Investigation of physicochemical transformation at mechanochemical, hydrothermal and microwave treatment of barium titanyloxalate

Journal of Alloys and Compounds 482 (2009) 229–234

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Investigation of physicochemical transformation at mechanochemical, hydrothermal and microwave treatment of barium titanyloxalate V.V. Sydorchuk a , V.A. Zazhigalov a , S.V. Khalameida a , K. Wieczorek-Ciurowa b , ˛ c,∗ , R. Leboda c J. Skubiszewska-Zieba a b c

Institute of Sorption and Problems of Endoecology, 13 General Naumov Street, 03164 Kyiv, Ukraine Faculty of Chemical Engineering and Technology, Cracow University of Technology, Warszawska Sq. 24, 31155 Kraków, Poland Faculty of Chemistry, Maria Curie-Skłodowska University, M.C-Skłodowska Sq.3, 20031 Lublin, Poland

a r t i c l e

i n f o

Article history: Received 17 January 2009 Received in revised form 23 March 2009 Accepted 25 March 2009 Available online 5 April 2009 Keywords: Barium metatitanate Barium titanyloxalate Mechanochemical Hydrothermal and microwave treatment

a b s t r a c t The effect of mechanochemical (in air and water) as well as hydrothermal and microwave treatment on physicochemical transformations of barium titanyloxalate was studied. The samples were examined using XRD and thermal analysis, FTIR spectroscopy, argon thermodesorption and granulometry. Barium metatitanate is already formed during mechanochemical treatment of barium titanyloxalate in air atmosphere and the following thermal treatment at 550 ◦ C improves its crystal structure. At the same time barium metatitanate is only formed at 800 ◦ C in the case of usual thermal decomposition of barium titanyloxalate. Hydrothermal and microwave treatment of barium titanyloxalate promotes formation of the barium orthotitanate (Ba2 TiO4 ) and barium carbonate (BaCO3 ) mixture. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Barium metatitanate BaTiO3 (BMT) is one of the most prevalent and popular electroceramic materials. It is applied for preparation of multilayer capacitors, thermistors, sensitive elements of gas and water vapour sensors, catalyst supports, photocatalyst and for other purposes [1–3]. Recent possibilities of temperature decrease at BMT synthesis as well as preparation of this substance in the nanocrystalline powder form, which should improve servicing characteristics of the products, arouse great interest. The analysis of the current methods of BMT synthesis is presented in [4,5]. Taking into account these data and the recent research results the following classification of BMT preparation methods can be proposed. All these processes proceed in the liquid and solid phases. One can distinguish the following liquid-phase processes: homogeneous deposition of oxides, sol–gel and citric methods, low temperature aqueous synthesis, hydrothermal treatment including that in the stream of steam under sub- and super-critical conditions [6], and its analoguesolvothermal treatment [7] and microwave procedures. These methods including hydrothermal and microwave techniques are well studied. They allow to prepare BMT at low temperature; however, further high-temperature treatment of the material

∗ Corresponding author. ˛ E-mail address: [email protected] (J. Skubiszewska-Zieba). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.03.163

(700 ◦ C and more) is indispensable in order to remove admixtures. The techniques proceeding in solid phases include emulsion combustion, interactions of barium and titanium compounds at ≥1000 ◦ C with convection and microwave heating [8], selfpropagating high temperature synthesis [9] and mechanochemical treatment (MChT). As known [3,10], the last method can be used either as mechanochemical activation (MChA) or mechanochemical synthesis (MChS). In the first case preliminary mechanochemical treatment of reagents promotes lowering of the synthesis temperature of the final product in the stage of post-annealing of activated reaction mixture. The latter is result of dispersity and imperfectness increase of components as well as improvement of mixture homogeneity [11]. In the second case chemical reaction and formation of a new compound take place in process of the same MChT. Literature reports the examples of using both MChA and MChS for synthesis of BMT from mixtures of different barium and titanium compounds. On the other hand, various types of apparatus are used in treatment. It should be noted that most of the available investigations concern MChA of either reaction mixture [11–19] or its components [20,21]. Such an approach allows to reduce annealing temperature from 1000–1100 ◦ C to 600–700 ◦ C at which interaction of reagents and BMT formation begins. On the other hand, it was also shown that mechanochemical activation of a barium carbonate and anatase mixture, carried out in planetary ball mill, lowers the complete conversion temperature of components into BMT by 150 ◦ C as compared to that using non-activated reagents

