DRYING OF Y-Ba-Cu-O COMPLEX OXIDE SUSPENSION BY SUPERCRITICAL CARBON DIOXIDE

Abstract

Fine and homogeneous powders of Y-Ba-Cu complex oxides are necessary to improve the quality of YBCO bulk high temperature superconductors. The powders which contain BaCO₃ must be sintered for producing YBCO super-conductors at temperature above 850 °C. Therefore, complex oxide is desired. In order to prepare fine complex oxides powders, suspension was synthesized by simultaneous hydrolization of 2-propanol solution of metal isopropoxides. However, the suspension was not dried by a hot air drying method, because aggregation occurs due to cohesion of particle along with evaporation. Also, the suspension could not be freeze-dried, because the freezing point of 2-propanol is ?88.5°C. So, supercritical fluid drying method using supercritical carbon dioxide was applied. The results from these experiments suggest that this method is useful for drying of the suspension prepared by the alkoxide route.     

Supercritical Fluid Drying of Aqueous 2-Propanol Suspension

Abstract

To prepare fine powders of oxides, supercritical fluid drying was applied. As the case of point, Y–Ba–Cu complex oxide was taken up. A suspension of Y–Ba–Cu complex oxide was synthesized by simultaneous hydrolyzation of 2-propanol solution of yttrium triisopropoxide–barium diisopropoxide–copper diisopropoxide. The supercritical fluid drying by carbon dioxide was confirmed to dry the suspensions, expecting no aggregation of particles throughout the drying procedure. First of all, solubilities of 2-propanol, water, and aqueous 2-propanol solution in high pressure carbon dioxide were measured. Judging from the observed solubilities, temperatures, and pressures in drying conditions were determined. The drying rates were measured and the dried powders were characterized by DTA, TGA, and SEM. The results of these experiments suggested that the supercritical fluid drying process is useful for drying of the suspensions of superconducting compound prepared by hydrolyzation of alkoxides.     

Synthesis of yttrium aluminum garnet (YAG) powder by homogeneous precipitation combined with supercritical carbon dioxide or ethanol fluid drying

YAG precursors were synthesized by the urea method in aqueous solution using supercritical carbon dioxide and ethanol fluid drying technique, respectively. The composition of the precursors, the phase formation process and the properties of the calcined powders were investigated by means of XRD, IR, TG/DSC, BET, TEM and SEM. Compared with the classically prepared powders at room temperature in air, the amorphous precursor dried by supercritical CO₂ fluid was loosely agglomerated and directly converted to pure YAG at about 900 °C. The resultant YAG powders showed good dispersity with an average crystallite size about 20 nm and specific surface area of 52 m² g⁻¹. However, the precursor dried by supercritical ethanol fluid was crystalline. Extensive phase segregation occurred during the drying process and resulted in the formation of separate phases such as monoclinic Y(OH)₃ and pseudoboehmite. YAM and YAP phases appeared in the calcination process and phase pure were not detected until 1200 °C.     

Synthesis, formation and characterization of ZnTiO3 ceramics

Zinc titanate (ZnTiO₃) powders of perovskite structure were synthesized by conventional solid state reaction using metal oxides. Powders of ZnO and TiO₂ in a molar ratio of 1:1 were mixed in a ball mill and then heated at temperatures from 700 to 1000 °C for various time periods in air. The crystallization temperature of ZnTiO₃ powder was 820 °C, activation energy for crystallization was 327.14 kJ/mol and for grain growth was 48.84 kJ/mol. A transition point was observed when the electrical resistivity was measured versus temperature. Like some ferroelectric materials, a PTCR behavior above the transition temperature was observed with Curie temperature of 5 °C.     

DRYING KINETICS AND CHARACTERISTIC DRYING CURVE OF LIGHTLY SALTED SARDINE (SARDINELLA AURITA)

Drying of lightly salted sardine (Sardinella aurita) was accomplished using three air temperatures (35°C, 40°C, 50°C) and three air velocities (0.5 m/s, 1.5 m/s, 2 m/s); the effects of drying conditions on drying kinetics were studied. As for all biological products, air temperature is the main factor influencing the drying kinetics. However, over a given temperature which seems to correspond to protein modification (50°C), and at a high air flow rate (2 m/s and 2.5 m/s) a crust formation on the surface of the fish, due to the combined effect of heat and salt was observed. This phenomenon inhibited the drying rate. From the drying curves, two falling rate periods were observed. The dimensionless drying rate versus a dimensionless moisture content data were regressed by the Marquardt Levenberg non-linear optimization method to obtain an empirical equation describing the salted sardine characteristic drying curve.     

