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Binary borate and polytitanates compounds have currently been of considerable interest to the scientific community due to unique properties of barium titanate, barium polytitanates [Kong, 2010; Wakino, 1990], betta barium borate[Chen, C. & Liu, 1986] and recently revealed high negative thermal expansion of crystallized barium di-borate glass samples [Hovhannisyan, 2006]. It has also been revealed recently, that the single crystal of barium di-tanate is high-temperature ferroelectric with Curie temperature equal to 450-500°C [Waghmare et al., 2004; Akishige et al., 2006]. Park with co-authors reported a new barium titanium oxoborate crystal of Ba3Ti3O6(BO3)2 and found out that the second harmonic generation (SHG) efficiency of this crystal is equal to 95% of the well known LiNbO3 [Park et al., 2004]. These data have even strengthened the interest of studying the ternary BaO-TiO2-B2O3 system even more.
Binary BaO-TiO2 and BaO-B2O3 systems are investigated rather in detail by various authors. The following eight binary barium titanates known in the BaO-TiO2 system are: Ba2TiO4 (2BaT), BaTiO3 (BaT), BaTi2O5 (Ba2T), Ba6Ti17O40 (6Ba17T), Ba4Ti13O30 (4Ba13T), BaTi4O9(Ba4T), Ba2Ti9O20(2Ba9T) and BaTi5O11 (Ba5T) [Rase & Roy, 1955; O’Bryan & Thomson, 1974]. Levin with co-authors have studied the BaO-B2O3 system and constructed its melting diagram. They discovered four Ba3B2O6 (3BaB), BaB2O4 (BaB), BaB4O7(Ba2B) and BaB8O13 (Ba4B) compounds [Levin & McMurdie, 1949; Levin & Ugrinic, 1953]. Hubner synthesized three new Ba4B2O6 (4BaB), Ba2B2O5(2BaB) and Ba2B10O17 (2Ba5B) compounds [Hubner, 1969]. However, these compounds are not visible on the BaO-B2O3 systems diagram constructed earlier by Levin with co-authors [Levin & McMurdie, 1949; Levin & Ugrinic, 1953].
Pavlikov with co-authors have studied the TiO2-B2O3 system and didn’t reveal binary compounds in it [Pavlikov et al., 1976]. Simple eutectic which is very close to B2O3 (~2.9 mol% TiO2) was found and the presence of liquid immiscibility is supposed in the field of compositions containing 25-55 mol% of TiO2 [Pavlikov et al., 1976]. One binary oxygen containing titanium boron compound- titanium borate, synthesized by Schmid is known now [Schmid, 1964]. Also the formation of TiBO3 was revealed during interaction between borate glass melts and titanium alloy [Brow & Watkins, 1987].
Vicat & Aleonard for the first time have obtained BaTi(BO3)2 (BaTB) ternary compound related to “Nordenskiöldine” group borates [Vicat & Aleonard, 1968]. Millet and co-authors studied BaTiO3-TiO2-BaB2O4 subsystem in BaO-TiO2-B2O3 system and have found the second incongruent melting at 950°C ternary Ba2Ti2B2O9 (2Ba2TB) compound. Between 950-960 °C it decomposes with formation BaTiO3 and liquid [Millet et al., 1986]. Authors [Zhang et al., 2003] have studied subsolidus phase relations in the ternary BaO-TiO2-B2O3 system. They confirmed only the existence of two known BaTB and 2Ba2TB ternary compounds in this system. The pure 2Ba2TB phase has not been obtained under authors experiment conditions [Zhang et al., 2003]. It was in equilibrium with BB, BT, B2T and 4B13T.
However, Park with co-authors considered, that the Ba2Ti2B2O9 composition was formulated incorrectly and should be re-formulated as Ba3Ti3B2O12, or more precisely Ba3Ti3O6(BO3)2 [Park et al, 2004]. Kosaka et al. have confirmed the data of Barbier's group and have shown necessity of reformulation of Ba2Ti2B2O9 (2Ba2TB) compound as Ba3Ti3O6(BO3)2 (3Ba3TB) [Kosaka et al., 2005]. They have synthesized new glass ceramic composition with 3Ba3TB crystalline phase in the BaO-TiO2-B2O3 system and have found that its powder sample SHG intensities is 68 times as large as a-quartz powders.
Sholokhovich & Varicheva have studied [50PbO+50B2O3, mol%]-PbTiO3 -BaTiO3-Ba(BO2)2 section of PbO-BaO-B2O3-TiO2 fourfold system and observed eutectic at 32 mol% BaTiO3 (m.p.906°C ) in the pseudo-binary Ba(BO2)2-BaTiO3 system [Sholokhovich & Varicheva, 1958]. Goto & Cross studied pseudo-binary BaTiO3-BaB2O4 system for BaTiO3 single crystals growth and also found simple eutectic with m.p.942°C at 32 mol% BaTiO3 [Goto & Cross, 1969]. Simple eutectic with m.p.1010°C has also been found in BaB2O4-BaTi(BO3)2 pseudo-binary system at 32 mol% BaTi(BO3)2 [Hovhannisyan, 2004].
Interest to glass formation in ternary barium titanium borate system is mainly connected with developing the new composition of glass ceramics on the basis of barium titanates [Matveev et al, 1966;
Bhargava et al., 1988a, 1988b, 1988c; Cerchez et al., 2000; Boroica et al., 2004], betta barium borate [Pernice et al., 1998; Feitosa et al., 2006] and 3Ba3TB [Kosaka et al., 2005]. We are fully confident, that experts and researchers will show interest to ceramics and glass ceramics on the basis of binary barium titanates and ternary barium boron titanates for a long time. However, it will be difficult to them to develop new practical compositions without presence of the first of all the phase diagram and glass forming diagram.
Hovhannisyan with co-workers have made the first attempt of the ternary BaO-TiO2-B2O3 system both glass forming and phase diagram construction [Hovhannisyan et al., 2008]. A large area of glass formation has been revealed in the BaO-TiO2-B2O3 system depending on melt's cooling ways. The new incongruent melted ternary Ba2TiB2O7 (2BaTB) compound has been reveled during the same composition glass crystallization. Clear correlation between glass forming ability and eutectic and peritectic areas has been observed in the investigated BaO-TiO2-B2O3 system [Hovhannisyan et al., 2008].