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[18]. Another result indicates that long-term mechanical treatment of initial substances (78–100 h) in attritor or ball mills leads to production of small amounts of weakly crystallized perovskite phase BMT [15] which arises in oxide matrix. Its crystalline structure is improved after successive calcination in all above mentioned examples. The synthesis at lower temperature positively affects some properties of BaTiO3 hindering enlargement of its crystallites. On the other hand at MChA in air atmosphere (particularly during long-lasting treatment) aggregation of particles of the final product and formation of admixture phases—barium carbonate (BaCO3 ) and barium orthotitanate Ba2 TiO4 (BOT) occur [15]. Examples of BMT direct mechanochemical synthesis are seldom described in literature [3,22–25] and cited investigations are rather contradictory which can be explained by different conditions of MChT carrying out. Thus, author of paper [22] reports that the reaction between barium and titanium oxides and formation of BMT was practically completed within 100 h if activation was realized in a ball mill. In accordance with the results, obtained in work [25], BMT phase appears after 5 h of oxides mixture grinding and its formation ends after 50 h. Oguchi et al. [24] performed solid-state reaction of BaCO3 and TiO2 through vibro-milling in the presence of glycine that allowed to prepare BMT for 3 h. Cubic BMT with good crystalline structure, formed directly during grinding, was also described by authors [3] who used BaO and TiO2 (rutile) for synthesis. Mechanochemical treatment was performed for 6 h in air using a planetary ball mill. At the same time additional calcination at 1200 ◦ C of mechanically activated product was necessary in order to obtain the tetragonal modification of BMT. Alternative method for producing high purity BMT at relatively low temperatures consists in using of barium titanyl oxalate BaTiO(C2 O4 )2 ·4H2 O (BTO) as precursor [5,8,26–32]. This raw material possesses the following advantages: (i) this reagent contains barium and titanium in stoichiometric ratio which is necessary for BMT synthesis that eliminates problem of components dosage at initial mixture preparation; (ii) both components (barium and titanium oxides) arise owing to destruction of parent material (BTO) and therefore they have higher reactivity (Hedwall effect). The authors [33] offered the following scheme of BTO thermal transformations in air: 225◦ C

465◦ C

BaTiO(C2 O4 )2 · 4H2 O −→ BaTiO(C2 O4 )2 −→ BaCO3 +TiO2

700−720◦ C

−→

BaTiO3

(1)

As can be seen BMT formation takes place on the third stage at 700–720 ◦ C (accordingly DTA data). In reality to produce pure BaTiO3 (without BaCO3 admixture) possessing perfect structure a 2-step heating process is used with temperature of second stage 900 ◦ C and above [34,35]. Application of microwave heating reduces the temperature of BMT formation below 700 ◦ C [8]. Both above considered approaches (mechanochemical and oxalate methods) allow to lower BMT synthesis temperature noticeably in comparison with usual solid-state process. As a result the prepared BMT powders consist of smaller particles. Thus from the scientific and applied points of view the combination of two above mentioned methods is of interest since milling as it is well known [36] assists destruction of chemical compounds. On the other hand formed products of BTO decomposition (BaCO3 and TiO2 ) possess increased reactivity that can lead to lowering the synthesis temperature of BMT. Therefore the aim of presented work is first of all the investigation of physicochemical transformations which occur during mechanochemical treatment of barium titanyl oxalate under various reaction conditions. Based on the obtained results we also studied principle possibility of BMT formation at lower temperature, suitable for its use both as catalysts (or support of

