Synthesis and Research of Alumina Ceramics Properties

The article describes in detail alumina powder synthesis by different methods at varying parameters. The technique of obtaining ceramics and the research of the optical properties for determining the materials with the maximum luminescence efficiency is presented. The concentration of the luminescence intrinsic centers and various defects differ for ceramics synthesized by different methods. It is determined that ceramics based on the powder synthesized by a sol-gel method has the maximum thermoluminescence intensity in the F-center peak, whereas for the peak of 360 °C it is obtained with the powder prepared by precipitation of aluminum nitrate with a PEG-20000 stabilizer.


Introduction
Material optical properties are studied to create on their basis high-performance luminophores, in particular, based on aluminum oxide, which is widely used in various fields of science and technology. Aluminum oxide is used in production of ceramics obtained from the artificially synthesized substances (pure oxides, nitrides, carbides, etc.) by forming the powder followed by sintering. One of the most important stage in preparing ceramic samples is obtaining the powders which must meet a number of requirements for morphology, agglomeration, impurity and phase composition [1]. For obtaining powders with a high purity degree such synthesis methods as a precipitation method, a method of thermal decomposition and a sol-gel method are used.
The precipitation method is based on the selective distribution of components between liquid and solid phases, accompanied by the separation of one or more components from the solution in the form of a precipitate. The principle of the precipitation method is that differences in the solubility of the compounds employed are used for effective separation. Optimum separation conditions are mainly determined by the value of the solubility product. The precipitation method [2,3] can include additional thermal processing. Moreover, inoculating can be used for the alumina particle agglomeration process and PEG-20000 can be employed as a stabilizer [3].
Thermal decomposition of aluminum nitrate (Al(NO 3 ) 3 •9H 2 O) is possible in two versions: using the original salt or its saturated solution. Obtaining Al 2 O 3 by the decomposition is reported in [4], according to which the phase transfer in modification occurs at a temperature higher than 1200 °С. The problem of particle sintering during nano-structures formation by the annealing method [5] under high temperatures is actively studied.
A sol-gel process was developed specifically for obtaining oxide ceramics. The process involves the following stages: preparation of alkoxide solutions, their catalytic interaction with subsequent hydrolysis, condensation polymerization, further hydrolysis [6][7][8][9]. An oxide polymer (gel) is obtained as a product. The authors of [10] describe the process of polycondensation into a gel after the hydrolysis of the polymer chains as a result of their formation upon dissolution of aluminum isopropoxide in isopropanol with the formation of complexes. After polymer chains formation their hydrolysis is carried out which results in their polycondensation into a gel. Then the gel undergoes aging, flushing out, drying and thermal processing. In [11] a modified sol-gel method for obtaining Al 2 O 3 is described where urea is used as a sol stabilizer (NH 2 ) 2 CO. The disadvantage of the method is a complexity of the hardware design, and its advantage is the high purity and homogeneity of the synthesized compounds, as well as the possibility of obtaining various Thus, the purpose of the study is to obtain aluminum oxide by various synthesis methods, to produce ceramics on its basis and to research the optical properties of the ceramics obtained.

Experimental
In the present work the experimental samples were obtained by the three methods described above. The main stages and their characteristics for each method are presented in Table 1. Next, each of them will be considered in more detail.
Precipitation method. The alumina oxide was obtained by the method of aluminum nitrate precipitation by the alkali (potassium hydroxide) up to pH = 9-10. Then, the "aging" took place -the exposure of the resulting mixture to air for a certain period of time. Two durations of the sample aging were taken: 2 and 48 h. A part of the samples was prepared both with inoculating and with PEG-20000. To analyze the influence of each component added, the samples were additionally prepared either only with inoculating or only with PEG-20000. However, it should be noted that the sample prepared only with inoculating was hygroscopic and its use for luminescent properties analysis was impossible. At the next stage the obtained suspensions of the samples were first filtered on a water-jet pump, and then dried at 100 °C for 12 h. The last step was a sintering process, which was conducted in two stages. The alumina synthesized powders were annealed at 700 °C for 30 min and then at 1150 °C for 4 h. The first stage is carried out for the full decay of the coprecipitation product up to the aluminum oxide. The second stage is necessary for the transition of the alumina into -phase. After two-stage sintering, the powders were pressed into compacts by cold static pressing at a pressure of 0.42 GPa. The compacts were of a disk shape with a diameter of 6 mm and a thickness of 1 mm. To increase the mechanical strength of the compacts, they were additionally thermally tempered in air at a temperature of 450 °C for 30 min. For these compacts, the thermoluminescence

Results and discussion
The luminescent properties of the materials under study are determined by the presence and concentration of the luminescent centers and defects, which are responsible for the luminescence at various peaks of TL curves. Fig. 1 shows ceramic TL curves annealed in air at 1150 °C produced from the alumina powder synthesized by the precipitation method at various synthesis parameters.
It is observed that two distinct peaks with maxima in the ranges 225-230 °C and 360-400 °C are recorded. The first TL peak is mostly likely to correspond to the main dosimetric peak for which various F-type alumina centers are responsible [13]. The second peak is usually associated with chromium ions which, as a rule, is found in super low concentrations in the studied oxide and has a high luminescence [14]. It should be noted that for single-crystal alumina these peaks are observed with maxima at 170 and 300 °C [15] and for the ceramics synthesized from nanopowder obtained  As the previous experiments showed, the transfer to alumina -phase (i.e. annealing at a temperature of 1200 °C) leads to a significant increase of luminescence in all the peaks recorded. In this regard, for the ceramics, for which the initial powder Al 2 O 3 was obtained by the thermal decomposition, TL curves were also measured after the last stage of annealing at 1200 °C for 4 h (Fig. 3). It is shown that the maximum concentration of F-centers occurs in a sample synthesized from a solid phase. In addition, compared to the precipitation method the position of the maximum of this peak shifts to the low-temperature region and corresponds to the range 200-215 °C. For a high-temperature peak a similar situation is observed when the peak maximum shifts to a lowtemperature region and corresponds to the range 350-360 °C.
To compare luminescent properties of ceramics synthesized by various methods, Fig. 4 shows TL curves of the samples obtained by the methods of precipitation, thermal decomposition and solgel method, which have the highest luminescence intensity in F-centers luminescence band. The graph demonstrates that TL peaks of the samples mentioned differ in shape, which indicates a different nature of these centers or a possible contribution to this luminescence of additional defects of the structure obtained. Thus, the ceramics obtained by the precipitation method has the smallest peak half-width at half maximum, and the largest value is recorded for the ceramics synthesized by the sol-gel method.  The XRF analysis was carried out to assess the influence of the structural condition on the luminescence at the recorded TL peaks. Its results are shown in Fig. 5.

Conclusions
During the study the alumina powders were obtained by various methods, such as thermal decomposition, chemical precipitation and sol-gel. The position of FWHM TL peaks with their maxima within the ranges of 200-215 °C and 350-370 °C for the ceramics obtained by various methods is different. This fact can evidence that along with the F-centers for the first peak and the luminescence centers of Chrome ions for the second peak additional defects of the obtained structure are found. Thus, in the study at the synthesis of the alumina ceramics by various methods it is determined that powder-based ceramics synthesized by the sol-gel method has the maximum TL intensity during annealing under vacuum at 1200 °C at 200 °C peak, whereas for the peak of 360 °C it occurs when the powder is prepared by the aluminum nitrate precipitation with a PEG-20000 stabilizer.