UDK 666.3/.7:621.762.5 ISSN 1580-2949 Professional article/Strokovni članek MTAEC9, 49(3)447(2015) PREPARATION AND DIELECTRIC PROPERTIES OF THERMO-VAPOROUS BaTiO3 CERAMICS PRIPRAVA IN DIELEKTRIČNE LASTNOSTI TERMO-PARNO POROZNE KERAMIKE BaTiO3 Anastasia Kholodkova1, Marina Danchevskaya1, Nellya Popova2, Liana Pavlyukova2, Alexandr Fionov3 1Chemistry Department, Moscow State University, GSP-1, Leninskie Gory 1-3, 119991 Moscow, Russia 2D. Mendeleev University of Chemical Technology of Russia, Geroev Panfilovtsev 20, 125047 Moscow, Russia 3Kotel'nikov Institute of Radio Engineering and Electronics of RAS, Mokhovaya 11-7, 125009 Moscow, Russia anastasia.kholodkova@gmail.com Prejem rokopisa - received: 2013-11-10; sprejem za objavo - accepted for publication: 2014-07-17 doi:10.17222/mit.2013.276 A crystalline BaTiO3 powder was synthesized at 350 °C for 0-20 h from TiO2 (> 99 % purity) and BaO (> 98 % purity) with water vapour acting as the reaction media. According to the XRD and SEM results, the BaTiOs synthesized for 3 h proved to be the most adequate raw material for ferroelectric ceramics among the obtained samples as long as it consisted of pure crystalline sphere-shaped BaTiOs particles with the average size of 156 nm. Pellets were pressed at (100, 150 and 200) MPa and sintered at 1300 °C for 1 h. The influence of the compacting pressure on the dielectric characteristics of BaTiO3 ceramics was studied by monitoring the permittivity and loss-tangent values of the pellets at 20 Hz-2 MHz. Keywords: barium titanate, thermo-vaporous synthesis, microstructure, dielectric ceramics Kristalni prah BaTiO3 je bil sintetiziran pri 350 °C od 0 do 20 h iz TiO2 (čistost: > 99 %) in BaO (čistost: > 98 %) z vodno paro kot reakcijski medij. Iz rentgenogramov in SEM-posnetkov izhaja, da je BaTiO3, sintetiziran 3 h, najprimernejša surovina med vsemi vzorci za feroelektrično keramiko, dokler sestoji iz čistih kristalnih okroglih delcev BaTiO3 s povprečno velikostjo 156 nm. Peleti so bili stiskani pri (100, 150 in 200) MPa in sintrani 1 h pri temperaturi 1300 °C. Vpliv tlaka pri stiskanju na dielektrične lastnosti keramike BaTiO3 je bil preiskovana s spremljanjem permitivnosti in velikosti izgube tg d peletov pri 20 Hz-2 MHz. Ključne besede: barijev titanat, termo-parno porozna sinteza, mikrostruktura, dielektrična keramika 1 INTRODUCTION suitable. The development of a simple and low-cost method for the industrial BaTiO3 production remains to Since the 1940s barium titanate, BaTiO3, has been be a pending problem. known for its extremely high values of ferroelectric cha- In the present work water vapour at 350 °C was used racteristics which make it widely used in the production as the medium for a BaTiO3 synthesis from simple of multilayer ceramic capacitors (MLCCs), resistors with oxides. A similar technique combined with the treatment a positive temperature coefficient of resistivity (PTCR), in supercritical water fluid was previously successfully temperature/humidity/gas sensors, piezoelectric transdu- used for the production of MgAl2O4, ZnAl2O4, Y3Al5O12, cers and actuators, ultrasonic and electro-optic devices, BaFei2Oi9, LiNbO315,16 and also BaTiO3.17,18 As ceramic IR-detectors, etc.1-4 Generally, the BaTiO3 powder is manufacturing is one of the main application areas for obtained as the raw material for the bulk-ceramic manu- the BaTiO3 powder, steps were taken to develop this facturing, as well as thin-film and composite-material technique for obtaining the BaTiO3 with the thermo- production.4 For this purpose a homogeneous, well-dis- vaporous process. We prepared ceramic samples in the persed pure BaTiO3 powder, consisting of spherical same conditions, but varied the compressing pressure particles up to 200 nm in size, is required.1,5,6 Various and studied the phase content, the microstructure and synthesis routes for the as-characterized powder have dielectric properties of the pellets in order to determine been developed over several decades. In addition to the the most appropriate value of the pressure for the cera- conventional solid-state method, the techniques such as mic-manufacturing route. pyrolysis (Pechini, citrate processes), dispersion (cate-cholate synthesis, spray pyrolysis, sol-gel), precipitation (oxalate, hydrothermal and solvothermal synthesis) are 2 EXPERIMENTAL WORK widely used for the fine-crystalline BaTiO3 process- ing.1,7-14 But, as some of them are multistage and require The synthesis of BaTiO3 was performed in laboratory the use of auxiliary substances, mostly in the solid state, stainless-steel autoclaves using