However, our further studies of glasses and glass ceramics in this system have shown necessity of both glass forming and phase diagram correction in the ternary BaO-TiO2-B2O3 system. Another aim of this work is both known and novel stoichiometric ternary barium titanium borates compounds investigations in glassy, glass ceramic and ceramic states. On the other hand we are seriously interested in giving additional information concerning the existence of two Ba2Ti2B2O9 and Ba3Ti3O6(BO3)2 compounds.
About two hundred samples of various binary and ternary compositions have been synthesized and tested in BaO-TiO2-B2O3 system. Compositions were prepared from “chemically pure” grade BaCO3, H3BO3 and TiO2 at 2.5-5.0 mol % intervals. The most part of samples has been obtained as glasses by various cooling rates depending on melts glass forming abilities: as bulk glass plates with thickness 6,5 ÷7mm by casting on metallic plate (up to 10 K/s), as monolithic glass plates with thickness up to 3mm by casting between two steel plates(~102 K/s), and glass tapes through super cooling method ( 103÷104 K/s). The glass melting was performed at 1400-1500°C for 30-60 min with a 25–30 g batch in a 50 ml uncovered Pt crucible, using an air atmosphere and a “Superterm 17/08” electric furnace. Chemical composition of some glasses was controlled and corrected by results of the traditional chemical analysis. The final analysis results indicate a good compatibility of calculated and analytical values of B2O3, BaO and TiO2.
Samples of compositions laying outside of a glass formation field or having high melting temperature, have been obtained by solid-phase synthesis. Mixes (15-20 g) were carefully frayed in an agate mortar, pressed as tablets, located on platinum plates and passed the thermal treatment in “Naber”firm electric muffles. After regrinding powders were tested by DTA and X-ray methods. The synthesized samples of binary barium borate system compositions containing 60 mol% and more of BaO and also compositions containing over 90mol % B2O3 had very low chemical resistance and were hydrolyzed on air at room temperature. In this connection the synthesized samples were kept in a dryer at 200°C.
DTA and X-ray diffraction data of glass and crystallized glass samples have been used for phase diagram construction in the ternary BaO-TiO2-B2O3 system. The DTA analysis (platinum crucible, powder samples weight ~600 mg, heating rates 7.5 or 15K/min) on Q-1500 type derivatograph were carried out. Glass transition -Tg, crystallization peaks -Tcr, melting -Tm and liquidus -TL temperatures have been determined from DTA curves. Reproducibility of temperatures effects on DTA curves from melting to melting was ±10K. The accuracy of temperature measurement is ±5 K.
Thermal expansion coefficient (TEC) and glass transition temperature (Tg) measurements were made on a DKV-4A type vertical quartz dilatometer with a heating rate of 3K/min. Glass samples in the size of 4×4×50 millimeters have been prepared for TEC measurement. The dilatometer was graduated by the quartz glass and sapphire standards. The TEC measurement accuracy is ±(3÷4)∙10-7K-1, Tg ±5 °C.
X-ray patterns were obtained on a DRON-3 type diffractometer (powder method, CuKα–radiation, Ni-filter). Samples for glass crystallization were prepared with glass powder pressed in the form of tablets. Crystallization process was done in the electrical muffles of “Naber” firm by a single-stage heat treatment. This was done within 1-60 hours around a temperature at which the maximum exothermal effects on glasses by DTA were observed.
Crystalline phases of binary and ternary compounds formed both at glasses crystallization and at solid-phase synthesis have been identified by using JCPDS-ICDD PDF-2 release 2008 database [ICDD, 2008].
Computerized methodic of ferroelectric hysteresis test and measurement of ferroelectric properties such as coercive field and remanent polarization at wide temperature (up to 250°C and frequency (10-5000Hz) ranges was used. Methodic based on the well known Sawyer – Tower’s [Sawyer & Tower, 1930] modified scheme, which is allowing to compensate phase shifts concerned with dielectric losses and conductivity. The desired frequency signal from waveform generator is amplifying by high voltage amplifier and applying to sample. The signals, from the measuring circuit output, proportional to applied field and spontaneous polarization are passing throw high impedance conditioning amplifiers, converting by ADC and operating and analyzing in PC. The technique allows to perform tests of synthesized glass ceramics obtained by means of controlling crystallization of thin (above 30 micrometer thick) monolithic tape (film) specimens by applying up to 300kV/cm field to our thin samples (~50 micrometer thick) and obtain hysteresis loops for wide diversity of hard FE materials.
3.1. Glass forming and phase diagrams of the of the BaO-TiO2-B2O3 system
The traditional method of phase diagram construction based on solid-phase sintered samples investigation takes long time and is not effective. The glass samples investigation technique is progressive, because the DTA curves have registered all processes taking place in glass samples, including the processes of glass crystallizations, quantity of crystal phases and temperature intervals of their formation and melting.
However, inadequate amount of glass samples restrict their use during phase diagram construction. The super-cooling method promotes the mentioned problem solving and open new possibilities for phase diagrams construction.
3.1.1. Glass forming diagram of the BaO-TiO2-B2O3 system
Figure 1 shows the experimental data on glass formation in the BaO-TiO2-B2O3 system obtained by different authors from 1957 to 2008 [Imaoka & Yamazaki, 1957; Matveev et al., 1966; Bhargava et al, 1987; De Pablos & Duran, 1993; Kusumoto & Sekiya, 1994; Hovhannisyan et al., 2008]. For defining the glass forming ability of the pointed system, the authors of the mentioned works used different amounts of melt, glass melting crucibles, temperature–time melting regimes, and technological methods of melt cooling. The obtained data are hardly comparable and are “torn away” from the two main factors in glass formation: the liquidus temperature (TL) and melt cooling rate.
Figure 2 shows corrected glass formation diagram in the BaO-TiO2-B2O3 system based on phase diagrams of the BaO–B2O3, BaO–TiO2, and B2O3–TiO2 binary systems and controllable melt cooling rates. Using the term “diagram,” but not the glass formation region, we take into account the interrelation between the phase diagram and the glass forming ability of the system.