Table 1 Effect of BTO treatment conditions on some characteristics of the obtained products. Sample

Phase

I1 0 1 /I0 0 2

D1 0 1 (nm)

D0 0 2 (nm)

S (m2 g)

TT-700 TT-800 MChT-A-2 MChT-A-2-550 MChT-A-2-700 MChT-A-2-800 MChT-A-5 MChT-A-5-550 MChT-A-5-700 MChT-A-5-800 MChT-W-5 MChT-W-5-550 MChT-W-5-700 MChT-W-5-800

BMT, BTO BMT BMT BMT BMT BMT BMT BMT BMT BMT BTO, BMT BTO, BMT BMT BMT

100/31 100/26 0/100 100/27 100/30 100/32 0/100 100/29 100/31 100/36 0/35 0/39 100/33 100/35

15.0 16.0 – 19.5 22.0 24.5 – 15.0 16.5 18.5 – – 16.0 19.5

12.0 12.5 34.5 17.0 20.5 22.0 25.5 12.0 13.5 16.5 24.0 25.0 12.5 15.5

7 5 17 15 13 10 23 20 17 14 13 10 8 6

Note: D1 0 1 and D0 0 2 —the size of crystallites calculated towards the planes (1 0 1) and (0 0 2) respectively, S—surface area, I1 0 1 and I0 0 2 —relative intensities of peaks from planes (1 0 1) and (0 0 2), accordingly.

catalysts) and electroceramic materials. The hydrothermal (HTT) and microwave treatments (MWT) were realized for BMT synthesis for comparison. Literature does not provide any information about MChT. HTT and MWT of barium titanyloxalate. 2. Experimental procedures Analytically pure barium titanyloxalate tetrahydrate produced by “Ferro” (Germany) was used as a starting reagent. Mechanochemical treatment was made for 2–5 h with 600 rpm using a planetary ball mill of the Pulverisette-6 type (Fritsch). Ten balls of silicon nitride of 15 mm diameter (total mass of balls—130 g) were used. Air and water were chosen as treatment medium (the samples are designated in the text and Table 1 as MChT-A-2(5) and MChT-W-2(5) correspondingly, i.e. MChT for 2 or 5 h). The amount of BTO undergone MChT was 10 g, the volume of vessel of silicon nitride was 250 ml. The mass BTO/water ratio was 0.1 (amount of added water was 100 ml). Hydrothermal treatment was carried out in steel autoclaves of 45 ml volume at 200 and 250 ◦ C for 5 h (the samples HTT—200 (250), Table 1). For microwave treatment there was used a high-pressure reactor “NANO 2000” (Plazmatronika, Poland) with power 650 W. The treatment temperature was 200 and 250 ◦ C, pressure—45 atm., time—0.5 h (the samples MWT–200 (250), Table 1). The HTT and MWT modifications were carried out in the suspension forms: the weighed amount of BTO was 3 g, the amount of water was 30 ml. Initial BTO was calcined at 700 and 800 ◦ C (samples TT-700 and TT-800 in Table 1). After MChT, HTT and MWT samples were subjected to thermal treatment in air at 550, 700 and 800 ◦ C for 2 h (designations of these samples also include calcination temperature, for instance MChT-A-5-550). The initial substance and the products of its transformations during MChT, HTT and MWT as well as thermally treated samples were subjected to XRD analysis in identical conditions using the diffractometer PW 1830 produced by Philips. Then the sizes of crystallites were calculated from broadening of the most intensive reflex on diffractograms using the Debay–Scherrer equation. Thermal analysis was made using the Derivatograph-C (MOM, Budapest) in the temperature range 20–800 ◦ C with the heating rate 10 ◦ C min−1 , sensitivity—100 mg. Weight of samples was 200 mg. The FTIR spectra in the range 2500–400 cm−1 were registered by using of the spectrometer “Spectrum – One” produced by PerkinElmer (tablets with KBr at the ratio 1:20). The specific surface area S was measured by means of thermal argon desorption method using of “NOVA instruments” (Quantochrome Instruments). Granulometric composition of powders was analyzed by sedimentation in ethylene glycol using the apparatus ZETASIZER 1000 produced by Malvern Instruments (Great Britain).

