C. B. Ponton

Magnetic properties of hydrothermally synthesized strontium hexaferrite as a function of synthesis conditions

Fine particles of strontium hexaferrite, SrFe12O19, with a narrow size distribution have been synthesized hydrothermally from mixed aqueous solutions of iron and strontium nitrates under different synthesis conditions. The relationship between the synthesis variables (temperature, time and alkali molar ratio) and the magnetic properties has been investigated. The results have shown that, as the synthesis temperature increases, the saturation magnetization of the particles increases up to a plateau and the coercivity decreases. As the alkali molar ratio R(=OH−/NO3−) increases, the coercivity decreases and goes through a local minimum, while the saturation magnetization increases and goes through a local maximum. Increasing the synthesis time from 2 h to 5 h has no significant effect on the saturation magnetization, but decreases the coercivity. An anisotropic sintered magnet with a high saturation magnetization value of 67.26 e.m.u g−1 (4320 G)‡ has been fabricated from the hydrothermally synthesized powders.

Effect of hydrothermal synthesis environment on the particle morphology, chemistry and magnetic properties of barium hexaferrite

Barium nitrate and iron nitrate have been used as precursors in the hydrothermal synthesis of barium hydroxide, iron oxide and barium hexaferrite sols under specified standard synthesis conditions (temperature, time, stirring, alkali concentration, amount of water and heating rate) as a function of the base species used during synthesis. The hydrothermal synthesis of barium hydroxide and iron oxide has been used to develop an understanding of the hydrothermal synthesis of barium hexaferrite from a mixture of their precursors. The investigation has shown that the nucleation and growth behaviour as well as the phase composition, thermal behaviour, particle size, particle-size distribution and magnetic properties are strong functions of the base species used. The electrostatic potential difference between the barium hydroxide and the iron oxide decreases with increasing cation size in the order NaOH, KOH, (C2H5)4NOH and NH4OH. Note the potential difference between the two sol species determines their tendency to coagulate into clusters; hence, the heterocoagulation will be greater when using NaOH or KOH than (C2H5)4NOH or NH4OH. Under the standard synthesis conditions, only NaOH and KOH are able to facilitate the formation of plate-like particles of barium hexaferrite. In contrast, ultrafine particles of iron oxide (10–20 nm) together with only a small amount of barium hexaferrite are produced when either NH4OH or (C2H5)4NOH base is used. The samples synthesized in the presence of the NaOH and KOH exhibit relatively higher saturation magnetization (i.e. 258 mT (39 e.m.u.g−1) and 215 mT (32 e.m.u.g−1), respectively) than those samples synthesized in the presence of NH4OH or (C2H5)4NOH which exhibit negligible saturation magnetization owing to the small amount of magnetic phase (BaFe12O19) present.

A study of Nd-substituted Sr hexaferrite prepared by hydrothermal synthesis

Nd-substituted Sr hexaferrite (Sr/sub 1-x/Nd/sub x/Fe/sub 12/O/sub 19/) plate-like particles were synthesized hydrothermally and then calcined at temperatures ranging from 1100/spl deg/C to 1250/spl deg/C for 2 h in air. The effects of the Nd-Sr ratio and the calcination temperature on the phase stability and magnetic properties were investigated. Nd substitution up to a Nd-Sr ratio of 1/8 increases the coercivity without causing any significant deterioration in either the saturation magnetization or the remanence.

Characterization and optimization of the coercivity-modifying nitrogenation and re-calcination process for strontium hexaferrite powder synthesized conventionally

