Conventional Hydrothermal Research Articles

Effect of Microwave Irradiation on Hydrothermal Treatment of Blast Furnace Slag

The effect of microwave irradiation on the hydrothermal treatment of blast furnace (BF) slag was investigated using a 2.45 GHz microwave irradiation system. The comparison of the features of a microwave-hydrothermal (M-H) reaction with those of a conventional hydrothermal (C-H) reaction showed that in both these reactions, tobermorite (Ca5Si6O16(OH)2·4H2O) was formed as the major phase in the treated samples. When the BF slag was hydrothermally treated by the C-H reaction, tobermorite was formed in the vapor state as the major phase at 200°C after a holding time of 48 h; in the M-H reaction, tobermorite was rapidly formed within 3 h at the same temperature. It was elucidated that microwave irradiation promoted the hydrothermal reaction to a significant extent.Furthermore, effects of microwave irradiation on BF slag subjected to hydrothermal hot-pressing were investigated. After microwave irradiation, the compressive strength of the slag compact and rate of porosity in the compact after treatment increased and decreased, respectively, in comparison with those before treatment. Moreover, microwave irradiation was advantageous over conventional heating at the same temperature in that it enhanced the compressive strength of the BF slag.

Preparation of Multicomponent Metal Oxides Using Nozzle Spray and Microwaves

AbstractMulticomponent metal oxide (MMO) crystallites are prepared by spraying a reactant solution into a receiving solution or into air under microwave radiation at atmospheric pressure. The injection of a nitric acid solution through an ultrasonic nozzle into a receiving solution of metal precursor and the use of microwave radiation are combined to form a novel preparation technique called the nozzle‐spray/microwave (NMW) method. The inclusion of an additional step, the in situ mixing of precursor solutions prior to their injection through the ultrasonic nozzle spray, leads to another procedure called the in situ/nozzle‐spray/microwave (INM) method. For comparison, MMO materials with the same metal constituents as those prepared by our novel techniques are prepared by conventional hydrothermal (CH) methods. Fresh materials prepared by NMW, INM, and CH methods were heat treated to study the effect of calcination. All materials were characterized before and after calcination using X‐ray diffraction, scanning electron microscopy, Brunauer–Emmett–Teller surface area, and inductively coupled plasma elemental analysis. The NMW method produces particles with rodlike morphologies different from those obtained using CH methods. The INM method produces an amorphous material that crystallizes after calcination into small (ca. 200 nm) particles with interesting morphologies. Notably, calcination of materials prepared by both NMW and INM reduces the particle size and increases the surface area. The work presented in this paper paves the way to use NMW and INM to prepare MMOs with unique morphologies.

Effects of chloride ions on hydrothermal synthesis of tetragonal BaTiO 3 by microwave heating and conventional heating

Effects of the chloride ions on the synthesis of tetragonal BaTiO 3 in the microwave-hydrothermal (MH) and conventional hydrothermal (CH) processes were investigated. Such effects were found to be dependent of the synthesis method and temperature. In the MH process performed at 240 °C, tetragonal BaTiO 3 powders of c/ a ratios larger than 1.009 were readily prepared whether the chloride ions were present or not. At the lower temperature of 220 °C, however, the c/ a ratio in the MH BaTiO 3 was only 1.0061 in the absence of the chloride ions but increased to 1.0084 aided by the chloride ions. In the CH process, the chloride ions were observed to play an invariably important role in the preparation of tetragonal BaTiO 3. In contrast, the chloride ions appeared to have less influence on the particle size of the hydrothermal BaTiO 3 in a relatively short period of time.

