The magnetic properties of glass-ceramics in the CoO-Fe2O3-B2O3 system

The preparation of glasses in the CoO-Fe₂O₃-B₂O₃ system is described and it is shown that the magnetic spinel cobalt ferrite (CoFe₂O₄) can be obtained in precipitate when the glasses are heat-treated. The identity of the crystalline phase is established by means of Mössbauer spectroscopy. It is shown that the magnetic properties of the glass-ceramics can be controlled by means of the heat-treatment schedule and intrinsic coercivities as high as 1900 Oe are recorded. The magnetic properties of the as-cast and heat-treated material are discussed generally in terms of superparamagnetic, single domain and multi-domain behaviour. The highest values of the intrinsic coercivity and remanence are compared to those calculated for an assembly of non-interacting single domain particles with positive cubic magnetocrystalline anisotropy.     

Transformation of Na2O-HfO2-B2O3 glass into a material having interconnected pores

Substitution of SiO₂ in the ternary sodium borosilicate system with HfO₂ was found to produce glasses, which after heat treatment decomposed into immiscible microphases, one of which was water soluble. The structure of the leached material after heat treatment was either glassy (mainly in the presence of Al₂O₃) or crystalline. Crystalline forms found during X-ray diffraction analysis of heat treated and leached material (melted in Pt/Rh crucibles) were monoclinic HfO₂. Monoclinic HfO₂ was also found in heat treated, leached and then fired materials melted in Pt/Rh or Al₂O₃ crucibles, in the latter an additional 9Al₂O₃ · 2B₂O₃ phase was detected. The higher solubility of HfO₂ in a Na₂O-B₂O₃ matrix than that of ZrO₂ (30 wt% against 15 wt%) resulting in clear glasses is of interest. The specific surface areas of the leached materials ranged between 41.3 and 290 m²g^–1, while the mean radii of interconnected pores were calculated to be 1.2 and 15.2nm. A firing temperature between 1450 and 1500° C is estimated from void volume and bulk density measurements.     

Transformation of Na2O-CeO2-B2O3 glass into a material with interconnected pores

Substitution of SiO₂ with CeO₂ in the ternary sodium borosilicate system was found to produce phase-separable glasses. Heat treatment of these glasses resulted in separation into two different phases. The one phase enriched in sodium borate was then leached out leaving a CeO₂-rich framework. The structure of the leached material was crystalline (Pt/Rh crucible melt) which changed to a rather net-like appearance if Al₂O₃ resulting from erosion of alumina crucibles was added. B₂O₃ remained partially in the insoluble CeO₂-skeleton. X-ray diffraction analysis of leached material proved the presence of crystalline cubic CeO₂ and cerium borate (metaborate of the aragonite type) in Pt/Rh crucible melts, whereas cubic CeO₂, 2Al₂O₃ · B₂O₃ and traces of sodium borate were detected in Al₂O₃ containing melts. The specific surface areas of the leached materials ranged between 25 and 120m²g^–1 while the main radii of interconnected pores were calculated to be between 0.5 and 17nm. A sintering temperature of about 1500° C was estimated from void volume and bulk density measurements.     

Phase formation kinetics in SrO-Al2O3-SiO2-B2O3 glass

Phase formation in Sr-celsian glass, containing 3 wt% B₂O₃ (SA2SB), was investigated by using non-isothermal and isothermal kinetic analyses. While stoichiometric celsian (SA2S) glass showed two-stage crystallization of glass, from glassy state to hexacelsian and from hexacelsian to monocelsian, SA2SB glass showed direct crystallization of glass to monocelsian, resulting in considerable decrease in the temperature of monocelsian formation. The activation energy for monocelsian formation in SA2SB glass was 390 kJ/mol, which is lower than those for hexacelsian and monocelsian formation respectively in SA2S glass. The Avrami exponent of SA2SB glass was 2.0, indicating two-dimensional crystal growth with interface-controlled mechanism at a zero nucleation rate.     

