Combined microwave–hot-air drying of burdock slices with feedback temperature control at surface and core

During combined microwave–hot-air drying, the surface and the core temperatures of the sample have great influence on the process. To investigate the influence systematically, drying system with feedback control strategy of the two temperatures was proposed. Then various pairs of the two temperatures were applied in the drying mode 1. However, it was found difficulty to achieve both short drying time and high product quality with fixed temperature pair, because the interaction between microwave and the sample changes as the moisture content decreases in the drying process. Different temperature pairs were applied during the three drying stages in drying mode 2, so that better product can be obtained in shorter drying time. To further improve the product quality, the drying rate was controlled by a feedback loop within a desired range in drying mode 3. The change of drying rate was realized by adjusting the two temperatures continuously. To omit the weighing scale, a feedforward control strategy for the drying rate was put forward in drying mode 4, where the temperatures were controlled along with preset lines. The results showed that the product quality and the drying time were similar to those in drying mode 3.     

Influence of convective intermittent drying schemes on drying induced stress–strain of a 3D clay object

Drying process of industrial green in-process products especially those susceptible to cracking, need great care, and optimally arrangement of parameters of convective drying. Intermittent drying is a new technique in drying area and is a promising solution for product quality enhancement. The intermittent drying with variable air temperature and the intermittent drying with variable air humidity are the most used techniques. The current study is devoted to 3D modeling and simulation of intermittent drying with variations of both air humidity and temperature and it is then compared with each of the cases of the intermittent drying with variable air temperature and the intermittent drying with variable air humidity. It was observed that the best dried product quality was obtained in intermittent drying with periodic changes of air temperature. Vapor condensation in the intermittent drying with variable air humidity is an undesirable phenomenon that significantly reduces the effectiveness of this process.     

Dielectric,mechanical and thermal properties of ZrO2–TiO2 materials obtained by microwave sintering at low temperature

The sinterability of 3Y-TZP/TiO₂ materials using micrometre-sized ZrO₂ and nanometre-sized TiO₂ (16 wt%) by one-step fast microwave sintering at low temperature (1200–1300 °C) was investigated. Firstly, in situ detailed analysis of the dielectric properties of the material with temperature was carried out in order to measure the capacity of the material to transform microwave energy into heat. Another related parameter associated to microwave sintering is the penetration depth of the microwave radiation into the material, which showed great homogeneity from 400 °C. Secondly, the effect of sintering conditions on microstructure, density, hardness and coefficient of thermal expansion was evaluated. The X-ray diffraction study and microstructural characterization demonstrate that it is possible to obtain fully dense pieces (>99%) by microwave sintering, a condition yielding to a coarse-grained (~1–2 μm), quite hard (~13.7 GPa) 3Y-TZP/TiO₂ material. However, the most important feature is the significant reduction of the thermal expansion coefficient (8·10⁻⁶ K⁻¹) as compared to that of 3Y-TZP. In addition, the results from conventional sintering at 1400–1500 °C with 2 and 6 h of dwell time are examined and compared. The materials obtained at 1500 °C showed high density with grain size and hardness similar to those obtained by microwave but with a dramatic difference in the power consumption of the sintering cycle, since the materials obtained by microwave used a maximum absorbed power of 120 W and a heating cycle of only 40 min.     

Ultrasonication as pretreatment for drying of tomato slices in a hot air–microwave hybrid oven

This study aims to investigate the effect of ultrasonic pretreatment on drying time and quality properties of tomato slices dried by microwave combined with hot air at 60°C. The influence of ultrasound pretreatment (0, 20, and 40?min) and microwave power (120, 150, and 180?W) on drying time, color, total phenolic content, lycopene, vitamin C, and rehydration capacity of dried slices of tomato was studied. Results showed that as the microwave power level increased, drying time decreased significantly (about 46.4%). Ultrasound pretreatment decreased the drying time by 7.38% only at 120?W microwave power and 40?min of pretreatment compared to those without ultrasound pretreatment at the same microwave power. Depending on drying conditions, vitamin C and lycopene contents reduced from 433.94 to 81.89?mg AA/100?g dry solids and 3920.57 to 415.40?mg/100?g dry solids, respectively. The change in total phenolic content was not severe as much as vitamin C contents. Rehydration capacity of pretreated samples was larger than nontreated samples. The color values of dried tomato slices were in the acceptable range. Both microwave power and ultrasound pretreatment affected the quality of the final product significantly.     