Strontium hexaferrite powder synthesized conventionally in-house from strontium carbonate and hematite (Fe2O3) without using additives has been treated in a static nitrogen atmosphere and subsequently calcined in static air. The phase identification studies by means of X-ray diffraction (XRD) and thermal magnetic analysis (TMA) indicated the decomposition of the strontium hexaferrite and the reduction of the resultant iron oxide (Fe2O3) during the reaction with nitrogen. High-resolution scanning electron microscopy (HRSEM) studies show that the reduction occurring during nitrogenation results in the conversion of some of the large grains into much finer sub-grains. Strontium hexaferrite, Fe3O4, and Sr7Fe10O22 were the main phases obtained after reduction. However, weak traces of other phases, such as Fe2O3, were also detected. The hexaferrite phase re-formed on subsequent calcination. The magnetic measurements indicated a significant decrease in the intrinsic coercivity during nitrogenation due to the formation of Fe3O4. However, after a re-calcination process, the remanence and maximum magnetization (i.e., magnetization at 1100 kA/m) exhibited values close to the initial values before treatment, but the value of the intrinsic coercivity was higher than that prior to nitrogenation. Examination of the re-calcined microstructure showed that this could be attributed to the fine grains that originated from the fine sub-grain structures formed in the powder particles during nitrogenation.The optimum time, initial gas pressure, and temperature of nitrogenation and the optimum temperature of re-calcination were investigated using a vibrating sample magnetometer (VSM), XRD, and HRSEM. The optimum temperature for nitrogenation was 950 and 1000 °C for re-calcination. The optimum time and initial nitrogen pressure were 5 h and 1 bar, respectively. The highest intrinsic coercivity obtained after re-calcination was ∼340 kA/m.

Colloidal processing of a mullite matrix material suitable for infiltrating woven fibre preforms using electrophoretic deposition

Abstract Commercially available alumina and silica precursors for the preparation of mullite ceramic via colloidal processing and viscous transient sintering have been identified, including fumed nanosize powders and colloidal suspensions. These materials were chosen due to the fact that they can be used in the form of a sol, as mullite matrix precursors, to infiltrate woven fibre preforms using electrophoretic deposition. The sintered density of the mullite matrices sintered for 2 h, at the upper temperature for fabricating SiC-fibre reinforced composites (1300 °C) is only ≈ 90% of theoretical. However, by exploiting a viscous flow densification mechanism, it is envisaged that hot-pressing can be used to produce fully dense mullite matrix composites at the required temperatures. Additionally. using a simple pressureless sintering route, almost fully dense (98% of theoretical density) monolithic mullite has been obtained from the pre-mullite powders. A very homogeneous and fine microstructure was achieved by sintering for 5 h at a temperature of ≈ 1450 °C.

Heat treatment of strontium hexaferrite powder in nitrogen, hydrogen and carbon atmospheres : a novel method of changing the magnetic properties

Strontium hexaferrite powder has been treated in nitrogen, hydrogen and carbon atmospheres. The results show that the phase composition and morphology, and hence, the magnetic properties of the strontium hexaferrite are affected significantly by these gas/vapour treatments. Generally, the coercivity decreased to below 0.8 kOe (regardless of the initial coercivity) and the magnetization at 14 kOe increased significantly, when strontium hexaferrite powder had been treated in a nitrogen, hydrogen or carbon atmosphere. However, it was found that a post-gas treatment of calcination in air, under appropriate conditions, resulted in a recovery of the hexaferrite structure (i.e. it is a reversible reaction). However, the particle/grain sizes of the calcined samples were significantly smaller than those of the non-treated samples, and it is believed that they were single domain particles/grains. In some cases, the coercivity increased by about 400%. The magnetization at 14 kOe and the remanence were either not affected or sometimes increased; magnetic measurements indicated a preferred orientation of the grains.

Hydrothermal processing and characterisation of doped lanthanum chromite for use in SOFCs

The perovskite powders Ca0.3La0.7CrO3 and Sr0.16La0.84CrO3 have been prepared using hydrothermal processing. The solid solutions were not formed directly in the autoclave, but the hydrothermally produced powders required calcination at a greatly reduced temperature to form the perovskite phase, reducing the tendency to produce hard agglomerates. Pellets with densities in excess of 95% TD were produced.