BaO (> 98 % purity) and hydrothermal and oxalate techniques are industrially TiO2 (> 99 % purity) as the starting reagents. As BaO interacts with CO2 in the air, forming BaCOs, in order to avoid a lack of Ba2+ ions in the reaction mixture, an amount fraction of excess of BaO 5 % was used. After a thorough mixing by means of grinding in an agate mortar with a pestle, the reagents were placed into a special container inside the autoclave, separated from the distilled water. Hermetically closed autoclaves were heated up to 350 °C (the water-vapour pressure of 16 MPa), kept at this temperature for (0, 0.5, 1, 2, 3, 4, 5, 20) h and then cooled so that the water vapour condensed at the bottom of the autoclaves separated from the product. The product was first washed with acetic acid solution to avoid a BaCOs contamination and then with distilled water. To produce a ceramic powder, the sample synthesized for 3 h (named BT-3h) was mixed with 1 % PVA and uniaxially pressed into pellets at (100, 150 and 200) MPa at room temperature. The pellets were sintered at 1300 °C for 1 h. The phase contents of the powders and ceramics were identified with an X-ray diffraction analysis (STOE STADI P) using the Cu-Ka radiation in a range of 20 ° < 20 < 80 The crystallite size was calculated with the Scherrer equation. The morphologies of the powder and ceramic samples were studied with scanning electron microscopy (JSM-6390 LA). The dielectric permittivity and loss tangent of the pellets were calculated from the capacity and the conductivity, respectively, of the plane condenser, in which each pellet was used as a dielectric layer. Dielectric characteristics were obtained with a precision LCR meter Agilent E4980a with a frequency range of 20 Hz-2 MHz. 3 RESULTS AND DISCUSSION During the thermo-vaporous BaTiO3 synthesis, H2O molecules from the vapour became incorporated into the '" t " A " A "1 " L' " '3 " I" " -I " j" f:^^ A Figure 2: Crystal-size distributions of BaTiO3 synthesized in water vapour at 350 °C and 16 MPa for 0-20 h Slika 2: Razporeditev velikosti kristalov BaTiO3, sintetiziranih od 0 do 20 ur, v vodni pari pri 350 oC in 16 MPa TiO2 structure due to the dissociative absorption manifested in the breaking of the Ti-O bonds and the creation of the Ti-OH bonds. In these conditions the TiO2 structure becomes more flexible, interacting with the Ba2+ ions and reorganising into BaTiO3. The XRD analysis of the powders prepared at 350 °C in the water-vapour atmosphere over the periods of 0-20 h showed that the powders consisted of crystalline BaTiO3 (Figure 1). The formation of BaTiO3 from TiO2 and Ba(OH)2 occurred already during the heating, thus, the sample synthesized for 0 h contained only crystalline BaTiO3. The interaction of the newly formed BaTiO3 phase with water vapour led to an elimination of lattice defects and to a perfection of the crystalline structure. Figure 2 shows the BaTiO3 crystallite-size dependence on the duration of the thermo-vaporous synthesis, calculated from the Scherrer equation. The crystallite size of the samples synthesized for 0-4 h fluctuates in a range of 35-45 nm, while, in the case of a longer synthesis, the crystallite Figure 1: XRD patterns of the BaTiO3 powders synthesized in water vapour at 350 °C and 16 MPa for 0-20 h Slika 1: Rentgenogram prahov BaTiO3 sintetiziranih od 0 do 20 ur, v vodni pari pri 350 °C in 16 MPa Figure 3: Box charts of the crystal-size distribution of BaTiO3 synthesized in water vapour at 350 °C and 16 MPa for 0-20 h Slika 3: Škatlasti diagram razporeditve velikosti kristalnih zrn BaTiO3, sintetiziranih od 0 do 20 ur, v vodni pari pri 350 oC in 16 MPa Figure 4: SEM images of BaTiOs synthesized in water vapour at 350 °C and 16 MPa for: a) 0 h, b) 3 h, c) 4 h, d) 20 h Slika 4: SEM posnetki BaTiOs, sintetiziranega v vodni pari pri 350 oC in 16 MPa po: a) 0 urah, b) 3 urah, c) 4 urah, d) 20 urah size is reduced. This effect can be explained with the interaction of the excessive amounts of Ba(OH)2 in the reacting mixture with the already formed BaTiO3. It is known that the crystallite size from the Scherrer equation is sensitive to phase inhomogeneities. In the SEM images of the synthesized samples the crystals of BaTiO3 exhibit a narrow size distribution. The average crystal size slightly varies in a range of 150-188 nm without a distinct relation to the duration of the synthesis (Figure 3). There is a clear effect of the reaction time on the shape of the crystals. The samples processed for 0-3 h consist of sphere-shaped particles (Figures 4a and 4b). A longer processing leads to a (0