Drying Suspensions in a Pulsed Fluidized Bed of Inert Particles

The drying of carbohydrate suspensions on polypropylene particles in a pulsed fluidized bed was studied by means of a 2⁵ experimental design, to determine the effect of the air flow and temperature, suspension flow rate, and free section and rotating speed of the rotary plate on the Nusselt number, the moisture content of the product, and the percentage of solids retained inside the bed (which were minimized to 4.9 and 14.4%, respectively) with an air flow of 600 m³/h at 90°C and 720 mmHg, a suspension flow rate of 6 L/h, and a plate with 6% free section, rotating at 50 rpm.

Additionally, the effects of temperature, air flow, and suspension flow rate on the residence time distribution (RTD) were determined, using the stimulus-response methodology. The RTD was represented by 1.1 to 2 tanks in series, according to this model. The mean residence time of the dried carbohydrate particles was between 5.4 and 8.2 min.

Finally, an egg suspension could be dried at 4 L/h, with air at 90°C, with a mean residence time about 50% longer that that found for drying carbohydrate suspensions.     

Accurate Determination of Moisture Content of Organic Soils Using the Oven Drying Method

The accuracy of moisture content values determined for organic soil using the oven drying method is dependent on the oven drying temperature. Some charring of the organic fraction occurs at the standard oven drying temperature of 110 ± 5°C; pore water remains in the soil when temperatures below 100°C are used. A new technique to determine the specific oven drying temperature that yields the correct value of the moisture content for organic soils is presented. Routine moisture content tests should be conducted at the standard oven drying temperature; moisture content values determined on the basis of the recommended oven drying temperature of 60°C include a larger error. A moisture content parameter is introduced to facilitate direct comparison of moisture content values calculated on the basis of different oven drying temperatures.     

Surface Characterization of Ti and Ti (6% AI-4%, V) Metal Powders and Interaction with Primer Solutions

The surfaces of Ti and Ti (6% Al-4% V) powders were characterized by several techniques. BET surface areas as a function of temperature were measured using nitrogen adsorption. Heats of immersion (δ_wH) of these metal powders in water were measured after evacuation over the temperature range 100°-400°C. The δ_wH in water increased with increasing evacuation temperature and an anomalous increase was observed between 300° and 400°C. This was attributed to exposure of water to elemental titanium by cracking of the oxide layer at 400°C. XPS analysis did not support the possibility of metal migration through the oxide layer. Higher heats of immersion in water were determined for chemically pretreated compared to untreated Ti 6-4 powders. Water vapor adsorption isotherms were measured after evacuation of the metal powders at 100°C. Partial irreversibility of the water adsorption was observed on both powders. Water adsorption on Ti was temperature dependent. Heats of immersion measurements were used to study the interaction of primer solutions with these metal powders. Polyimide (LARC-13) and polyphenylquinoxaline (PPQ) interacted preferentially compared to the solvents with both powders. This polymer-metal interaction improved significantly after pretreatment of the Ti 6-4 powder by the Turco® 5578 and phosphate-fluoride processes. Again, an anomalous increase in the heat of immersion of Ti 6-4 evacuated at 400°C in the primer solution/solvents was observed. Anatase and rutile TiO₂ powders are not satisfactory models for the surface oxide layer on either Ti or Ti 6-4 powders.     

Effect of the Treatment Temperature on the Surface Drying Set of Sugi Boxed-Heart Square Timber

High-temperature and controlled humidity treatment before drying has proven to be highly effective in preventing surface checks by forming drying sets in the surface layer of boxed-heart square timber in Japan. In this study, we examined the differences of the surface drying sets on sugi timber under different treatment temperatures. As a result, the width of a surface check after drying was much smaller at a treatment temperature of 120°C than at 80°C, and this indicates that the drying set with 120°C treatment would be larger than that with 80°C treatment. Also we observed that treatment temperature had a greater effect on the drying set as the drying advanced.     