Optimization of the coercivity-modifying hydrogenation and re-calcination processes for strontium hexaferrite powder synthesized conventionally

Strontium hexaferrite powder, synthesised conventionally in-house from strontium carbonate (SrCO3) and hematite (Fe2O3) without additives, has been treated in a static hydrogen atmosphere and subsequently calcined in static air under different conditions. The optimum time, temperature, and initial pressure of hydrogenation and the optimum temperature of re-calcination for a fixed time of 1 h were determined using a combination of X-ray diffraction, vibrating sample magnetometer, and high-resolution scanning electron microscope techniques.Increasing the temperature, initial pressure, and time of hydrogenation up to the determined optimum values resulted in the decomposition of the strontium hexaferrite into Fe2O3 and Sr7Fe10O22, together with a more marked reduction of the resultant Fe2O3 to Fe. This was accompanied by the conversion of the initial single-crystal particles into very fine sub-grains, which is the reason for the higher coercivities obtained after re-calcination. Increasing the hydrogenation and re-calcination parameters beyond the optimum values, however, generally resulted in grain growth, which decreased the final magnetic properties. Increasing the re-calcination temperature to 1000 °C resulted in completion of the hexaferrite reformation. Beyond this temperature, however, the coercivity decreased due to grain growth.The optimum conditions were as follows: hydrogenation at 700 °C for 1 h under an initial pressure of 1.3 bar and then re-calcination in air at 1000 °C for 1 h. The highest coercivity obtained after re-calcination was around 400 kA/m. The remanence and saturation magnetization values were very similar to their initial values before the hydrogen treatment.

Oxidation of SiC powders for the preparation of SiC/mullite/alumina nanocomposites

The oxidation behaviour of two types of SiC powder of differing particle size and morphology distribution has been studied in the present work; one submicron-sized and the other micron-sized. It has been observed that the onset-temperature for significant oxidation of the SiC powder of smaller particle size is much lower than that for the SiC powder of larger particle size; namely, about 760 °C as compared with about 950 °C. Furthermore, the rate and extent of oxidation of the former SiC powder is much higher than that of the latter SiC powder. Interestingly, however, the SiC powder of smaller particle size exhibits more controllable oxidation behaviour in the context of the preparation of SiC/mullite/alumina nanocomposites, i.e., in terms of the extent of oxidation, and hence the amount of silica formed as an encapsulating outer layer and the resulting core SiC particle size, than the SiC powder of larger particle size. The SiO2 layer formed was amorphous when the SiC powders were oxidized below 1,200 °C, but crystalline in the form of cristobalite when they were oxidized above 1,200 °C. Since the presence of amorphous silica can accelerate the sintering of the nanocomposite, oxidation of the chosen SiC powder should thus take place below 1,200 °C.

Comparative effects of the hydrogen and nitrogen gas treatment and re-calcination (GTR) routes on the composition, microstructure, and magnetic properties of conventionally synthesized Sr-hexaferrite

Optimized static hydrogen treated and recalcined (HTR) and static nitrogen treated and recalcined (NTR) Sr–hexaferrite powders synthesized conventionally in-house are compared with one another. The phase identification studies and lattice parameter measurements showed first that the Sr–hexaferrite decomposed, forming iron oxide (Fe2O3), which was then reduced during the static hydrogen or nitrogen treatment, and, second, that the hexaferrite phase was recovered albeit with a small change in the composition (as indicated by the lattice spacings) after the re-calcination treatment in static air. These effects were more pronounced in the hydrogen process than in the nitrogen process. The main effect of this gas-treatment and re-calcination (GTR) process on the microstructure of the Sr–hexaferrite was the transformation of the single-crystal particles into particles with a very fine sub-grain structure during the gas treatment, which resulted in the formation of polycrystalline hexaferrite particles with a much finer grain size during subsequent recalcination, compared to that of the initial hexaferrite powder. This finer structure was responsible for the higher coercivities observed after re-calcination. With regard to the hydrogen and nitrogen processes, the former resulted in a higher degree of oxide reduction and hence a higher coercivity on re-calcination. The coercivity of the initial Sr–hexaferrite increased from 310 kA/m (3.9 kOe) to ∼400 kA/m (5 kOe) after HTR and to 342 kA/m (4.3 kOe) after NTR. The initial magnetization behavior was also different for the HTR- and NTR-processed powders, with the former exhibiting behavior characteristic of single domains. This was consistent with the grain size being significantly less than the single-domain size (∼1 μ).