Super cooling technique constructed by our group allowed to expand the borders of glass formation in studied system. The largest glass forming area have been obtained under high melts cooling rates equal to (103-104 ) K/s (Fig.2-1). It includes: compositions content 2.5÷3.0 mol% TiO2 around eutectic area e9 (~2.9 mol% TiO2) with m.p. ~450°C in the binary B2O3–TiO2 system [Pavlikov et al., 1976]; compositions content 30÷35 mol% BaO around eutectic area e7 (~31.5 mol% BaO) with m.p. 1317°C in the binary BaO–TiO2 system[Rase & Roy, 1955; O’Bryan &Thomson, 1974], which include BaTi2O5 composition. Area of glass formation from both these areas moves to eutectic areas e1 (16,5 mol% BaO) with m.p. 878°C [Levin & McMurdie, 1949; Levin & Ugrinic, 1953] and ~ 31.5 mol%B2O3 content compositions in the binary BaO–B2O3 system and includes Ba2B2O5 and BaB2O4 compositions (Fig.2-1). The transparent glass tapes have been obtained firstly for Ba3Ti3B2O12 (3Ba3TB) composition by its melt high cooling rate (103-104 ) K/s (Fig.2-1). The 3Ba3TB glass composition (Table 3) is located practically on the border of glass formation (Fig.2-1).
Figure 1.
Glass forming regions in the BaO-TiO2-B2O3 system according to the data of the authors: 1- [Imaoka & Yamazaki, 1957]; 2- [Matveev et al., 1966]; 3-[Bhargava et al, 1987]; 4-[De Pablos & Duran, 1993]; 5- Kusumoto & Sekiya, 1994]; 6-[Hovhannisyan et al., 2008].
Loss of melts cooling speed to ~102 K/s has naturally led to narrowing of glass formation area (Fig.2-2). However, this cooling rate is enough for monolithic glass plates with thickness up to 3mm fabrication by melts casting between two steel plates. The ternary BaTi(BO3)2 (BaTB), Ba2Ti2B2O9 (2Ba2TB) and Ba2TiB2O7 (2BaTB) compounds have been obtained as bulk glass samples by this way. It was big surprise, that monolithic glass samples have been obtained for ternary glass compositions close to e5 eutectic area(37.5 mol%B2O3) with m.p.915°C in the binary BaO–B2O3 system and containing about 3÷4 mol% TiO2 (Fig.2-2).
The further reduction of melts cooling rate to ~ 10 K/s has allowed to reveal field of glass compositions with low crystallization ability and stable glass formation in the studied ternary BaO-TiO2-B2O3 system (Fig.2-3).
Figure 2.
Glass forming diagram in the BaO-B2O3-TiO2 system depending of melts cooling rates: (103-104 ) K/s; 3~102 K/s; 2-up to 10K/s;
Earlier we have reported about transparent glass sheets formation in the field of compositions being between eutectic e1 in the binary B2O3–TiO2 system and ternary eutectic E1[ Hovhannisyan et al., 2008]. However, the present studies have not confirmed previous data. It was possible to obtain transparent glass sheets only in the narrow field of compositions close to eutectic e1 area (Fig.2).
3.1.2. DTA study of the stoichiometric glass compositions in the BaO-TiO2-B2O3 system
The glass nominal compositions in the BaO–TiO2–B2O3 system examined in the present study and their DTA and dilatometric characteristics are given in Table 1. DTA curves for glasses corresponding to ternary and binary stoichiometric compositions are shown in Fig. 3, giving the peaks due to the glass transition, crystallization, melting, and liquidus temperatures.
On the DTA curve of the 33.33BaO ∙ 33.33TiO2 ∙ 33.33B2O3 (mol%) glass composition strong exothermic effect with maximum at 680ºС and endothermic effect with minimum at 1080ºС were observed, which show the crystallization and congruent melting of one well known crystalline BaTiB phase (Fig. 3, curve 1).
The similar picture is seen on the DTA curve of the 42.85BaO ∙ 42.85TiO2 ∙ 14.28 B2O3(mol%) glass composition corresponding to stoichiometric 3Ba3TB crystalline compound: we have strongly expressed exothermic effect at 625 ºС and endothermic effects at 975ºС, which show the crystallization and melting of one crystalline phase (Fig. 3, curve 3).
On the DTA curve of the 40BaO ∙ 40TiO2 ∙ 20B2O3 (mol%) glass composition three exothermic effects clear observed: two effects at 640ºС(small) and at 660ºС(high) are combined and third is weakly expressed at 690ºС, which show processes of glass crystallization and probably of three phases formation (Fig.3, curve 2). Endothermic effect at temperature interval 955-1020 ºС (minimum at 975 ºС) is connected with the formed crystalline phases melting.
Table 1.
Chemical compositions, derivatographical (glass transition -Tg, crystallization peak -Tcr, melting -Tm, liquidus temperature-TL) and dilatometric characteristics (glass transition temperature -Tg, thermal expansion coefficient -TEC) of BaO – TiO2 – B2O3 system glasses.
Two exothermic effects is seen on the DTA curve of the 50BaO ∙ 25TiO2 ∙ 25B2O3 (mol%) glass composition: strongly expressed effect at 585 ºС and small diffused effect with maximum at 740°C(Fig. 3, curve 4). Both effects are connected with two crystalline phases formation. According to [Hovhannisyan et al., 2008] the first phase is new crystalline compound, formulated by us as Ba2TiB2O7. The 2BaTiB composition melted incongruently at 940 ºС with melt and BaTiO3 formation. The second weakly expressed endothermic effect on DTA curve at 1110ºС is associated with BaTiO3 dissolution in a melt (Fig. 3, curve 4).
On the DTA curve of the 33.3BaO ∙ 66.67TiO2 (mol%) glass composition strongly expressed exothermic effect with maximum at 740ºС and endothermic effect with minimum at 1320ºС were observed, which show the crystallization and melting of one crystalline phase (Fig.3, curve 5). According to [Rase & Roy, 1955] the 33.3BaO ∙ 66.67TiO2 (mol%) composition is melted incongruently at 1310ºС with melt and BaTiO3 formation.
3.1.3. TEC study of the stoichiometric glass compositions in the BaO-TiO2-B2O3 system
The isolines diagram of BaO-TiO2-B2O3 system glasses TEC values is given on Fig.4. It is clear observed common regularity, that the increase of barium oxide amounts in glasses of binary BaO-B2O3 system leads to increase of glasses TEC values. The same tendency is observed for glasses of ternary compositions: increasing of BaO amounts leads to increase glasses TEC values from 60 to 120 ∙ 10-7К-1. The substitution of B2O3 for TiO2 in the area of low BaO content glass compositions (20-25 mol%) practically does not influence on the their TEC value. The same tendency is observed for the high BaO content glass compositions (55-60 mol%). It is seen in the central area of compositions that TEC values increase with the substitution of B2O3 for TiO2. However, the major factor influencing on TEC value of studied glasses is the BaO amount in their compositions (Fig.4).