3. Results and discussion Fig. 1 shows XRD spectra of starting BTO and products of its mechanochemical transformation on air and in water as well as BMT obtained from BTO by usual thermal decomposition in air according to the method described in [32]. As can be seen due to MChT in air complete destruction of crystalline structure of the starting substance—BTO takes place already during 2 h of treatment (curves 1 and 2). Simultaneously the formation of cubic BaTiO3 is observed during of this process that is evidenced by XRD spectra which are typical for this phase. Increase of MChT duration to 5 h

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231

Fig. 2. FTIR spectra of the sample TT-800 (1) and the initial BTO (2).

Fig. 1. Diffractograms of the initial BTO (1) and the samples MChT-A-2 (2) MChT-A-5 (3), MChT-A-5-550 (4), MChT-W-5 (5), and TT 800 (6).

(Fig. 1, curve 3) only insignificantly elevates intensity of all reflexes on diffractograms. Formation of any other phase was not registered. The obtained results indicate the following distinctive features of crystalline structure of MChT products in air (Fig. 1, curves 2 and 3, Table 1): 1) Complete absence of the reflex corresponding to plane (1 0 1) with the interplanar spacing d = 0.284 nm (2 = 31.55◦ ), which usually possesses the maximal intensity I1 0 1 for cubic BMT [3–5] and absence of some other reflexes. In our case the most intensive is the reflex from plane (0 0 2) with d = 0.203 nm (2 = 45.22◦ ). Usually intensity of this peak I0 0 2 averages 30–35% of I1 0 1 . The reflexes corresponding to the values d = 0.234 nm (2 = 38.85◦ ), 0.143 and 0.122 nm are also presented (the last two reflexes are at 2 > 60◦ , therefore they are not presented in the figure). 2) The presence of quite strong background and small halo in the range 2 = 10–40◦ on the diffractograms may indicate the presence of amorphous state either the starting reagent or part of the product obtained due to MChT that was found often in mechanochemical transformations [10,36]. The above mentioned pecularities may be explained as follows. Probably under MChT conditions when the solid phase has undergone strong shear forces a crystalline BMT structure in the strongly stressing state appears. This may be the reason for the absence of some reflexes, even the most intensive, on the diffractograms. Some explanation is provided by the results of the study of these samples using the FTIR method as shown in Figs. 2 and 3. As can be seen the spectrum of the product obtained by MChT in air during 5 h (Fig. 3, curve 1) differs significantly from that of the starting spectrum of BTO (Fig. 2, curve 2): all absorption bands of BTO (wide band 560,

859 and 1056 cm−1 ) are absent in the spectrum of the obtained product. At the same time in the spectrum of this sample—MChTA-5 (analogous spectrum was obtained for the sample treated for 2 h), the absorption bands characteristic of BMT (Fig. 3, curve 1) i.e. 488, 527 and 908 cm−1 are present. Thus the results of XRD and IR spectroscopy can indicate the fact that part of amorphous BMT is present in the products of MChT of barium titanyloxalate. It should be noted that all powders after MChT on air are white in a color that is indirectly evidence of absence of carbonization processes during MChT. Analogous conclusions can be drawn from the data of thermal analysis. Thus Fig. 4 shows the TG and DTA curves for the starting BTO. Their forms are in good agreement with the scheme of BTO thermal decomposition reaction (1) presented above. Thus three stages of mass loss and corresponding endothermic effects with the minima at 183, 370 and 712 ◦ C are observed. At the same time, on the TG and DTA curves of the sample obtained at MChT in air for 5 h there are present two distinct stages of mass loss: the endothermic effect with the minimum at 214 ◦ C corresponds to the first stage and the exothermic effect with the maximum at 540 ◦ C corresponds to the second one (Fig. 5). The presence of exothermic effect at 540 ◦ C allows to suggest that it is connected with crystallization of the amorphous part of BMT which is formed as a result of mechanical treatment of BTO in air atmosphere. The above hypothesis is confirmed by the XRD spectrum of the sample MChT-A-5-550 which was additionally calcined in air at 550 ◦ C for 2 h after MChT (Fig. 1, curve 4). The XRD indicates that due

Fig. 3. FTIR spectra of the sample MChT-A-5 (1) and MChT-W-5 (2).