Effect of the addition of Fe2O3 and heat treatment duration on the magnetic susceptibility of V2O5-P2O5-B2O3 glass system

The effect of the addition of Fe₂O₃ and heat treatment duration on the magnetic susceptibility of vanadium borophosphate glass were studied. The magnetic susceptibility of glass samples was found to increase with increasing Fe₂O₃ content, which may be explained by the formation of the FeO₆ group and the change of Fe²⁺ to Fe³⁺ which has higher paramagnetic properties. No detectable changes in the magnetic susceptibility with heat treatment for the samples containing 0.0, 0.5 and 1.0 mol% Fe₂O₃ was observed. The magnetic susceptibility for the heat treated samples containing 2.5, 5.0 and 7.5 mol% Fe₂O₃ decreases sharply with increasing duration of heat treatment up to 6 h and then remains almost constant. The sharp decrease in magnetic susceptibility of 2.5 mol% Fe₂O₃ is attributed to the increase in the number of ferrous ions. The sharp decrease for samples containing 5.0 and 7.5 mol% Fe₂O₃ is attributed to the increase in the number of Fe³⁺ in tetrahedral co-ordination. The rate of crystallization owing to the heat treatment was calculated and was found to increase with increasing iron oxide content. The geometry of crystallization was found to be in three-, two-and one-dimension(s) for samples containing 2.5, 5.0 and 7.5 mol% Fe₂O₃, respectively.     

DC electrical conductivity of Na2O-ZnO-B2O3 glass system

The d.c. electrical conductivity of Na₂O-ZnO-B₂O₃ glass system has been measured as a function of temperature in the range of 350–600°K. The conductivity data show that the activation energy of Na⁺ ions is dependent on ZnO concentration. The results have been discussed in the light of the cluster model of glasses.     

Mössbauer studies of the effect of nucleating agents on the crystallization of YIG in the Na2O-SiO2-Y2O3-Fe3O3 glass system

The effect of 2 mol % of P₂O₅ or TiO₂ on the crystallization of yttrium-iron-garnet (YIG) in Na₂O-SiO₂-Y₂O₃-Fe₂O₃ glass ceramics has been investigated by X-ray and Mössbauer spectroscopic studies. These studies indicate that the glass ceramics containing P₂O₅ are more favourable for the crystallization of YIG. Analysis of the results suggest that an optimum growth temperature for crystallization of the YIG phase is 800° C in the sample with P₂O₅ as the nucleating agent. The sample heat-treated at the growth temperature for 40 h is not as favourable for crystallization of the YIG phase as the sample which is given a two-stage heat-treatment.     

Dielectric behaviour of glasses and glass ceramics in the system BaO-PbO-TiO2-B2O3-SiO2

Glasses with varying molar ratios of PbO/BaO in the system BaO-PbO-TiO₂-B₂O₃-SiO₂ were prepared keeping (BaO + PbO)/TiO₂ ratio equal to one. The glasses were ceramized by two-stage heat treatment. X-ray diffraction indicates that PbTiO₃ crystallizes in lead-rich glasses while BaTiO₃ precipitates in barium-rich compositions. Solid solution (Ba, Pb)TiO₃ does not seem to crystallize over the entire range of compositions. Simultaneous presence of PbO and BaO in the initial glass composition reduces the yield of ferroelectric phase. Dielectric properties have been interpreted in terms of microstructural features.     

Immiscibility in the TeO2-B2O3 system

Experimental investigations have been carried out to study the phase separation process in the TeO₂-B₂O₃ system. It was found that the phase diagram is of a eutectic-like type (monotectic invariant reaction), with a wide region of stable and metastable phase separation. During the cooling of the melts two phases are obtained. One has a high TeO₂ content and is a transparent glass. The other, richer in B₂O₃, is opaque and in structure constitutes a complicated microheterogeneous system.     