TEC values of glasses corresponding to the ternary barium titanium borates given in Table 3. The glass composition corresponds to BaTiB2O6 (33.33BaO∙33.33TiO2∙33.33B2O3, mol%) has TEC=88∙10-7К-1 and Tg=555°C calculated from dilatometric curve. Reduction the B2O3 and TiO2 amount together with increasing of BaO amounts in glass compositions leads to increase TEC and reduction Тg values: for glass composition 40BaO∙40TiO2∙20B2O3, mol % (Ba2Ti2B2O9) TEC=107∙10-7К-1 and Тg= 570°C; for glass composition 50BaO∙25TiO2∙25B2O3, mol % (Ba2TiB2O7) TEC=115∙10-7К-1 and Тg=512°C (Table 3).
3.1.4.1. Phase diagram of the pseudo-binary BaTiO3 -BaTi(BO3)2 system
The introduction of BaTB compound in pseudo-binary BaTiO3 -BaTi(BO3)2 system sharply reduced the melting point of initial barium titanate, reduced the crystallization abilities and resulted in the formation of simple eutectic area at 72 mol% BaTB content (38 BaO ∙ 38 TiO2 ∙ 24 B2O3, mol%) with melting point 1020°C(Fig.5 A).
Available inconsistent data about existence of Ba2Ti2B2O9 crystalline compound promoted more careful study of the BaTi(BO3)2 –BaTiO3 pseudo binary system. High sensitivity of our DTA equipment have allowed to reveal temperatures intervals of processes taking place in initial glass powder samples. Existence on DTA curve of 42.85BaO ∙ 42.85TiO2 ∙ 14.28B2O3 (mol%) glass composition corresponding to stoichiometric 3Ba3TB crystalline compound only one strongly expressed exothermic (625 ºС) and endothermic (975 ºС) effects showed on existence of one crystalline phase. Really, X-ray analysis of products of 42.85BaO ∙ 42.85TiO2 ∙ 14.28B2O3 (mol%) glass powder samples crystallized in an interval 600-900 ºС has revealed presence of only one 3Ba3TB crystalline phase (Fig.6, curves 1-4). X-ray diffraction patterns of glass crystallization products identification have shown their full conformity with the known references data [Park et al., 2004;ICDD, 2008 , File # 074-4273]. 3Ba3TB compound is stable up to 950 ºС. It decomposes on BaT and BaTB in temperature interval 950-1020 ºС (Fig.5, A; Fig. 6, curve 5). The BaTiO3 and melt formation is the result of 3Ba3TB composition incongruent melting at temperature higher 1020 ºС (Fig 6, curve 6). We have revealed that the 3Ba3TiB crystalline compound melted incongruently at 975 ºС, with the formation of melt and barium titanate. The dissolution of these phases in a melt lead to the appearance on a DTA curve of the second, weakly expressed endothermic effect in an interval 1020-1260ºС (Fig.3, curve 3).
Figure 5.
Phase diagram of the pseudo-binary BaTiO3 - BaTi(BO3)2 system (A) and temperatures intervals of ternary Ba3Ti3B2O12 (B) and Ba2Ti2B2O9 (C) compounds existence on diagram.
Figure 6.
XRD-patterns of the crystallized powder glass samples corresponding to Ba3Ti3B2O12 composition: curve 1- 600°C 60h; curve 2- 660°C 24h, curve 3- 700°C 24h, curve 4- 900°C 24h, curve 5- 950°C 24h, curve 6- 1050°C 24h (samples 2-6 have been water quenched from heat treatment temperature).
The other picture was observed at the 40BaO ∙ 40TiO2 ∙ 20B2O3 (mol%) glass composition crystallization. X-ray identification of products of 40BaO ∙ 40TiO2 ∙ 20B2O3 (mol%) glass crystallization at temperature interval 640÷660 ºС (first and second exothermic effects expressed on its DTA curve) within 24h has shown presence of new unknown crystalline phase in both samples. In our point of view it is new crystalline Ba2Ti2B2O9 compound, which formed as single phase at the same composition glass crystallization. The x-ray powder diffraction patterns of new crystalline Ba2Ti2B2O9 phase could be indexed on a orthorhombic crystal symmetry with lattice cell as follows : a=9.0404 Å, b=15.1929 Å, c=9.8145 Å; unit cell volume V=1348.02ų, Z =6, calculated density (D calc.)= 3.99g/cm³; D exp.=3.25g/cm³; α;β;γ =90,00°(Table 2).
Table 2.
X-ray characteristics of Ba2Ti2B2O9 crystalline compound obtained at 40.0BaO ∙ 40.0TiO2 ∙ 20.0B2O3(mol%) glass composition crystallization at 640°C, 24 hours.
3.1.4.2. Phase diagram of the pseudo-binary BaB2O4-BaTiO3 system
Study of pseudo-binary system BaB-BaT has revealed some interesting regularity. The liquidus curve constructed by us is the same as constructed by the Goto & Cross [Goto & Cross, 1969] (Fig.7). We have confirmed presence of pseudo-binary eutectic point with m.p.940°C containing 32 mol% BaTiO3 (Fig.7). We have confirmed also information about existence of new crystalline Ba2TiB2O7 (2BaBT) compound, which has been reported earlier at this system glasses crystallization [Hovhannisyan et al., 2008]. Strong exothermic effect at narrow temperature interval 560-590°C with maximum at 585°C is observe on DTA curve of 50.0BaO ∙ 25.0TiO2 ∙ 25.0B2O3 (mol%) glass composition (Fig.3, curve 4). The 2BaBT composition has incongruent melting at 940°C and decomposes on BaT and melt. Its liquidus temperature obtained from DTA curve and is equal to 1150°C(Fig.3, curve 4). The 2BaBT compound is not stable and is observed in narrow temperature interval (570-650°C) (Fig.7. A).
Figure 7.
Phase diagram of the pseudo-binary BaB2O4- BaTiO3 system (A) and position of Ba2TiB2O7 compound on it (B).
The pure 2BaBT phase crystallizes from the same glass composition crystallization at 585°C, 24 hours. The X-ray characteristics of Ba2TiB2O7 were determined and are given in Table 3. The X-ray powder diffraction patterns of 2BaBT could be indexed on a rhombic crystal symmetry with lattice cell as follows : a=10.068 Å, b=13.911 Å, c=15.441 Å; unit cell volume V=2629.17ų, Z =12, calculated density (D calc.)= 4.23g/cm³; D exp.=4.02g/cm³; α;β;γ =90,00°.