Microwave-hydrothermal synthesis of tetragonal barium titanate

Tetragonal barium titanate with c/a ratio of 1.0093 and average particle size of 240 nm was synthesized by the microwave-hydrothermal (MH) method at 240 °C in only 12 h. Temperatures above 200 °C were first introduced to the MH process in this study, since the temperature has a critical effect on the formation of tetragonal BaTiO3, shown by the experiments here. Hydrous titanium oxide and Ba(OH)2 were used as precursors, without halide anions and alkali-metal cations to avoid contamination. The kinetics of tetragonal phase formation in BaTiO3 was considerably promoted in the MH processing, in comparison with the conventional hydrothermal (CH) route.

Microwave-hydrothermal synthesis of tetragonal BaTiO 3 under various conditions

Tetragonal BaTiO 3 powders were synthesized by microwave-hydrothermal (MH) method. The effects of reaction time, reaction temperature, and reactant composition on the MH synthesis were investigated. Typical experiments were performed at 240 °C, a temperature above 200 °C in MH processing was not reported by previous workers, and used hydrous titanium oxide and Ba(OH) 2 as precursors, in the absence of chloride ions and alkali metal ions to avoid contaminations. In MH process, the formation of tetragonal BaTiO 3 was strongly enhanced. Tetragonal BaTiO 3 formed in only 3 h and approached full tetragonality ( c/ a = 1.010) when aged for 20 h. At lower temperature of 220 °C, the rate of the formation of tetragonal BaTiO 3 and the growth of particles slowed down, substantially. It was found that the excess Ba 2+ in hydrothermal solution played an important role on promoting the formation of tetragonal BaTiO 3. A few conventional hydrothermal (CH) experiments were carried out for comparison with the MH experiments.

ZSM-5 zeolite/porous carbon composite: Conventional- and microwave-hydrothermal synthesis from carbonized rice husk

The formation of novel ZSM-5 zeolite/porous carbon composite from carbonized rice husk has been investigated using microwave- and conventional-hydrothermal reaction at 140–160 °C. ZSM-5 zeolite crystals were formed on porous carbonized rice husk by the dissolution and in situ crystallization of silica in the carbonized rice husk. Compared to the conventional-hydrothermal (C-H) formation of ZSM-5 zeolite, the microwave-hydrothermal (M-H) reaction led to increased rate of formation by 3–4 times at 150 °C. In addition M-H reaction yielded small particles of 0.3–5 μm. The surface area of ZSM-5 zeolite (without template)/porous carbon composite is 485.4 m 2/g, and this composite has both micropores and mesopores.

Rapid Synthesis of Monodispersed .ALPHA.-Fe2O3 Nanoparticles from Fe(NO3)3 Solution by Microwave Irradiation

Monodispersed α-Fe 2 O 3 nanoparticles were prepared at 100°, 120°, and 140°C using 0.02, 0.05, and 0.1 M-Fe (NO 3 ) 3 .9H 2 O solutions by microwave-hydrothermal (M-H) and conventional-hydrothermal (C-H) reactions. Compared to the formation of monodispersed α-Fe 2 O 3 particles using the C-H reaction, the M-H reaction led to increased rate of formation by 3 times at 120°C and 5 times at 140°C from 0.1 M-Fe(NO 3 ) 3 solutions. α-Fe 2 O 3 nanoparticles of 23-25 nm in diameter were obtained from 0.02 M solution at 100°C after treatment for 4 h by applying microwaves, which led to the formation of smaller sized particles with a narrow particle size distribution. The coagulation of nano-sized single crystals of 5 to 6 nm in diameter was enhanced by increasing the concentration of Fe(NO 3 ) 3 solutions.

Enhancement of crystallization rate by microwave radiation: synthesis of ZSM-5

ZSM-5 type of zeolite was synthesized using two different techniques, first by conventional hydrothermal (C-H) method and another by exposing the hydrogel to microwave radiation followed by C-H method. In the case of microwave treated gel the crystallization period is reduced substantially (nearly half) to that of crystallization period by C-H method. ZSM-5 zeolites synthesized by these two methods have been characterized by different characterization techniques like X-ray diffraction, IR, TG/DTA, sorption and MASNMR. By both the techniques the ZSM-5 synthesized was similar in all the respects however; the M-T synthesis technique is time saving.