Application of pulse discharge sintering (PDS) technique to rapid synthesis of Ti3SiC2 from Ti/Si/C powders

To synthesize Ti₃SiC₂ samples, pulse discharge sintering (PDS) technique was utilized to sinter elemental powders of Ti/Si/C with stoichiometric and off-stoichiometric ratios in a temperature range of 1200–1500 °C. The results showed that high purity Ti₃SiC₂ could not be obtained from the Ti/Si/C powder with molar ratio of 3:1:2, and Ti₃SiC₂ preferred to form at relatively low sintering temperature for a short time. When 5Ti/2Si/3C and 3Ti/1.5Si/2C powders were sintered for 15 min, the TiC content was respectively decreased to 6.4 and 10 wt.% at 1250–1300 °C. The corresponding relative density of the samples sintered from 5Ti/2Si/3C powder was calculated to be as high as 99% at the temperature above 1300 °C. It is suggested that low-temperature rapid synthesis of Ti₃SiC₂ would be possible through the PDS technique, provided that the composition of the starting powders should be adjusted to be off-stoichiometric ratio from 3:1:2.     

DRYING CHARACTERISTICS OF THE HAZELNUT

Equilibrium moisture content isotherms for Spanish hazelnut (Corylus avellana L.) at different temperatures (30°C-80°C) were determined using static gravimetric method. Thin layer drying experiments were done with forced air circulation and were conducted with different operating conditions to determine the drying characteristics of hazelnuts. The effect of air temperature (30°C-70°C), air velocity (0.5 m/s - 2 m/s) and drying bed loading density (50 kg/m² - 150 kg/m²) on drying of unshelled and shelled hazelnuts was studied. Six mathematical models were used to fit the experimental equilibrium moisture content data, from which the G.A.B. model was found to give the best fit. Diffusion coefficients were determined by fitting experimental thin-layer drying curves to the Fick's diffusion model. Variation of the effective diffusion coefficient with temperature was of the Arrhenius type. The Page equation was found to describe adequately the thin layer drying of hazelnut. Page equation drying parameters k and n were correlated with air temperature and relative humidity.     

MICROFLORA CHANGES IN TUNA MINCE DURING CONVECTION AIR DRV1NG

Microorganisms grow in tuna during drying at low temperatures. The drying temperature of 50°C or below is not lethal to the microflora. The decimal reduction time (D-value) varied from 12.66 to 2.63 hr when drying temperature increased from 60 to 100°C, respectively. These values can be used to estimate the lethal effect of drying on the natural microflora in tuna.     

The SMSI between supported platinum and CeO2–ZrO2–La2O3 mixed oxides in oxidative atmosphere

CeO₂–ZrO₂–La₂O₃ (CZL) mixed oxides were prepared by citric acid sol–gel method. The as-received gel was calcined at 500, 700, 900 and 1050 °C to obtain the so-called C5, C7, C9 and CK, respectively. The C5, C7 and C9 powders were impregnated with H₂PtCl₆ and then calcined at 500 °C to prepare P5C5, P5C7 and P5C9, respectively. The impregnated CK powders were calcined at 500, 700 and 900 °C to prepare P5CK, P7CK and P9CK, respectively. The XRD and XPS analyses show that the surface distribution of Pt is evidently influenced by the structural and textural properties of the support. The CO adsorption followed by FTIR reveals that the dispersion and the chemisorption sites of Pt are reduced as the calcination temperature of CZL support increases. The chemisorption ability of the CK samples is even completely deactivated. The encapsulation mechanism, which has been applied to explain the so-called strong metal–support interaction (SMSI) after reductive treatment, is introduced here to demonstrate the abnormal observations though the samples were prepared in oxidative atmosphere. The HRTEM results also confirm this explanation. The effects of oxygen vacancies, the chemisorption sites on the Pt surface and Pt/Ce interfacial sites on the three-way catalytic activities are discussed.     

Synthesis of barium titanate using supercritical CO2 drying of gels

BaTiO₃ powders from sol-gel derived gels were prepared using two different drying methods. In addition to the conventional drying of gels in air at 90°C the supercritical CO₂ drying method was also used. Results showed that the properties of BaTiO₃ powder produced by supercritical drying with CO₂ are better. The grain surface is less contaminated as a result of the supercritical drying and the microstructure development during sintering leads to a homogeneous fine-grained microstructure.     