Combining osmotic–steam blanching with infrared–microwave–hot air drying: Production of dried lemon (Citrus limon L.) slices and enzyme inactivation

The different pretreatments were given to lemon slices to inactivate pectinesterase and peroxidase enzymes and to dry the product rapidly using infrared–microwave hot air combination. Osmotic pretreatment followed by 1-min steam blanching was found to reduce moisture in the product, increase solid content, and inactivate enzymes in lemon slices while maintaining negligible dry matter and juice sac loss. The infrared hot air was found effective in partial drying of pretreated lemon slices up to 1 hour without entering in drastic falling-rate period. Therefore, after 1?h microwave hot air was used to complete the drying process. The optimum infrared drying condition was found at 3000?W/m² radiation intensity, 90°C air temperature, 100?mm distance between lamp and product, and 1.5?m?s^?1 air velocity. In microwave finish drying, the power density of 0.30?W?g^?1, 89.9°C air temperature, and 0.5?m?s^?1 air velocity were found to result in the best product. The hybridization of osmotic–steam blanching and the two drying methods overcame the problems of browning, extended falling-rate periods, improper power distribution, and quality deterioration. Also, the higher values of moisture diffusivities were observed during hybrid drying.     

Mesoporous copper–cerium–oxygen hybrid nanostructures for low temperature catalytic oxidation of carbon monoxide

Mesoporous copper–cerium–oxygen hybrid nanostructures were prepared by one-pot cetyltrimethylammonium bromide surfactant-assisted method, and were characterized by thermogravimetry, X-ray diffraction, transmission electron microscopy, nitrogen adsorption–desorption, X-ray photoelectron spectroscopy and temperature-programmed reduction techniques. Low temperature carbon monoxide oxidation was used as probe reaction to investigate the application of the prepared mesoporous copper–cerium–oxygen hybrid nanostructures in catalysis. The product calcined at 400 °C, with disordered wormlike mesoporous structure, high specific surface area (SSA) of 117.4 m²/g and small catalyst particle size of 8.3 nm, shows high catalytic activity with the 100 % CO conversion at 110 °C, indicating its potential application in catalysis. Catalytic activity results from the samples calcinied at different temperature suggested that high SSA, small catalyst particle size, finely dispersed CuO species and synergistic effect between CuO and CeO₂ were responsible for the high catalytic activity of the catalysts.     

Drying uniformity analysis of pulse-spouted microwave–freeze drying of banana cubes

This article reports the investigation of the drying uniformity of a new drying technique called pulse-spouted microwave–freeze drying. A computer vision technique and mathematical statistics were used to evaluate the drying uniformity. The results show that low microwave power results in prolonged drying time (the drying time of 1 W/g was 6 h, which was longer than that for 2 and 3 W/g), whereas spouting time and the time interval show less influence on the drying time. Analysis from infrared camera photos reveals that lower microwave power, longer spouting interval, and longer spouting time could improve the temperature distribution. Sample 4 (power: 2 W/g, time: 3 s, interval: 300 s) had the best temperature distribution uniformity.     

Temperature stability and high-Qf of low temperature firing Mg2SiO4–Li2TiO3 microwave dielectric ceramics

In this work, a series of low-temperature-firing (1−x)Mg₂SiO₄–xLi₂TiO₃–8 wt% LiF (x = 35–85 wt%) microwave dielectric ceramics was prepared through conventional solid state reaction. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses showed that the Li₂TiO₃ phase was transformed into cubic phase LiTiO₂ phase and secondary phase Li₂TiSiO₅. Partial substitution of Mg²⁺ ions for Ti³⁺ ions or Li⁺Ti³⁺ ions increased the cell volume of the LiTiO₂ phase. The dense microstructures were obtained in low Li₂TiO₃ content (x ≤ 65 wt%) samples sintered at 900 °C, whereas the small quantity of pores presented in high Li₂TiO₃ content (x ≥ 75 wt%) samples sintered at 900 °C and low Li₂TiO₃ content (x = 45 wt%) sintered at 850 and 950 °C. Samples at x = 45 wt% under sintering at 900 °C for 4 h showed excellent microwave dielectric properties of ε_r = 10.7, high Q × f = 237,400 GHz and near-zero τ_f = − 3.0 ppm/°C. The ceramic also exhibited excellent chemical compatibility with Ag. Thus, the fabricated material could be a possible candidate for low temperature co-fired ceramic (LTCC) applications.     

