for 2 h. The geometry for the product is rhombohedral based upon the calculated XRD patterns.

Figure 2:

X-ray diffraction patterns of amorphous Al₂O₃, micro crystalline LaOCl,Al₂O₃, and LaOCl precursors.

Figure 3:

X-ray diffraction patterns of LaAlO₃.

TG and DTA were measured to study the endothermic and exothermic effects of Al₂O₂ and LaOCl precursors, and the respective curves are shown in Figure 4. Three weight loss steps were observed in the TGA curve. The first weight loss between 300 and 500 K in the TGA curve is due to the desorption of physically absorbed water. The second sharp weight loss step between 880 and 1070 K corresponds to the crystallization processes of amorphous Al₂O₃ into the γ-Al₂O₃ crystallization processes. The third weight loss step between 1070 and 1270 K corresponds to the reaction of Al₂O₃ with LaOCl to generate LaAlO₃. An obvious weight loss appeared at 1160 k; this is explained by an increase in temperature increasing the intensity of the reaction.

Figure 4:

TGA, DTG, and DTA curves for the precursors powder.

Figure 5A and B shows TEM micrographs and SAED patterns of amorphous Al₂O₃. The diffraction pattern shows an amorphous ring, which is consistent with the conclusion that alumina crystals did not grow. The mean diameter of the nanoparticles was 20 and 100 nm for LaOCl, respectively (Figure 5C and D). From the larger size particle dark field TEM image (Figure 5D) can be found that the particles are composed of a plurality of small grain aggregates. So these values are consistent with the XRD results. The particles of Al₂O₃ and LaOCl synthesized by spray pyrolysis have a size distribution of 0.5–15.0 μm (Figure 5E). In Figure 5F, the dark field TEM image shows that Al₂O₃ and LaOCl precursor particles are composed of small grains; energy spectrum analysis of the different positions of the precursor particles, lanthanum, aluminum, chlorine, and oxygen and differences of content show that the reaction of Al₂O₃ and LaOCl occurs in the grain boundary.

Figure 5:

TEM images and SAED patterns of the amorphous Al₂O₃, micro crystalline LaOCl, Al₂O₃, and LaOCl precursors.

The SEM images and EDS patterns of the Al₂O₃ and LaOCl precursors are shown in Figure 6. It can be seen from the SEM images (Figure 6A) that the particles comprise hollow spheres or flake particles with a particle size distribution between 0.5 and 15.0 μm. The EDS pattern (Figure 6B) confirms the presence of La, Al, O, and Cl. Figure 6C shows that the LaAlO₃ powder consists of highly porous agglomerates of round-shaped particles. The porous structure could be caused by the large volume of gases (Cl₂) released during the reaction. On the other hand, the round shape of the particles might be related to the presence of precursors, which melt during the reaction process and forces LaAlO₃ particles to adopt a spherical shape. This type of particle morphology had a positive influence during the pressing and sintering stage. The EDS (Figure 6D) pattern confirms the presence of La, Al, and O in the sample.

Figure 6:

SEM images and EDS patterns of the Al₂O₃ and LaOCl precursors, LaAlO₃ product.

4 Conclusion

Thermodynamic calculations reveal that the initial reaction temperature of LaOCl and Al₂O₃ is 2244K. This synthesis method provides LaAlO₃ powders from metal chlorides at remarkably low temperatures (1073 K). This method has several advantages over existing techniques: (1) with metal chloride as raw materials, the vast majority of rare earth products common to industrial production exists in the form of rare earth chloride and fluoride; (2) compared with other low temperature syntheses, spray pyrolysis is a greener method, which occurs via decomposition of single inorganic materials (metal chloride) in air; and (3) the LaAlO₃ purity was more than 99.9%, as confirmed through measurements of chloride ion content.

The XRD, TEM, and SAED methods showed that the precursors synthesized by spray pyrolysis were microcrystalline LaOCl and amorphous Al₂O₃. These particular forms of the precursors were achieved at significantly lower reaction temperatures. Using this method, perovskite structure compounds LaCrO₃ and LaCoO₃ were also successfully synthesized.