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Fig. 4. TG and DTA curves for initial BTO.

to such thermal treatment following changes occur: (i) the removal of mechanical stresses from the BMT structure that is evidenced by appearance of all reflexes of BaTiO3 but not single ones and correspondence of their intensities to those described in literature earlier (Table 1. ratio I1 0 1 /I0 0 2 ); (ii) crystallization of amorphous part of BMT formed in the MChT process: decrease in background size on the diffractograms. Well-crystallized BMT of cubic form is created due to growth of crystallites. It should be noted that absolute intensity of reflex from plane 1 0 1 for sample MChT-5-550 (Fig. 1, curve 4) exceeds significantly that for sample TT-800, prepared by means of usual calcinations of BTO (curve 6) although diffractogramms were recorded in identical conditions. This experimental fact can indicate more perfect crystal structure of preliminary activated samples in comparison with that for samples which were not subjected to preliminary MChT. The presence of a single reflex in the range 2 = 45◦ but not a double one (as in the tetragonal modification) indicates the cubic BMT [4]. It should be noted that for the sample calcined at 550 ◦ C on the diffractogramms all reflexes characteristic for BMT with the standard ratio of their intensities I1 0 1 /I0 0 2 are present (Table 1). Moreover, not very intensive (less than 5%) peaks for d = 0.372 and 0.262 nm occur on the diffractogramms of the samples subjected to thermal treatment in air at 550–800 ◦ C. They can be assigned to not decomposing BTO as well as BaCO3 which is usually formed in the synthesis of BMT in air atmosphere [3,5,18]. For decomposition of barium carbonate admixture the authors [5] propose either thermal treatment at the temperature higher

Fig. 5. TG and DTA curves for sample MChT-A-5.