Ibuprofen–polymer precipitation using supercritical CO2 at low temperature

A process for the SAS coprecipitation of ibuprofen with the polymers poly(l-lactic acid) and Eudragit L100 was successfully carried out. The particle size was reduced to micrometer and near nanometer ranges. The morphology of the raw material changed to spherical upon processing for both poly(l-lactic acid)/ibuprofen particles and eudragit/ibuprofen particles. The eudragit-based particles were significantly smaller than those obtained with poly(l-lactic acid). Ibuprofen release profiles were determined for simulated gastric and intestinal fluids in order to study the effect of the polymer and to identify the appropriate systems for different administration routes. The in vitro release profiles for both polymer/drug systems showed a slower and more controlled release in comparison to the unprocessed ibuprofen. Moreover, the effects of pressure, temperature, initial concentration of the solution and drug-to-polymer ratio on the particle size and morphology of these drug/polymer systems have been evaluated. According to the XRD, DSC and FTIR data, physicochemical interactions do not occur between ibuprofen and the polymers and a proportion of the ibuprofen molecules probably remained on the microparticle surface.     

Hydrothermal synthesis of ZrO2–Y2O3 solid solutions at low temperature

Ultrafine powders of ZrO₂–Y₂O₃ solid solutions have been synthesized by hydrothermal treatment at 110°C. Zirconia gel, crystalline Y₂O₃ and various mineralizing solutions have been utilized as precursors for the hydrothermal synthesis. Yttria-stabilized zirconia (YSZ) with different Y₂O₃ content and characterized by different crystallite sizes have been produced by changing the hydrothermal treatment temperature, and the nature and concentration of the mineralizer solution. The role of mineralizer solutions on the crystallization-stabilization of zirconia gel at low temperature of hydrothermal treatment is discussed.     

The production of β-SiAlON ceramics with low amounts of additive,at low sintering temperature

In the present work nano-sized powder of β-SiAlON was produced using a wet milling process. Different milling times and mediums (methyl ethyl keton, ethanol and toluene as solvents, polyethyleneglicol, oleic acid, sodium tripolyphosphate and polyvinylpyrrolidon as dispersants) were performed for the determination of the most efficient milling system. The powders were produced using a conventional process (the ball to powder ratio was 1:1.5, at 300 rpm, for 1.5 h) having a few hundred nanometer particle size, and these were used as standard powders in this study. The nano-sized β-SiAlON starting powders (<100 nm) were sintered at lower temperatures than that of the conventional powders. The amount of Y₂O₃ in powders (～130 nm), produced by high energy milling process, was fewer than conventional powders (5 wt.%). The results of the powder size, sintering behavior and mechanical properties of this sample were compared to those of the standard powder and its sintered sample. This sample, produced using the nano-powder, was investigated, and densified at 150 °C lower than that of the standard sample. Even though the amount of Y₂O₃ was decreased, the hardness of the samples was better than that of the standard sample.     

Fast-low temperature microwave sintering of ZrSiO4–ZrO2 composites

Today, many industrial applications require components that work under extreme conditions, especially at very high temperatures (>1200 °C) for a long time. An excellent combination of properties such as low thermal conductivity, low coefficient of thermal expansion and high chemical resistance are required for such applications. Advanced ceramic materials based on zircon-zirconia composites (ZrSiO₄–ZrO₂) possess these properties, thus making them attractive for, i.e., high-level radioactive waste immobilisation. The main drawback of these materials are the high temperatures and long residence times required to sinter them and obtain high densities, which entails high energy consumption and costs. Therefore, non-conventional microwave sintering is a very powerful and efficient technique capable of reducing sintering temperatures and holding times. The objective of this study is to evaluate the microwave sinterability of zircon-zirconia powders obtained by colloidal methods (80–20 vol% and 20–80 vol% ZrSiO₄–ZrO₂). A stability study of the phases present was carried out by X-ray diffraction and the mechanical and microstructural properties were evaluated in order to obtain the best materials with outstanding final properties.     