3.1.4.3. Phase diagram of the BaO-TiO2-B2O3 ternary system
First of all we have deleted eutectic point e1, which has been for the first time wrongly put by Levin with co-workers on the binary BaO-B2O3 diagram [Levin & McMurdie, 1949; Levin & Ugrinic, 1953], and then is repeated in our recent publications [Hovhannisyan, R. et al, 2008, Hovhannisyan, M. et al, 2009]. Such imperfect data very often committed many authors first of all at binary borate system diagram constructions [ ACerS & NIST, 2004]. Because, this point indicate only sharp increase of liquidus temperature which is connected with stable phase separation, typical for many binary borate systems (Fig.8).
Table 3.
X-ray characteristics of Ba2TiB2O7 crystalline compound obtained at 50.0BaO ∙ 25.0TiO2 ∙ 25.0B2O3(mol%) glass composition crystallization at 585°C, 24 hours
Figure 8.
Phase diagram of the BaO-TiO2-B2O3 system
Seven ternary eutectic points E1-E7 have been reveled as result of phase diagram construction (Fig.8, table 3). The phase diagram evidently represents interaction of binary and ternary compounds taking place in the pseudo-ternary systems. The ternary eutectic E1 with m.p. 850°C has been determined among Ba4B and BaTB compounds and TiO2; ternary eutectic E2 with m.p. 835°C has been formed among Ba4B, 2Ba5B and BaTB compounds; ternary eutectic E3 with m.p. 850°C has been formed among 2Ba5B, Ba2B and BaTB compounds; ternary eutectic E4 with m.p. 860°C has been formed among Ba2B, BaB and BaTB compounds; ternary eutectic E5 with m.p. 865°C has been formed among BaT, 2BaB and BaB compounds; ternary eutectic E6 with m.p. 930°C has been formed among BaT, BaB and BaTB compounds; ternary eutectic E7 with m.p. 1000°C has been formed among BaT, BaTB compounds and TiO2 (Fig8, Table 4).
Clear correlation between glass forming ability and both binary and ternary eutectic areas has been observed in the investigated ternary system (Fig.2).
3.2. Crystallization behavior of the stoichiometric glass compositions in the BaO-TiO2-B2O3 system
3.2.1. Crystallization behavior of the stoichiometric glass BaTi(BO3)2 composition
The BaTi(BO3)2 ternary compound (BaTB) is related to “Nordenskiöldine” group borates with common formula Me2+Me4+B2O6 with well known dolomite-type structure [Vicat & Aleonard, 1968; Bayer, 1971]. The “layer-type” structure of calcite and dolomite is
Point
Tm , (°C)
Composition, mol% B2O3:BaO:TiO2
E1
850
76.0:20.0:4.0
E2
835
75.0:21.0:4.0
E3
850
63.5:32.0:4.5
E4
860
58.0:38.0:4.0
E5
865
35.0:62.0:3.0
E6
930
31.5:45.0:23.5
E7
1000
22.6:32.1:45.3
Table 4.
The melting temperature (Tm) and compositions for ternary eutectic points in the BaO-B2O3-TiO2 system
responsible for the strong anisotropy of the “Nordenskiöldine” group borates [Bayer, 1971]. It is very stable compound occupied dominating position in BaO-TiO2 -B2O3 system phase diagram (Fig.8). It has congruent character of melting at 1080oC (Fig.3, curve1). We have given a preference to study the process of directed crystallization of BaTiB composition based on above stated.
Thermal treatment at 670-690°C, 1 h is enough for full crystallization of the pressed powder glass samples. X-ray diffraction patterns of crystallization products (Fig.9, curve1) are identical to the references data [Vicat & Aleonard, 1968; ICDD, 2008, File # 35-0825].
The other pictures were observed for monolithic samples (Fig.9, curve 2). X-ray diffraction patterns determined from crystallized (630°C 4h+ 690°C 24h) tape samples surface indicated reorientation of crystalline structure, leading to increase of intensity of following reflexes : 5.47 Å (003) from 7% up to 70% (10 time), 2.51 Å (110) from 35 to 100%(3 time), 3.85 Å (102) from 75 to 100%.
At monolith glass sample crystallization at 630°C 4h+ 690°C 12h under direct current (DC) voltage 3.0 kV/cm X-ray diffraction patterns of samples surface again indicate reorientation of crystalline structure: reflexes 5.47 Å (003) decrease from 70% to 46%, 3.85 Å (102) decrease from 100 to 60%, 2.51 Å (110) from 100 to 42%, and reflex 2.97 Å (003) again began 100% (Fig.9, curve3).
X-ray diffraction patterns of BaTB glass tape sample surface crystallized at 630°C 12h+ 690°C 12h (Fig.8, curve4) indicate very strongly change of crystalline structure in relation to a sample received by a traditional powder method: reflex 5.47 Å (003) increase from 7% to 100 %, 3.85 Å (102) decrease from 100 to 10%, 2.51 Å (110) decrease from 100 to 42%, 2.97 Å (003) decrease from 100 to 35%, 2.73 Å increase from 3 to 51%, 1.82 Å increase from 3 to 43% again began 100% (Fig.9, curve4). Part of reflexes practically disappeared (are not visible) at the given regime of X-ray record: 2.73, 2.28, 2.15, 2.10, 2.06, 1.92, 1.61, 1.52 Å.
X-ray diffraction patterns of BaBT glass tape sample surface crystallized under DC 3kV/cm at 630°C 1h+ 690°C 4h again indicate reorientation of crystalline structure (Fig.9, curve5).
Processes of reorientation of the crystal structure, similar occurring with a monolithic sample are observed: reflexes 5.47 Å (003) sharply decrease from 100% to 38%, reflex 2.97 Å (003) again began 100%, 3.85 Å (102) increase from 10 to 58%, 2.51 Å (110) increase from 42 to 78% (Fig.9, curve5). Well observable reflexes 2.72, 2.62, 2.15, 2.10, 1.92, 1.61, 1.60, 1,52 Å have again appeared. Such impression is created, that under DC action the re-oriented structure tends to return to structure inherent in the initial sample received on powder technology.
Figure 9.
