Received on: August 25, 2017
Accepted on: September 05, 2017
Published on: September 19, 2017
Material science institute
It is shown that ferrite of bismuth synthesized on the basis of precursors - iron and bismuth oxides melted on a solar furnace has a denser structure, a low coefficient of thermal expansion, in comparison with the traditionally synthesized bismuth ferrite.
As the analysis shows, the materials of perovskite (Ti (Sr) BaO3) and pyroxene (CaMgSi2O6) compositions synthesized from melt in the Big Solar Furnace (BSP) have a stable stable structure and increased mechanical and dielectric properties [1, 2, 3]. However, the synthesis of perovskite structures based on bismuth ferrite BiFeO3 with magnetic properties using BSP has not been studied.
Synthesis and properties of bismuth ferrite BiFeO3 are investigated quite widely [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16]. However, the preparation of a single-phase sample of BiFeO3 still presents a serious problem. For example, a material synthesized from a mixture of Bi2O3 + Fe2O3 always contains impurities Bi25FeO39 and Bi2Fe4O9 not depending on the synthesis method [5, 6, 7, 8]. The use of a mixture with a large excess of Bi2O3 also did not lead to a single-phase BiFeO3 ferrite . In  it is noted the difficulty of preparation of single-phase BiFeO3, which is associated with the features of the system state diagram Bi2O3-Fe2O3 (the presence of three compounds), volatility Bi2O3 above the point of their melting point  and thermodynamic BiFeO3 instability in air in the absence of Bi2O3 equilibrium molten solution - Fe2O3 . Analysis of the literature data shows the impossibility of obtaining a single-phase compound BiFeO3 by solid-phase synthesis .
Thus existing thermal (solid-phase reaction at high temperatures (Tsin
chemical (reaction solution) methods do not allow to obtain a single-phase ferrite bismuth. In this aspect, the scientific interest is the use of solar technology, i.e. synthesis of a melt obtained Exposure to concentrated high-density light radiation at which compounds are formed during reactions in melts, and then this state is fixed by quenching.
This work is aimed at studying the synthesis of BiFeO3 from a mixture of bismuth (Bi2O3) oxides and iron (Fe2O3) fused on the solar furnace.
At the first stage of the experiments, the oxides of bismuth (Bi2O3) and iron (Fe2O3) were melted on the focal plane of the solar furnace under the action of a concentrated light flux of 450 W / cm2 density and held in the melting state for 15 minutes. The melts are quenched in water (104 deg / s). Molten melts in a ball mill by a wet method (material: water: grinding media = 1: 1: 1) for 10 hours. Sieved through a sieve of 0.05. On the basis of fused oxides, a mixture of Bi2O3 + Fe2O3 was produced in a stoichiometric ratio. Imagery-tablets are pressed by pressing at an effort of 1 ton on a C-100 press. The firing was carried out in an electric resistance furnace with silicate heaters at various temperatures. The samples were designated A-type.
In the second stage, bismuth ferrite was synthesized from a mixture of oxides without melting on a solar furnace (B-type samples) was used as control.
The thermogravimetric analysis of the Bi2O3 + Fe2O3 mixture was carried out in the temperature range 100-10000С on a Q-1500D derivatograph at a heating rate of 15оС / min.
X-ray diffraction patterns were taken on powders using a DRON-3M diffractometer with copper anode. The determination of the apparent density of the samples, ρ, was carried out by hydrostatic weighing in octane, the calculation of the X-ray density, the ρent, was carried out according to the formula: ρrent = 1.66 X M / V (M weight of the formula unit in grams, V-volume of the perovskite cell in Å 3) , Ρot-on the formula (ρкж / ррент) X 100%. The structural looseness was determined by the formula: ω = M/ (nρ); Where M is the molecular weight equal to the sum of the atomic weights of the elements of the compound, n is the number of structural units (atoms, ions, complexes or radicals) in the formula unit of the compound, ρ is its density.
The coefficient of linear thermal expansion was determined on a cathetometer in the temperature range 25-6000С. Micro structural studies were carried out on transparent (in transmitted light) and polished sections (in reflected light).
Results and Discussion
Figure 1 shows the differential-thermal analysis curve for the Bi2O3 + Fe2O3 mixture in the temperature range 100-10000С.