than 900 ◦ C or washing off with nitric acid. In presented paper mechanochemically activated samples were calcined for 2 h at 700–800 ◦ C in the argon stream. This procedure leads to complete disappearance of the above mentioned reflexes. A more complicated picture was obtained for sample undergone MChT in the aqueous medium (Fig. 1, curve 5). As can be seen even after treatment in water for 5 h the diffractogram shows only separate reflexes of barium titanate (BMT) corresponding to d = 0.233, 0.203 nm (2 = 31.83, 45.22◦ ) e.a. with relatively low intensity equal to 10–20% (only the peak corresponding to the interplanar spacing 0.203 nm (2 = 45.22◦ ) possesses 35% intensity). At the same time the strongest reflexes of BMT, i.e. with d = 0.284 and 0.164 nm (2 = 31.55 and 56.28◦ ) are absent (or concealed by the reflexes coming from BTO) as in the case of MChT in air atmosphere. All other reflexes on the diffractogramms refer to the BTO structure whereas sufficiently strong shift of interplanar spacing values compared to the starting BTO samples (in some cases by 0.5–0.8%) as well as significant decrease of their relative intensity, i.e. by 10–30% are observed. These experimentally obtained facts may indicate only partial (contrary to dry activation) destruction of crystalline structure of BTO and its deformation in the course of aqueous MChT. The next thermal treatment of this sample in air at 550 ◦ C (MChT-W5-550, Table 1) causes disappearance of majority of the BTO lines (the most intensive is its reflex with d = 0.335 nm) from the diffractogram and increase of BMT reflex intensity. After calcination at 700 ◦ C (the sample MChT-W-5-700) pure and well-crystallized BMT with the common set of reflexes and ratio of their intensities) is formed (Table 1; diffractograms are not shown here). For the last sample (MChT-W-5-700) the FTIR spectrum with the absorption bands 484, 532, 806, and 905 cm−1 (Fig. 3, curve 2) corresponding to pure BMT was obtained. At the same time for the sample MChTW-5 obtained directly after MChT the spectrum analogous to that of starting BTO was recorded (not shown here). The fact that changes in titanyloxalate structure with MChT in water are smaller compared with the results of its MChT in air atmosphere may be caused by the following reasons: 1) consumption of the large part of energy supplied owing to MChT for heating of medium (water); as a result little part of mechanical energy is used for activation of physicochemical transformations in the solid phase; therefore only partial decomposition of BTO is observed; 2) other mechanism of BTO transformation cannot be realized because barium titanyloxalate is practically insoluble in water and did not undergo hydrolysis in the activation conditions. Table 1 presents the treatment conditions of the obtained samples and some of their characteristics including those for the samples TT-700 and TT-800 synthesized through the thermal decomposition of BTO in air without preliminary MChT. As can be seen during MChT of BTO the BMT is formed which is characterized by much larger specific surface area (S) compared with the sample prepared by usual thermal decomposition of this substance (BTO). With the increase of successive calcination temperature the value S decreases in all cases and the size of crystallites D1 0 1 and D0 0 2 calculated from broadening of reflexes from the planes (0 0 1) and (0 0 2) increases due to recrystallization processes (enlargement of crystallites). This was also observed in the papers of other authors [3,5]. On the other hand, primary crystallites are aggregated to a greater extent, though their sizes are 12–35 nm (Table 1, columns 4 and 5). Thus as follows from the granulometric measurements the sample TT-800 has the grain (aggregate) size in the range 800–1000 nm and the sample MChT-A-5-800 in the range 150–500 nm. With the help of ultrasonic action (apparatus UZDN-1, Ukraine, 0.5 h in the n-pentane medium, frequency—20 KHz) aggregates were disintegrated: for the sample MChT-A-5-800 the grain

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size in the range 80–300 nm was obtained. These results agree with data of work [37]. Ability of mechanochemically activated samples to aggregation has advantage too: according to [38] such samples are characterized by very good capability of compression and sintering which is very important in preparation of electroceramic materials on their basis. From a scientific point of view the comparison of the results of barium titanyloxalate MChT in water with those of its hydrothermal and microwave treatments is important. It follows from the fact that during mechanochemical activation in the aqueous medium the simulation hydrothermal conditions are possible [39,40]. Therefore additional investigations of hydrothermal treatment 200–250 ◦ C (water vapour pressure 16 and 45 atm. for 5 h), and microwave treatment at 200–250 ◦ C (water vapour pressure—45 atm.) of BTO were carried out. In the second case duration of treatment was only 1 h because at MWT the reaction medium is quickly and uniformly heated that leads to intensification of physicochemical transformations [4,41]. Table 2 includes the results of XRD for BTO subjected to HTT and MWT at 200 ◦ C (here interplanar spacings d and relative intensities of reflexes I for all obtained phases are indicated). As follows from the presented results at MWT and HTT drastic changes of crystalline structure are observed: 1) partial destruction of BTO structure though the diffractograms show reflexes of small intensity referring to this phase; 2) formation of low-temperature form of titanium dioxide – anatase; 3) formation of new phases: orthorhombic modification of barium orthotitanate (BOT-o) Ba2 TiO4 (JCPDS 38-1481) and rhom-

Table 2 XRD of the samples prepared by hydrothermal treatment and microwave treatment of barium titanyl oxalate (BTO) at 200 ◦ C. HTT 200 ◦ C

MWT 200 ◦ C

d (nm)

I (%)

Phases

d (nm)

I (%)