Surface tension of PbO-B2O3 and Bi2O3-B2O3 glass melts

The surface tensions of xPbO-(100?x) B₂O₃ (x = 30–80 mol%) and xBi₂O₃-(100?x) B₂O₃ (x = 0–100 mol%) melts were measured using the ring method over the temperature range 973 to 1373 K. The compositional and temperature dependences of surface tension were investigated. Addition of PbO and Bi₂O₃ to B₂O₃ increased the surface tensions of their respective PbO-B₂O₃ and Bi₂O₃-B₂O₃ melts. The surface tension showed a maximum at 60 mol% PbO in the PbO-B₂O₃ melts and at 70–80 mol% Bi₂O₃ in the Bi₂O₃-B₂O₃ melts. The temperature coefficient of surface tension was examined on the basis of its relationship to the structure, and it was suggested that the temperature coefficient of surface tension decreases with an increasing content of four-coordinated boron.     

Microstructural and dielectric behaviour of glass ceramics in the system PbO-BaO-TiO2-B2O3-SiO2

Glasses in the system PbO-BaO-TiO₂-B₂O₃-SiO₂ with and without P₂O₅ as nucleant have been prepared. The glass samples were ceramized based ondta studies. The ferroelectric phase crystallizing out has been found to be BaTiO₃ fromxrd. The optical and scanning electron micrographs show the presence of BaTiO₃ as major phase. In these glass ceramic samples, dielectric constant and dissipation factor are approximately constant with temperature and frequency upto the glass transition temperatureT _g and thereafter increase sharply with temperature and finally level off. The addition of P₂O₅ as nucleant and molar ratio of (PbO + BaO) to TiO₂ has marked influence on the dielectric behaviour and composition of ferroelectric phase crystallizing out.     

An infrared study of the Na2O-V2O5-Fe2O3 glass system

Structural studies of Na₂O-V₂O₅-Fe₂O₃ glasses have been made from their IR spectra which show that vibrational bands characteristic of the vanadium-oxygen bonds in V₂O₅ are maintained in these glasses, but the addition of Na₂O to these glasses results in a shifting of the higher frequency peaks towards lower wave number due to structural changes produced in V₂O₅. It is inferred that Na⁺ ions make bonds interstitially with isolated V=O bonds and VO₅ polyhedra are destroyed, resulting in the formation of VO₄ polyhedra through intermediate complexes. The variation of Fe₂O₃, however, produces an insignificant structural change in these glasses. The IR spectra of samples heated to their temperature of crystallization confirm the formation of a series of complexes with several isolated V=O bonds.     

Fracture toughness of CaO-P2O5-B2O3 glasses and glass-ceramics determined by indentation

The fracture toughness, (K _IC) of CaO-P₂O₅-B₂O₃ glasses and glass-ceramics was investigated using both Vickers indentation and the notched beam technique (NBT). Five representative equations were applied and it was found that for the variation of K _IC with B₂O₃ content, the Lawn and Fuller equation showed the best correspondence with the NBT. The values of fracture toughness obtained from the Lawn and Fuller equation showed the same trend with B₂O₃ content as that determined by NBT, although the values from indentation were on average 33% lower. The determination of absolute fracture toughness by indentation requires a correction factor which can be obtained by calibration using NBT. A significant increase in K _IC occurred after a 37CaO-37P₂O₅-20B₂O₃-6Al₂O₃ (mol%) glass was converted to a glass-ceramic. The much higher K _IC for the glass-ceramic measured by NBT (1.32 MN m^–3/2) compared with that from indentation (0.89 MN m^–3/2) is attributed to internal stresses due to thermal expansion differences between the crystalline and residual glass phases leading to additional microcrack toughening.     

Glass formation and crystallization of barium ferrite in the Na2O-BaO-Fe2O3-SiO2 system

Amorphous ribbons of approximate composition 0.13Na₂O-0.30BaO-0.30Fe₂O₃-0.27SiO₂ were successfully fabricated using roller-quenching, after melting at 1400C. Devitrification of the amorphous phase by annealing at 710C for 2 h formed a uniform distribution of barium ferrite (BaFe₁₂O₁₉) particles with a mean diameter of 55.88 nm. Other crystalline phases formed on heat treatment were BaFe₂O₄ and Na₂Ba₂Si₂O₇. Devitrified ribbons showed a saturation magnetization (M _S) of 24.41 emu g^–1 and a coercivity (H _C) of 3.06 kOe.