Microwave hydrothermal preparation of submicron-sized spherical magnetite (Fe 3O 4) powders

Submicron-sized (0.15–0.2 μm) spherical agglomerates of magnetite (Fe 3O 4) powders have been prepared successfully by microwave hydrothermal (MH) reaction of ferrous sulphate and sodium hydroxide in the temperature range of 90–200 °C. X-ray powder diffraction patterns of all these powders indicated that the product is single-phase magnetite with cubic spinel structure having lattice parameter, a 0=8.392 Å. The Mössbauer spectra of these powders indicated that stoichiometric Fe 3O 4 particles are obtained only when molar ratio of Fe/NaOH≥0.133 is maintained in the solution. It is observed that Fe/NaOH ratio is an important parameter for the controlled oxidation of ferrous salts in alkaline media under MH condition to produce stoichiometric Fe 3O 4. Further, the kinetics of MH synthesis is one order faster than the reported conventional hydrothermal (CH) synthesis. The value of saturation magnetization M=70 emu/g is obtained in case of stoichiometric Fe 3O 4. However, when ferric salt is treated in alkaline medium single-phase α-Fe 2O 3 is obtained under the MH conditions of 200 °C (194 psi).

Microwave Versus Conventional-Hydrothermal Synthesis of NaY Zeolite

This paper focuses on the synthesis of NaY zeolite crystals by using microwave-hydrothermal (M-H) processing. NaY zeolites were synthesized from colloidal silica, sodium aluminate, sodium hydroxide and deionized water at 100°C, and some properties were studied. NaY zeolite crystals with high surface area, in the range of 439 to 716 m2/g, have been prepared at 100°C for 1 to 3 hrs. Compared to the formation of NaY zeolite via conventional-hydrothermal (C-H) treatment, the M-H treatment led to increased rate of formation by 3 to 4 times.

Processing of Pb(Zr 0.52Ti 0.48)O 3 (PZT) ceramics from microwave and conventional hydrothermal powders

Lead zirconium titanate (PZT) perovskite powders were synthesized without using any excess lead content by microwave hydrothermal method (M-H) at 122°C and conventional hydrothermal (C-H) method at 138°C. PZT powders prepared by both methods showed a particle size of less than 4 μm. Pellets prepared from these powders and sintered at 1250°C for 4 h, without any additives or sintering aids, showed densities in the range of 92–95% of theoretical density. Variation of relative permittivity and dissipation factor with temperature of these PZT pellets was studied. A maximum permittivity of 20,570, low dissipation factor of 0.002, and Curie temperature of 373°C were observed for sintered pellets prepared from PZT powders obtained by microwave hydrothermal method. The pellets prepared from PZT powders obtained by the conventional hydrothermal method showed a relative permittivity of 16,310, dissipation factor of 0.001, and Curie temperature of 351°C.

Barium titanate ceramics prepared from conventional and microwave hydrothermal powders

Microwave-hydrothermal (MH) synthesis of BaTiO 3 was done at 138°C with reaction time as low as 15 min, using Ba(NO 3) 2, TiCl 4 and KOH as starting chemicals. The resulting powder was pressed into pellets and sintered at different temperatures. BaTiO 3 ceramics prepared by the above method resulted in a density of about 93% upon sintering at 1300°C/2 h with larger grains and room temperature relative permittivity of 2000 at 1 kHz with a Curie temperature of 125°C. The above properties were compared with samples of the same composition prepared by conventional hydrothermal (CH) method using the same starting solutions.