XRD-patterns of the BaTB samples and [hkl]-indices attributed to the peaks of the BaTiB2O6 crystallized glasses:curve 1- pressed powder sample crystallized at 690°C 1h.curve 2- monolithic glass sample crystallized at 630°C 4h+ 690°C 12h; curve 3- monolithic glass sample crystallized at 630°C 4h+ 690°C 12h under DC 3kV/cmcurve 4- tape glass sample crystallized at 630°C 12h+ 690°C 12hcurve 5- tape glass sample crystallized at 630°C 1h+ 690°C 4h under DC 3kV/cm
3.2.2. Crystallization behavior of the stoichiometric glass Ba3Ti3O6(BO3)2 composition
Having confirmed existence of 3Ba3TiB there was a necessity to study temperature intervals of its stability. We’ll try to do it through glass powder samples crystallization using its DTA data (Fig.3, curve 3). Formation of pure 3Ba3TiB compound clear observed on X-ray diffraction patterns of the same composition glass powder samples crystallization at 600°C 60h - temperature of the crystallization beginning on the DTA curve (Fig.6, curve 1). The further increasing of thermal treatment temperatures (660, 700, 800 and 900°C) lead to indication only pure 3Ba3TiB crystalline compound in products of glass crystallization (Fig.6, curves 2-4). X-ray diffraction patterns of crystallization products (Fig.6, curves 1-4) are identical to the references data [Park et al., 2004; ICDD, 2008, File # 074-4273]. We have revealed that the 3Ba3TiB compound in an interval 950-1020°C decomposes with BaTiO3 and BaTB formation (Fig.6, curve5). The 3Ba3TiB compound is melted incongruently at 975°C (Fig.3, curve 3), with formation of melt and BaTiO3 at temperatures higher 1020 °C (Fig.6, curve 6).
3.2.3. Crystallization behavior of the stoichiometric glass Ba2Ti2B2O9 composition
Denying of existence of (2Ba2TiB) stoichiometric compounds from Barbier group [Park et al., 2004] after Ba3Ti3B2O12 (3Ba3TiB) synthesis and characterization have increased an intrigue around 2Ba2TiB compound. According to references data both compounds had Hexagonal cell and very closed cell parameters: a = 8.7110 Å, c = 3.9298 Å for 3Ba3TiB) [ICDD, 2008, File#074-4273] and a= 8.7210 Å, c = 3.933 Å for 2Ba2TiB [Millet et al., 1986]. It was impossible to pure synthesis of both compounds through solid phase reaction. We’ll try to do it through glass tapes crystallization.
It was big surprising for us, when on the DTA curve of the 40BaO ∙ 40TiO2 ∙ 20B2O3 (mol%) glass composition three exothermic effects have been observed: two effects at 640ºС (small) and at 660ºС (high) are combined and third is weakly expressed at 690ºС (Fig.3, curve 2). X-ray identification of products of 40BaO ∙ 40TiO2 ∙ 20B2O3 (mol%) glass composition crystallization at temperatures 640 and 660 ºС within 24h has shown presence of new unknown Ba2Ti2B2O9 crystalline phase in both samples(Fig.10, curves 2, 3). Its X-ray characteristics have been determined and are given in Table 2. X-ray identification of crystallization products have shown of 2Ba2TB phase formation starting from 600 ºС (Fig.10, curve1).
The third exothermic effect on 2Ba2TiB glass DTA curve (Fig.3, curve 2) at 690 ºС is connected with its decomposition and 3Ba3TiB, BaTiB compounds formation (Fig.10, curves 4, 5). Process of 2Ba2TB sample decomposition is continues up to 950 ºС 24h. At this temperature the 3Ba3TiB phase is disappear and the BaTiO3 phase starts to appear together with BaTiB phase (Fig.10, curve 6). And finally we observed disappearance of both 3Ba3TB and BaTB phases at temperatures higher 1020°C (Fig.10, curve7).
The strong endothermic effect of melting with minimum at 975 is observed on DTA curve (Fig.3, curve 2). According to references data [Millet et al., 1986] the 2Ba2TiB composition between 950-960 °C decomposes with BaTiO3 and liquid formation. In our cases together with BaTiO3 we have identified the BaTB in temperature interval 950-1020°C (Fig.10, curve 6), and BaTiO3+ melt us results of incongruent melting at temperatures higher 1020 °C (Fig.10, curves 7).
The same picture as for powder samples is observed for crystallized glass tapes of 2Ba2TB composition. We have X-ray amorphous transparent tape glass sample after thermal treatment at 600°C 6h (Fig.11, curve1). Only new, pure 2Ba2TB phase is formed at next steps of thermal treatment: 640°C 24h, and 660°C 24h (Fig.11, curves 2,3). The 2Ba2TB compound decomposes with 3Ba3TB and BaTB phases formation at thermal treatment at 700°C 24h (Fig.11, curve 4).
3.2.4. Crystallization behavior of the stoichiometric glass Ba2TiB2O7 composition
X-ray identification of glass powder samples of 50BaO ∙ 25TiO2 ∙ 25B2O3 (mol%) compositions thermal treated in an interval 570- 650 24 h have shown pure 2BaTB compound formation (Fig.12, curve1). The 2BaTB compound decomposes on two phases: BaTiO3 and BaB2O4 at temperature interval 650-940ºС (Fig.12, curves 2, 3).
3.2.5. Crystallization behavior of the stoichiometric glass BaTi2O5 composition
Using our super cooling technique and special two step melting technology we have obtained BaTi2O5 (B2T) compound in glass state (glass tapes with thickness 0.03 – 0.4mm) and have studied processes of it crystallization. The initial glass tape sample thermal treated
Figure 10.
XRD-patterns of the crystallized powder glass samples corresponding to Ba2Ti2B2O9 composition: curve 1- 600°C 60h; curve 2- 640°C 24h; curve 3- 660°C 24h; curve 4- 700°C 24h; curve 5- 740°C 24h; curve 6- 950°C 24h; curve 7- 1020°C 24h (samples 2-7 have been water quenched from heat treatment temperature).
Figure 11.
XRD-patterns of the crystallized tape glass samples corresponding to Ba2Ti2B2O9 (2Ba2TiB) composition:curve 1- 600°C 6h; curve 2- 640°C 24h; curve 3- 660°C 24h; curve 4- 700°C 24h
Figure 12.
XRD-patterns of the crystallized powder glass samples corresponding to Ba2TiB2O7 composition:curve 1- 585°C 24h; curve 2- 730°C 24h; curve 3- 800°C 24h;
at 680°C 12h is X-ray amorphous (Fig.13, curve1). The further increase of crystallization temperature (680°C 12h +740°C 12h) results in occurrence of BaTi2O5 peaks [ICDD, 2008, File # 34-0133] with preservation of a transparency of a tape sample (Fig.13, curves 2,3). The BaTi2O5 is dominating crystalline phase at studied crystallization regimes (Fig.13, curves 3-4). However, we do not exclude presence of barium polytitanates in glass crystallization products in quantities not influencing on the end-product properties. We have observe on X-ray patterns strongly reorientation of formed barium di-titanate crystalline phase: reflex 8.247 Å (hkl 002) increase from 9% and become 100% and 3.47 Å (hkl 401) decrease from 100% to 0 (Fig.13, curves 2,3).