Effect of Microwave-Vacuum Drying on the Carotenoids Retention of Carrot Slices and Chlorophyll Retention of Chinese Chive Leaves

Microwave-vacuum drying, and combined microwave-vacuum with air or vacuum drying were investigated as potential methods for drying green and high-carotenoid vegetables of Chinese chive leaves and carrot slices. The samples were dried by microwave-vacuum until the moisture content reached about 20% (wet basis), and then by conventional air drying at the temperature of 45-50°C or conventional vacuum drying at the temperature of 55-60°C or by continued microwave-vacuum drying at lower power level to final moisture content about 6% (wet basis). The carotenoid retention of carrot slices and chlorophyll retention of Chinese chive leaves, dried by these methods, was evaluated and compared with those dried by freeze-drying and conventional hot-air drying. The comparison showed that the carotenoid retention of carrot slices and chlorophyll retention of chive leaves, dried by the current methods, was very close to or even as much as those dried by freeze-drying and much better than those dried by conventional hot-air. Blanching was not necessary when using microwave-vacuum drying or combined microwave-vacuum with conventional hot-air drying or vacuum drying, because the activity of enzymes which were responsible for the degradation of color was greatly reduced with extremely rapid decreasing of moisture, and because oxygen was absent in microwave-vacuum drying.     

Drying Kinetics and Color Retention of Dehydrated Rosehips

Freshly harvested rosehips (Rosa canina L.) were dehydrated in a parallel flow type air dryer at six air temperatures (30, 40, 50, 60, and 70°C) at air velocities of 0.5, 1.0, and 1.5 m/s. Drying air temperature and velocity significantly influenced drying time and energy requirement. Minimum and maximum energy requirement for drying of rosehips were determined as 6.69 kWh/kg for 70°C at 0.5 m/s, and 42.46 kWh/kg for 50°C, 1.5 m/s. In order to reduce drying energy consumption, it is recommended that the drying air velocity must not be more than 0.5 m/s and drying air temperature should be 70°C. In addition, the influence of drying air temperature and air velocity on the color of dried rosehip has been studied. Hunter L, a, b values were used to evaluate changes in the total color difference (ΔE) on dried rosehips. 70°C drying air temperature and 1 m/s air velocity were found to yield better quality product.     

Activity-loss characteristics of spores of Bacillus thuringiensis during spray drying

Survival of spores of Bacillus thuringiensis was determined under various processing conditions for spray drying. The results indicated that the viable spores number of B. thuringiensis decreased with increased inlet air temperature, outlet air temperature and atomising air pressure. When the inlet air temperature ranged from 170 °C to 250 °C, the outlet air temperature (75 °C) and atomising air pressure (0.15 MPa) were fixed, the pseudo-z value (one logarithmic cycle reduction) was 238.1 °C; when the outlet air temperature ranged from 65 °C to 95 °C, the inlet air temperature (200 °C) and atomising air pressure (0.15 MPa) were fixed, the pseudo-z value was 85.5 °C, the activation energy calculated according to the outlet air temperature was 59.96 kJ mol⁻¹; when the atomising air pressure ranged from 0.10 MPa to 0.25 MPa, the inlet air temperature (200 °C) and outlet air temperature (75 °C) were fixed, the pseudo-z value was 3.49 MPa, the variance analysis showed that the atomising air pressure has no significant influence to the spores. The diluting solutions between Tween-80 solution and phosphate buffer have significant influence on the plate count of spores.

The B. thuringiensis powder prepared by spray drying with inlet air temperature 250 °C, outlet air temperature 97 °C, the spores count of powder decreased obviously. But when the inlet air temperature of 155–165 °C, outlet air temperature of 66–70 °C were employed, the spores count of powder approaches to that of freeze drying powder. The spores count of oven drying powder was lower than that of the freeze drying powder, but close to the spray drying powder which inlet air temperature was 200 °C, outlet air temperature was 75 °C.     

Photoactivity of anatase–rutile TiO2 nanocrystalline mixtures obtained by heat treatment of homogeneously precipitated anatase

Nanosized titanium dioxide photocatalysts with varying amount of anatase and rutile phases have been synthesized. Homogeneous precipitation of aqueous solutions containing TiOSO₄ with urea was used to prepare porous spherical clusters of anatase TiO_2. Photoactive titania powders with variable amount of anatase and rutile phases were prepared by heating of pure anatase in the temperatutre range 800–1150 °C. The structure evolution during heating of the starting anatase powders was studied by XRD analysis in overall temperature range of phase transformation. The morphology and microstucture characteristics were also obtained by HRTEM, BET and BJH. The spherical particle morphology of TiO₂ mixtures determined by SEM was stable in air up to 900 °C. The photocatalytic activity of the sample titania TIT85/825 heated to 825 °C in air, contained 77.4% anatase and 22.6% rutile was higher than that nanocrystalline anatase powder. Titania sample TIT85/825 reveals the highest catalytic activity during the photocatalyzed degradation of 4-chlorophenol in aqueous suspension.