Rapid synthesis of AlPO 4-11 and cloverite by microwavehydrothermal processing

This study focuses on the rapid preparation of AlPO 4-11 and cloverite by microwave-hydrothermal (M-H) heating and the comparison of this heating technique with the conventional-hydrothermal (C-H) heating technique in the synthesis of these phases. By M-H heating, pure AlPO 4-11 phase was synthesized from boehmite at 70 psi (~150 C) within 2.5 h without pretreatment of the hydrogels. C-H preparation of AlPO 4-11. on the other hand, requires an aging step for about a day at 90 C prior to crystallization at 200 C. In the M-H heating system, pure cloverite was crystallized at 30 psi (~120C) within 2.7 h. whereas C-H heating at 120 C required more than 24 h for its crystallization. Large single crystals of dense GaPO 4 phase were crystallized at 70 psi (~150 C) within 2.5 h by M-H treatment. This study shows that the crystallization kinetics of AlPO 4-11 and cloverite were enhanced by more than an order of magnitude in the M-H system compared with the C-H system.

Hydrothermal and microwave-hydrothermal preparation of silica gels

We report here the rapid preparation and characterization of silica gels under conventional-hydrothermal (CH) as well as microwave-hydrothermal (MH) conditions from tetraethoxysilane and water in a molar ratio of 1:4 but without adding any alcohol as a co-solvent. Silica gels were prepared at 138 °C in 2 h by both CH and MH processing while heating in an oven at 60 °C required 48 h for complete gelation. Gelation was not complete in a water bath at 60 °C even after 48 h. The rapid preparation of silica gels under CH and MH conditions is expected to save time, and maybe energy.

Microwave-hydrothermal processing of layered anion exchangers

We have compared the microwave-hydrothermal (M-H) processing with conventional hydrothermal (C-H) processing in the preparation of two layered anion exchangers, i.e., Mg3Al(OH)8NO3 · nH2O and Ni1-xZn2x(OH)2(CH3COO)2x · nH2O. Both these phases can be crystallized more rapidly (an order of magnitude) under M-H processing compared to C-H processing. The above layered mixed basic salt of Ni and Zn was found to exhibit very high selectivity for PO4= (Kd = 15,000). Its order of selectivity for various anions in the presence of 0.1 N̲ NaC1 (ratio of C1- to anion in question is 100) increases as follows: PO4= ≫ SO4= > NO3-.

Conventional- versus microwave-hydrothermal leaching of glass from sintered kaolinite to make porous mullite

A microwave-hydrothermal treatment with 1 and 5N-NaOH solution was used at 150°C and 190°C to dissolve excess silica glass and thus make porous mullite ceramics from a fired New Zealand kaolin honeycomb. The effect of microwave-hydrothermal (M-H) treatment time on the dissolution of the glass was examined and compared to the result of conventional-hydrothermal (C-H) treatment. As expected, the rate of the dissolution of glass was faster with M-H treatment compared to C-H treatment. The dissolution of 40–43% glass was almost complete after M-H treatment in 1N-NaOH for 6 hrs at 150°C, in1N-NaOH for 3 hrs at 190°C, in5N-NaOH for 1.5 hrs at 150°C, and in 5N-NaOH for 35 minutes at 190°C. When the M-H treatment in 5N-NaOH solution at 190°C was more than 45 minutes, nonporous prismatic crystals were formed on the honeycomb and these crystals decreased the specific surface area. There were no changes in the morphology of mullite whiskers or in the structure of porous mullite body after M-H treatments in NaOH solutions compared to that of C-H treatment. The M-H treatment has been shown to be a rapid technique to prepare porous mullite ceramics from fired kaolin honey comb.

Microwave‐Hydrothermal Processing of BiFeO3 and CsAl2PO6

Microwave‐hydrothermal (M‐H) processing was compared with conventional‐hydrothermal (C‐H) processing in the crystallization of BiFeO3 and CsAl2PO6 phases. The presence of the microwave field led to accelerated kinetics of the crystallization of both these phases as detected by powder X‐ray diffraction. The acceleration of reaction rates under microwave field is expected to lead to energy savings during ceramic processing.