Figure 13.
XRD-patterns of the crystallized BaTi2O5 glass tape samples obtained by super cooling technique:curve 1- initial tape sample thermal treated at 680°C 12h -transparent; curve 2- tape sample (680°C 12h +740 °C 12h)-transparent;curve 3- tape sample 740 °C 12h- transparent;curve 4- tape sample 900-1000 °C 12h, casting in water
4. Ferroelectric properties of stoichiometric glasses in BaO-TiO2-B2O3 system
The ferroelectric (polarization - electric field) hysteresis, is a defining property of ferroelectric materials. In the last twenty years it has become a subject of intensive studies due to potential applications of ferroelectric thin films in nonvolatile memories. In ferroelectric memories the information is stored as positive or negative remanent polarization state. Thus, the most widely studied characteristics of ferroelectric hysteresis were those of interest for this particular application: the value of the switchable polarization (the difference between the positive and negative remanent polarization, PR − (−PR), dependence of the coercive field Ec on sample thickness, decrease of remanent or switchable polarization with number of switching cycles, polarization imprint, endurance, retention [Damyanovich, 2005]. Electric field induced polarization measurement was used for ferroelectric characterization of known and revealed first time new ternary BaTi(BO3)2, Ba2Ti2B2O9, Ba3Ti3B2O12, Ba2TiB2O7 and binary BaTi2O5 stoichometric compositions glass ceramics.
4.1. Polarization behavior of BaTi(BO3)2, Ba3Ti3B2O12, Ba2Ti2B2O9, Ba2TiB2O7 glass ceramics
Electric field induced polarization (P) and remanent polarization(Pr) were measured at room temperature for BaBT, 3Ba3TiB, 2Ba2TiB, 2BaTiB glass tape samples crystallized using various regimes (Fig.14).
Linear P–E curves are observed up to fields of 40-120 kV/cm for all measured samples with thickness 0.04-0.08mm. The polarization becomes nonlinear with increasing of applied electric field, and at 140-400 kV/cm the remanent polarization 2Pr values were found 0.35, 3.89, 0.6 and 0.12 µC/cm2 for the BaBT (Fig.14, A), 3Ba3TiB (Fig.14, B), 2Ba2TiB (Fig.14, C) and 2BaTiB (Fig.14, D) crystallized glass tape samples respectively. According to obtained results it is possible to conclude that samples are ferroelectrics. The highest remanent polarization value (2Pr=3.89 µC/cm2 ) has 3Ba3TiB crystallized glass tape sample (Fig.14, B).
Figure 14.
Dependence of polarization (P) on electric field (E) for crystallized stoichiometric glass compositions:BaTiB2O6 glass tape sample of 0.08 mm in thickness crystallized at 700°C 24h Ba3Ti3B2O12 glass tape sample of 0.07 mm in thickness crystallized at 900°C 12h Ba2Ti2B2O9 glass tape sample of 0.08 mm in thickness crystallized at 640°C 24h Ba2TiB2O7 glass tape sample of 0.04 mm in thickness crystallized at 580°C 12h
4.2. Polarization behavior of BaTi2O5 glass ceramic
Electric field induced polarizations were measured at room temperature for BaTi2O5 glass tape samples crystallized at various regimes. The high value of polarization (P~10µCu/cm2) and remanent polarization (2Pr = 6,2 µCu/cm2) we observe for strongly oriented transparent glass ceramic at applied field 220 kv/cm (Fig.15).
Figure 15.
Dependence of polarization (P) on electric field (E) for BaTi2O5 crystallized glass tape (740°C 12h) of 0.08 mm in thickness
Revision of phase diagrams of very complex ternary BaO-TiO2-B2O3 system has allowed us to study it more precisely. For this purpose glass samples have been used as initial testing substance for phase diagram construction. It is a very effective method, because it is possible to indicate temperature intervals of all processes taking place in glass samples: glass transition, crystallization, quantity of formed crystalline phases and their melting. Whereas, samples prepared by traditional solid phase synthesis are less informative and often lose a lot of information.
On the other hand super cooling technique created by our group allowed us to expand borders of glass formation from stable glass forming barium tetra borate up to binary di-barium borate and up to barium di-titanates which, together with compositions corresponding to ternary BaTB, 2BaTB, 2Ba2TB, 3Ba3TB compounds, have been obtained as glass tape with thickness of 30-400 microns (Fig.2). Large area of glass formation has allowed to have enough quantity of samples for DTA and X-ray investigations and BaO-TiO2-B2O3 system phase diagram construction.
There are very stable congruent melted binary barium titanate and barium borate and ternary barium boron titanate (BaTB) compounds in the ternary system. They have dominating positions in ternary diagram and occupied the biggest part of it (Fig.8). However, mutual influence of these stable compounds and not-stable binary (Ba2T) and ternary compounds (2Ba2TB, 3Ba3TB and 2BaTB) lead to formation of seven ternary eutectic points (Table2), which have essential influence on liquidus temperature decrease and glass formation. Ternary eutectics E5, E6 and E7 together with binary eutectics e7 and e6 have allowed to outline the field of barium titanate crystallization on the BaO-TiO2-B2O3 system phase diagram. The BaTiO3 is very stable compound and occupies dominating position on the phase diagram (Fig.8). Ternary eutectics E5, E6 and E4 together with binary eutectics e4 and e5 have allowed to outline the field of barium borate crystallization. The BaB2O4 is very stable compound also and occupied enough position on the studied ternary phase diagram (Fig.8). Ternary eutectics E1, E2, E3, E4, E6 and E7 have allowed to determine the field of crystallization of BaTB ternary compound. The BaTB is very stable compound also and occupies dominating position in the central part of the studied ternary BaO-TiO2-B2O3 system phase diagram (Fig.8).
The clear correlation between glass forming and phase diagrams has been observed in studied system. The glass melting temperature and level of glass formation depending on the cooling rate of the studied melts are in good conformity with boundary curves and eutectic points (Fig.2 and 8). It is possible to ascertain confidently, that glass formation can serve as the rapid test method for phase diagram construction.