Phases

0.467 0.432 0.413

11 20 12

BTO BOT-o BC-r

0.380

13

BTO

0.357

100

0.323 0.317 0.311 0.307 0.296 0.284

16 44 18 18 4 5

BTO, BOT-o BTO, BOT-o BOT-o BOT-o, BTO BTO BOT-o

0.470 0.437 0.416 0.387 0.378 0.374 0.353 0.341 0.322 0.317

19 25 13 46 47 57 69 86 31 100

BTO BTO, BOT-o BTO, BC-r BTO BTO BC-o, BTO Anatase, BOT-o BC-r BTO, BOT-o, BC-o BTO, BOT-o

0.308 0.296 0.284 0.280

55 12 22 11

BOT-o, BTO BTO BOT-o BTO

0.272

12

BTO

0.262

6

BTO, BC-r

0.253 0.246

17 7

BTO, BOT BOT-o

0.267 0.263 0.257 0.252

21 31 7 40

BTO BTO, BC-o, BC-r BTO, BOT-o BTO, BOT-o

0.240

31

BTO

0.232 0.227 0.223 0.219 0.215 0.209 0.200

16 5 11 8 19 13 10

BTO, BOT-o BTO, BC-r BTO, BOT-o BTO, BOT-o, BC-r BOT-o BTO, CB-r BOT-o

0.189 0.181

9 8

0.227 0.222 0.219 0.216 0.209 0.201 0.199 0.195 0.193 0.188 0.181

34 29 33 46 40 17 11 18 23 15 12

BTO, BC-r BTO, BOT-o BTO, BOT-o, BC-r BOT-o, BC-o BTO, BOT-o, BCB-r BC-o, BOT-o BOT-o BTO, BOT-o, BC-o BTO, BOT-o, BC-o Anatase BOT-o

Anatase, BOT-o

Anatase BOT-o

Note: d—the interplanar spacing, I—the relative intensity of reflexes from XRD.

233

Fig. 6. Diffractogram of BTO after MWT at 200 ◦ C and the additional thermal treatment at 700 ◦ C.

bohedral BaCO3 (BC-r) (JCPDS 74-1625) during MWT and orthorhombic BaCO3 (BC-o) (JCPDS 44-1487) during HTT. Based on the reflexes intensity ratio on the diffractograms it can be stated that extent of BTO decomposition at HTT is larger than that at MWT and barium orthotitanate is a main phase in the mixture. Temperature increasing from 200 to 250 ◦ C leads to decomposition of BTO in larger extent and increase of Ba2 TiO4 content in the products of hydrothermal transformations. Besides the specific surface area of the samples reaches 35–55 m2 g−1 . The reaction scheme can be presented as follows: 4BaTiO(C2 O4 )2 → Ba2 TiO4 + 2BaCO3 + 3TiO2 + 8CO + 6CO2

(2)

Thermal treatment at 700 ◦ C of the samples undergone HTT and MWT leads to complete decomposition of BTO and to formation of the two-phase system namely the mixture of orthorhombic modification of barium carbonate and otrhotitanate (Fig. 6). Comparing the results of XRD for the products of BTO MChT in the aqueous medium on the one hand and for its HTT and MWT on the other hand it can be stated that decomposition of barium titanyloxalate proceeds according to different mechanisms. As a result at following thermal treatment in the first case practically pure cubic barium metatitanate and in the second case the mixture of orthorhombic modification of barium orthotitanate and carbonate are formed.

4. Conclusions Mechanochemical treatment of barium titanyloxalate in air for 5 h leads to formation of barium metatitanate with structural defects which transform at next calcination in air already at 550 ◦ C into well-crystallized cubic modification of barium metatitanate, whereas during the thermal decomposition without preliminary MChT this phase is formed only at 800 ◦ C. The product of barium titanyloxalate MChT in the aqueous medium transforms into barium metatitanate after next calcination at 700 ◦ C. All samples of barium metatitanate synthesized by means of MChT are characterized by much larger specific surface area compared with ones prepared by means of usual calcinations of barium titanyloxalate. Results obtained in presented work can be useful for synthesis of BMT which can be applied for different purposes. Preliminary hydrothermal and microwave treatments of barium titanyloxalate promote formation of barium orthotitanate and carbonate mixture.

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