Synthesis and Research of Alumina Ceramics Properties Chimica Techno Acta ARTICLE published by Ural Federal University 2021, vol. 8(1), № 20218102 journal homepage: chimicatechnoacta.ru DOI: 10.15826/chimtech.2021.8.1.02 1 of 5 Synthesis and Research of Alumina Ceramics Properties Evgeniy I. Frolov ab* , Polina V. Notina c , Sergey V. Zvonarev b , Evgeniya A. Il’ina d , Vyacheslav Yu. Churkin b a: Samara State Technical University, 244 Molodogvardeiskaya st., Samara, 443001, Russia b: Ural Federal University, 19 Mira st., Yekaterinburg, 620000, Russia c: Clausthal University of Technology, 2A Adolph-Roemer-Strasse, Clausthal-Zellerfeld, 38678, Germany d: Institute of High Temperature Electrochemistry of the Ural Branch of the Russian Academy of Sciences, 20 Academicheskaya st., Yekaterinburg, 620990, Russia * Corresponding author: frolov_zhenya@inbox.ru This article belongs to the PCEE-2020 Special Issue. © 2021, The Authors. This article is published in open access form under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). Abstract 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 mate- rials with the maximum luminescence efficiency is presented. The concentration of the luminescence intrinsic centers and various de- fects differ for ceramics synthesized by different methods. It is de- termined 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. Keywords aluminum oxide synthesis thermal decomposition chemical precipitation sol-gel method thermoluminescence Received: 27.10.2020 Revised: 18.12.2020 Accepted: 18.12.2020 Available online: 21.12.2020 1. 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 pro- duction of ceramics obtained from the artificially synthe- sized 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 com- position [1]. For obtaining powders with a high purity de- gree 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 dis- tribution 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 princi- ple of the precipitation method is that differences in the solubility of the compounds employed are used for effec- tive separation. Optimum separation conditions are main- ly 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(NO3)3∙9H2O) is possible in two versions: using the original salt or its saturated solution. Obtaining Al2O3 by the decomposition is reported in [4], according to which the phase transfer in modification occurs at a tempera- ture higher than 1200 °С. The problem of particle sinter- ing during nano-structures formation by the annealing method [5] under high temperatures is actively studied. A sol-gel process was developed specifically for obtain- ing oxide ceramics. The process involves the following stages: preparation of alkoxide solutions, their catalytic interaction with subsequent hydrolysis, condensation polymerization, further hydrolysis [6–9]. An oxide poly- mer (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 poly- mer 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 pro- cessing. In [11] a modified sol-gel method for obtaining http://chimicatechnoacta.ru/ https://doi.org/10.15826/chimtech.2021.8.1.02 http://creativecommons.org/licenses/by/4.0/ Chimica Techno Acta 2021, vol. 8(1), № 20218102 ARTICLE 2 of 5 Al2O3is described where urea is used as a sol stabilizer (NH2)2CO. The disadvantage of the method is a complexity of the hardware design, and its advantage is the high puri- ty and homogeneity of the synthesized compounds, as well as the possibility of obtaining various nanopowders [12]. 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. 2. Experimental In the present work the experimental samples were ob- tained by the three methods described above. The main stages and their characteristics for each method are pre- sented in Table 1. Next, each of them will be considered in more detail. 2.1. Precipitation method The alumina oxide was obtained by the method of alumi- num nitrate precipitation by the alkali (potassium hydrox- ide) up to pH = 9–10. Then, the “aging” took place – the exposure of the resulting mixture to air for a certain peri- od 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 pre- pared 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 pow- ders 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 α-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 (TL) curves were measured on an experimental setup “Gray” with linear heating within the temperature range from 25 to 450 °C at a rate of 2 °/s. In order to fill the luminescence centers, before measuring TL, the samples were exposed to pulsed electron irradiation with the following characteristics: the pulse width of ≈2 ns, the average electron energy of 130 keV, the number of pulses was 10. To compare TL curves for the samples annealed at various temperatures the compacts were subjected to the additional high tem- perature annealing at 1200 °С, corresponding to the