Figure 1: The Thermo Gravimetric Analysis of the Bi2O3 + Fe2O3 Mixture in the Temperature Range 100-10000С: a) With Components not Melted in the Solar Furnace, and b) With Components Melted in the Solar Furnace
The thermogravimetric analysis of the Bi2O3 + Fe2O3 mixture in the temperature range 100-10000С reveals 5 endothermic effects. It is clear that at 7400C a polymorphic Bi2O3 transformation occurs, the peak at 7900C is due to the melting of the eutectic in the Bi2O3 - Fe2O3 system at which the synthesis of BiFeO3 begins. As the analysis of the DTA curve shows, BiFeO3 begins to decompose at 9200C and 9500C. Analysis shows that on the DTA curve of a mixture of oxides melted on a solar furnace, there is no peak at 7400C (Figure 1b). This indicates that the preliminary melting of the Bi2O3 and Fe2O3 components on a solar furnace leads to an increase in the chemical activity of the oxides. In addition, the endothermic peaks are shifted toward high temperatures by 50oC.
Figure 2 shows X-ray diffraction patterns of calcined samples at different temperatures.
An analysis of the X-ray diffraction pattern of the calcined sample at a temperature of 8850oC showed that this picture describes a BiFeO3 compound with a rhombic lattice with lattice parameters a = 3.958, b = 3.78, c = 4.08 A (ITSM No. 20-169). There is also an impurity phase Bi2Fe4O9 (solid Fe2O32BiFeO3 solution) (ITSM No. 25- 90).
Figure 2: The X-ray Diffraction Pattern of Bismuth Ferrite BiFeO3 Synthesised at Various Temperatures (0С): a)700; (B) 800; C) 885. X-phase Corresponds to a Solid Solution of Bi2Fe4O9 = [Fe2O3 2BiFeO3]
As is known, bismuth ferrite is characterised by orthorhombic distortion of the perovskite cell  and temperature-dependent non-stoichiometry [19, 20], which complicate the synthesis of single-phase bismuth ferrite, as well as compounds containing BiFe03.
The x-ray density of bismuth ferrite was ρ = 8.39 g / cm3.
Figure 2 shows that, depending on the type of samples, the size and shape of the grains of ferrite of bismuth ceramics change. Thus, in the case of A-type samples, the grain size varies between 5-20 μm and 10-30 μm for B-type.
Table 1 shows the values of porosity, apparent density and coefficient of linear thermal expansion of the samples, depending on the synthesis method. It can be noted that the sinter ability of bismuth ferrite is somewhat improved when it is synthesized from fused oxides.
Figure 1(A&B): Microstructure of Bismuth Ferrite a) A- and b) B-types
Table 1: The Values of Aparent Density (ρ density), Porosity (П), Relative Density (ρot), Structural Looseness (ω) and Coefficient of Linear Thermal Expansion (α) of Ceramic Bismuth Ferrite Samples, Depending on the History of the Components
The coefficient of linear thermal expansion was 13x10-6K-1 for A-type samples with a predominance of rhombic phase and 11x10-6K-1 for B-type samples in which the orthorhombic phase prevailed. The difference between the structural looseness of materials is due to the fact that preliminary melting of oxides on a solar furnace promotes the synthesis of bismuth ferrite with a denser structure.
Table 2 presents the results of the synthesis of bismuth ferrite as a function of the prehistory of the Fe2O3 and Bi2O3 components. It can be seen that bismuth ferrite, obtained from fused oxides, is characterised by a small content of impurity phases. It can be assumed that the fused oxides are a more active form of the reagent, as a result of which the formation of impure Bi2Fe4O9 begins at a lower temperature than the formation of bismuth ferrite.
Table 2: Results of Synthesis of Bismuth Ferrite as a Function of the Prehistory of Fe2O3 and Bi2O3 Components
Thus, bismuth ferrite, synthesized on the basis of precursors - iron and bismuth oxides, melted on a solar furnace has a denser structure, a low coefficient of thermal expansion, in comparison with the traditionally synthesized bismuth ferrite. The technology of creating ceramic materials based on bismuth ferrite requires careful regulation of the physico-chemical state of the starting materials.
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