Common regularities of bulk glass samples TEC changes in studied BaO-TiO2-B2O3 system have been determined: increase of BaO amounts leads to increase glasses TEC values from 60 to 120 ∙ 10-7К-1. The substitution of B2O3 for TiO2 practically doesn't influence glasses TEC value (Fig.4).
We have tried also to answer in discussion among various scientific groups about the existence of 2Ba2TB and 3Ba3TB compounds [Millet et al., 1986; Zhang et al., 2003; Park et al, 2004; Kosaka et al., 2005]. We have revealed for the first time through glass samples of stoichiometric 3Ba3TB composition examination, that 3Ba3TB compound is very stable in an interval of 600-950°C. It decomposes in temperature interval 950-1020°C with BaTiO3 and BaTB phase formation. Then, at temperature higher than 1020°C, it has incongruent melting with melt and BaTiO3 formation (Fig.5A).
The next unexpected result was obtained at glass samples corresponding to 2Ba2TB composition crystallization. First of all we have revealed on its DTA curve the presence of three exothermic effects with maximums at 640, 660 and 690°C. We have confirmed the existents of 2Ba2TB compound in temperature interval 600-670°C. Its new X-ray powder diffraction patterns could be indexed on a orthorhombic crystal symmetry with lattice cell as follows : a=9.0404 Å, b=15.1929 Å, c=9.8145 Å; unit cell volume V=1348.02ų, Z =6, calculated density (D calc.)= 3.99g/cm³; D exp.= 3.25 g/cm³; α;β;γ =90,00°(Table 2). However, its X-ray characteristics don’t coincide with earlier reported data [Millet et al., 1986].
As a result of the pseudo-binary BaB2O4-BaTiO3 system reinvestigation, a new ternary Ba2TiB2O7 compound has been revealed and characterized at the same composition glass crystallization in the temperature interval of 570-650°C. The X-ray powder diffraction patterns of 2BaBT could be indexed on a rhombic crystal symmetry with lattice cell as follows : a=10.068 Å, b=13.911 Å, c=15.441 Å; unit cell volume V=2629.17ų, Z =12, calculated density (D calc.)= 4.23g/cm³; D exp.=4.02 g/cm³ ; α;β;γ =90,00°. X-ray characteristics of both 2Ba2TB and 2BaTB compounds were determined and are given in Tables 2 and 3.
Study of the directed crystallization processes have allowed to reveal, that at the given way of casting the oriented germs are induced in the glass tape, which at the further heat treatment results in oriented transparent and opaque GC formation (Fig.9). The impact of external electric field changes the direction of crystalline BaTiB2O6 phase growth, i.e. reorients them (Fig.9).
Electric field induced polarization (P) and remanent polarization(Pr) were measured at room temperature for BaBT, 3Ba3TiB, 2Ba2TiB, 2BaTiB glass tape samples crystallized at various regimes. All tested samples are ferroelectrics and shown loop of hysteresis.
Linear P–E curves are observed up to fields of 40-120 kV/cm for all measured samples with thickness 0.04-0.08mm. The polarization becomes nonlinear with an increase of applied electric field, and at 140-400 kV/cm the remanent polarization 2Pr values were found 0.35, 3.89, 0.08 and 0.12 µC/cm2 for the BaBT (Fig.14, A), 3Ba3TiB (Fig.14, B), 2Ba2TiB (Fig.14, C) and 2BaTiB (Fig.14, D) crystallized glass tape samples respectively. According to obtained results it is possible to conclude that samples are ferroelectrics. Tha 3Ba3TiB crystallized glass tape sample (Fig.14, B) has the highest remanent polarization value (2Pr=3.89 µC/cm2 ).
Studies of crystallization processes of barium di-tatanate compositions glass tapes also have led to unexpected results. As far as it is difficult to receive this composition in glassy state as appeared so difficultly to crystallized it. All time we obtained transparent glass ceramics, which has residual polarization equal to 6,2 µCu/ cm2 comes nearer to barium di-titanate single crystal (6,8 µCu/ cm2 [Akishige et al., 2006]). For comparison the value of residual polarization of known barium titanate is equal to 25µCu/ cm2. However, the barium di-titanate has Tc= 470°C [Akishige et al., 2006] as for BaT its value equal to 124°C.
The earlier published phase and glass forming diagrams of the ternary BaO-TiO2-B2O3 system have been revised and reconstructed. Seven ternary eutectics have been determined in it. Existence of three ternary Ba2TiB2O7, Ba2Ti2B2O9 and Ba3Ti3B2O12 incongruently melted compounds have been confirmed at the same glass compositions crystallization, temperatures borders of their existence and their X-ray characteristics have been determined.
The new Ba2TiB2O7 and Ba2Ti2B2O9 compounds have been characterized. The X-ray powder diffraction patterns of Ba2TiB2O7 could be indexed on a rhombic crystal symmetry with lattice cell as follows: a=10.068 Å, b=13.911 Å, c=15.441 Å; unit cell volume V=2629.17ų, Z =12, calculated density (D calc.)= 4.23g/cm³; D exp.=4.02g/cm³; α;β;γ =90,00°. It is stable in temperature interval 570-650 °C. The Ba2Ti2B2O9 X-ray powder diffraction patterns could be indexed on a orthorhombic crystal symmetry with lattice cell as follows a=9.0404 Å, b=15.1929 Å, c=9.81455 Å; unit cell volume V=1348.02ų, Z =6, calculated density (D calc.)= 3.99g/cm³; D exp.=3.25g/cm³; α;β;γ =90,00°. It is stable in temperature interval 600-670 °C. The Ba3Ti3B2O12 is very stable compound in temperature interval 600-900°C.
The influence of various methods of melts casting on glass forming ability in the ternary BaO-TiO2-B2O3 system is investigated. The expanded glass formation area changes from stable glass forming barium tetra borate up to binary di-barium borate and up to barium di-titanate. Clear correlation between glass forming ability and eutectic areas have been revealed in investigated system.
Common regularities of bulk glass samples TEC changes in studied BaO-TiO2-B2O3 system have been determined: increasing of BaO amounts leads to increase glasses TEC values from 60 to 120 ∙ 10-7К-1. The substitution of B2O3 for TiO2 practically don’t influence on glasses TEC value.
All synthesized tapes glass ceramics are ferroelectrics. The transparent barium di-titanate glass ceramics has high residual polarization value equal to 6,2 µCu/ cm2.
This work was supported by the International Science and Technology Center (Projects # A-952 & A-1486).
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Written By
Rafael Hovhannisyan, Hovakim Alexanyan, Martun Hovhannisyan, Berta Petrosyan and Vardan Harutyunyan
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