trans- fer of Al2O3 into -phase. The synthesized samples of Al2O3 were examined by X- ray diffraction method (XRD). The phase analysis was done using a Rigaku D/MAX-2200VL/PC diffractometer (Rigaku, Japan) at room temperature. A curved graphite crystal was used to monochromate Cu Kα radiation. The data were collected over a 2 range of 15–75 in a continu- ous mode at a scan rate of 3 /min. 2.2. Thermal decomposition Direct thermal decomposition of Al(NO3)3∙9H2O (“chemi- cally pure”) was conducted in two ways. The first way was to use the initial inorganic salt, to carry out thermal de- composition to amorphous alumina the temperature was first gradually raised to 250 °C (evaporation process) and then annealing took place in air at 700 °C for 30 min. The second way involved obtaining a saturated solution of aluminum nitrate by dissolving the original salt in water at its first stage. Table 1 Conditions of Obtaining Al2O3 Samples Sample No. Synthesis method Synthesis conditions Annealing stages, °С (h) aging, h additives aggregate state of the initial mixture thermal pre- processsing, °С (h.) 1 2 3 1 Precipitation method (with the drying stage after precipitation for 12 h at 100 °С) 2 - - - 700 (0.5) 1150 (4) 1200 (4) 2 48 3 inoculum + + PEG-20000 - - 4 PEG-20000 - - 5 Thermal decomposition - - solid salt 250 (0.5) 700 (0.5) - 1200 (4) 6 saturated solution 7 Sol-gel (with a stage of evaporation to a trans- parent gel) - - 200 (3) 700 (1) 800 (1) 1200 (4) Chimica Techno Acta 2021, vol. 8(1), № 20218102 ARTICLE 3 of 5 Then, similar to the previous sample, evaporation and the first stage of annealing were carried out. Before annealing at the second stage, the powders were compacted at a temperature of 1200 °C for 4 h by the method described above. TL curves were measured at a temperature of 1200 °C after annealing. 2.3. Sol-gel method As the initial compounds for the alumina synthesis, Al(NO3)3∙9H2O and C6H8O7∙H2O (“reagent grade”), previ- ously dissolved in a small amount of distilled water, were used. The obtained solutions were mixed and evaporated to a transparent gel. The gel was dried at ~200 °C for 3 h. The product obtained was annealed in air at temperatures of 700 °C (1 h), 800 °C (1 h) and 1200 °C (4 h) to remove any organic residues and soot, as well as to form the main phase of aluminum oxide. After each annealing stage the powder was ground in an agate mortar to homogenize the powder. At the final stage, similar to the previous meth- ods, the compacts were obtained, and TL curves were measured. 3. 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 pro- duced from the alumina powder synthesized by the precip- itation 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 lumi- nescence [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 na- nopowder obtained by the sol-gel method at 140 and 330 °C [16]. The sample under study obtained by alumi- num nitrate precipitation at a low aging duration without any additives has the maximum TL peak intensity at 225 °C. Moreover, the comparison of the samples with equal aging time shows that PEG-20000 additive allows the cre- ation of ceramics with a large number of intrinsic defects, such as F-centers. In addition, such synthesis method re- sults in creation of ceramics with the maximum lumines- cence intensity at the TL peak at 400 °C. Annealing at a temperature of 1200 °C leads to a sin- gle-phase material containing only alumina -phase. Fig. 2 demonstrates ceramic TL curves shown in the previous figure (samples No. 1–4) which were additionally annealed for 4 h at a temperature of 1200 °C. It is seen that for sample No. 1 only high temperature peak intensity changes. For the samples with the longer aging duration the intensity of the both peaks increases from 1.4 to 12.1 times. In this case, the position of the peaks does not change. 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 Al2O3 was obtained by the ther- mal 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 oc- curs in a sample synthesized from a solid phase. In addi- tion, 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 low-temperature re- gion and corresponds to the range 350–360 °C. Fig. 1 Ceramic TL curves synthesized by the precipitation method (samples No. 1–4 from Table 1) after the second annealing stage (1150 °C) Fig. 2 TL curves of alumina ceramics synthesized by the precipita- tion method at various parameters (samples No. 1–4) after addi- tional annealing for 4 h at a temperature of 1200 °C Chimica Techno Acta 2021, vol. 8(1), № 20218102 ARTICLE 4 of 5 To compare luminescent properties of ceramics syn- thesized by various methods, Fig. 4 shows TL curves of the samples obtained by the methods of precipitation, thermal decomposition and sol-gel 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 na- ture of these centers or a possible contribution to this lu- minescence 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 influ- ence of the structural condition on the luminescence at the recorded TL peaks. Its results are shown in Fig. 5. 4. 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. 10 20 30 40 50 60 70 0 4000 8000 12000 16000 * * * *** ** * *** c) b) In te n s it y , a .u . 2, o a) * Al2O3 Fig. 5 Diffractogram and bar-diffractogram of Al2O3 powders: a – precipitation method, b – thermal decomposition, c – sol-gel method; * – phases other than α-Al2O3 Acknowledgments This work was financially supported by the Russian Sci- ence Foundation, project No. 18-72-10082. References 1. Garanin SG, Rukavishnikov NN, Dmitryuk AV, Zhilin AA, Mikhaĭlov MD. Laser ceramic. 1. Production methods. Journal of Optical Technology. 2010;77(9):565–76. doi:10.1364/JOT.77.000565 2. Li H, Lu H, Wang S, Jia J, Sun H, Hu X. Preparation of a nano- sized α-Al2O3 from a supersaturated sodium aluminate solu- tion. Ceramics International. 2009;35(2):901–4. doi:10.1016/j.ceramint.2008.01.030 3. Bugaeva AYu. Ceramic matrix filled composite prepared by the sol-gel method. Glass Physics and Chemistry. 2012;38:149–54. doi:10.1134/S108765961106006X 4. Yakovlev IV, Volodin AM, Zaikovskii VI, Stoyanovskii VO, Lapina OB, Vedyagin AA. Stabilizing effect of the carbon shell on phase transformation of the nanocrystalline alumina par- ticles. Ceramics International. 2018;44:4801–6. doi:10.1016/j.ceramint.2017.12.066 5. Xu R, Xu Y. Modern Inorganic Synthetic Chemistry: Second Edition. Changchun (China): Elsevier; 2017. 785 p. Fig. 3 TL curves of alumina ceramics, synthesized by thermal decomposition (samples No. 5–6 from Table 1) after annealing for 4 h at 1200 °C Fig. 4 TL curves of alumina oxide synthesized by different meth- ods: the method of precipitation (sample No. 1), thermal decom- position (sample No. 5) and a sol-gel method (sample No. 7) https://doi.org/10.1364/JOT.77.000565 https://doi.org/10.1016/j.ceramint.2008.01.030 https://doi.org/10.1134/S108765961106006X https://doi.org/10.1016/j.ceramint.2017.12.066 Chimica Techno Acta 2021, vol. 8(1), № 20218102 ARTICLE 5 of 5 6. Rivera T, Azorin J., Barrera M., Soto AM. Nanostructural pro- cessing of advanced thermoluminescent materials. Radiat E Def Sol. 2007;162:731–6. doi:10.1080/10420150701482675 7. Pierre A. Introduction to Sol-Gel Processing. New York: Kluwer Academic Publishers; 1998. 685 p. 8. Bitencourt JFS, Ventieri K, Gonçalves KA, Pires EL, Mittani JC, Tatumi HA. Comparison between neodymium doped alumina samples obtained by Pechini and sol-gel methods using ther- mo-stimulated luminescence and SEM. J Non-Crystal. 2010;356:2956–59. doi:10.1016/j.jnoncrysol.2010.03.047 9. Khrushcheva АА. Sol-gel synthesis of composite nanoparticles based on aluminum, cerium and zirconium oxides [disserta- tion]. Moscow (Russia): Baikov Institute of metallurgy and materials science of the Russian Academy of Sciences; 2016. 170 p. 10. Pietrzyk B, Miszczak S, Kaczmarek Ł, Klich M. Low friction nanocomposite aluminum oxide/MoS2 coatings prepared by sol-gel method. Ceramics International. 2018;44:8534–9. doi:10.1016/j.ceramint.2018.02.055 11. Vinogradov VV, Agafonov AV, Vinogradov AV, Gulyaeva TI, Drozdov VA, Likholobov VA. Sol-gel synthesis, characteriza- tion and catalytic activity of mesoporous γ-alumina prepared from boehmite sol by different methods. Journal of Sol-Gel Science and Technology. 2010;56:333–9. doi:10.1007/s10971-010-2310-x 12. Rempel’ AA, Valeeva AA. Materialy i metody nanotekhnologiy [Materials and methods of nanotechnology]. Yekaterinburg: Ural. un-ta; 2015. 136 p. Russian. 13. Grigorjeva L., Zolotarjovs A, Millers D, Smits K, Krug P, Stol- lenwerk J, Osman A, Tenostendarp T. Magnetron sputtering fabrication of α-Al2O3:Cr powders and their thermolumines- cence properties. Radiation Measurements. 2018;119:140–3. doi:10.1016/j.radmeas.2018.10.009 14. Kortov V., Kiryakov A., Pustovarov V. Luminescent properties of alumina ceramics doped with chromium oxide. IOP Conf Series: J Phys: Conf Series. 2016;741:012195. doi:10.1088/1742-6596/741/1/012195 15. Kortov VS, Zvonarev SV, Pustovarov VA, Slesarev AI. Features of thermoluminescence in anion-defective alumina single crystals after highdose irradiation. Radiation Measurements. 2014;61:74–7. doi:10.1016/j.radmeas.2013.12.010 16. Zvonarev SV, Kortov VS, Shtang TV, Ananchenko DV, Pe- trovykh KA. Effect of structural changes on luminescent and dosimetric properties of nanoscale aluminum oxide. Applied Radiation and Isotopes. 2015;95:44–7. doi:10.1016/j.apradiso.2014.10.003 https://doi.org/10.1080/10420150701482675 https://doi.org/10.1016/j.jnoncrysol.2010.03.047 https://doi.org/10.1016/j.ceramint.2018.02.055 https://doi.org/10.1007/s10971-010-2310-x https://doi.org/10.1016/j.radmeas.2018.10.009 https://doi.org/10.1088/1742-6596/741/1/012195 https://doi.org/10.1016/j.radmeas.2013.12.010 https://doi.org/10.1016/j.apradiso.2014.10.003