Effect of composition on sinter-crystallization and properties of low temperature co-fired α-cordierite glass–ceramics

This article investigates effect of composition, including SiO₂ and impurity defined to contain K₂O, Na₂O, Fe₂O₃, etc., from K-feldspar, on sinter-crystallization and properties of the low temperature co-fired α-cordierite glass–ceramics. Increasing impurity content from 5.72 wt% to 9.16 wt% leads to enhanced crystallinity, formation of leucite and more pores but the crystallinity and porosity decreased with a further increase to 10.8 wt%. The main impurity K₂O is critical for formation of α-cordierite and leucite. Only α-cordierite was precipitated from the glasses with different SiO₂ contents but an increase of SiO₂ content slightly improves their densification. The impurity and SiO₂ contents greatly affect the properties of glass–ceramics. Notably, some glass–ceramics from K-feldspar show high densification at low temperature, low dielectric constant (6–8), low loss (about 0.005), appropriate linear CTEs (4.32–5.87 × 10⁻⁶ K⁻¹) and flexural strength (above 100 MPa), all of which meet the requirements of LTCC substrates.     

Long-term creep behavior of polypropylene/fumed silica nanocomposites estimated by time–temperature and time–strain superposition approaches

Nonlinear viscoelastic creep was studied on polypropylene/fumed silica nanocomposites. The free-volume theory of nonlinear viscoelastic creep was successfully applied to obtain generalized creep master curves using a tensile compliance vs. internal time superposition in the region of nonlinear viscoelasticity. Concurrently, a time–temperature superposition approach was also adopted for the construction of creep master curves. A good agreement between the time–strain and the time–temperature superposition approaches was assessed by comparing the master curves obtained from the two data reduction methods. Both approaches evidenced a remarkable stabilizing effect induced by the nanoparticles that was observed especially for higher creep stresses and at increased temperatures and, considering the correspondent superposition principle, at long loading times. At the same time, both storage and loss moduli measured through dynamic mechanical analyses, were enhanced in all nanocomposites. Activation energy values obtained from the analysis of dynamic multi-frequency tests were in good accordance with those referred to creep tests.     

Influence of uncertainty in heat–moisture transport properties on convective drying of porous materials by numerical modelling

The influence of uncertainties in heat–moisture transport properties, due to measurement errors and material heterogeneity, on the numerical simulation results of convective drying of two capillary-saturated porous materials is investigated by a transport-property parameter analysis (TP-PA), based on the Monte Carlo method. Here, the heat–air–moisture transfer model is evaluated many times, each time using a random set of one or multiple input parameters (i.e. transport properties), by which a stochastic model output is generated. The propagation of these transport-property uncertainties to the hygrothermal behaviour of the drying system is evaluated by statistical analysis of the model simulation output. The spread on the hygrothermal response is found to be strongly material dependent and is related to the dominant mode of moisture transport in the material, i.e. liquid or vapour transport. The TP-PA results clearly indicate that uncertainties in the heat–moisture transport properties can lead to significant differences in drying behaviour predictions, where differences of the total drying time with respect to its mean value up to 200% are found for the materials considered. Therefore, numerical modelling of heat and moisture transport in porous materials should preferably include a quantification of the propagation of these uncertainties, for example by means of the proposed transport-property parameter analysis. Such analysis, however, additionally requires detailed (a priori) experimental material characterisation to determine realistic uncertainty ranges.     

Elastic properties and damping behavior of alumina–zirconia composites at room temperature

Alumina–zirconia composite ceramics (AZ composites) have been prepared in the whole range of compositions from pure alumina to zirconia (in steps of 10 vol.%) by slip casting, followed by sintering at 1350 °C and microstructural characterization via the Archimedes method (relative densities 0.93–0.99). Young's modulus has been measured at room temperature via the impulse excitation technique (IET) and, after appropriate porosity correction (linear, power-law, exponential), found to be in good agreement with the Hashin–Shtrikman bounds. The damping factor (internal friction), which has been measured for dense AZ composites (also via IET at room temperature), is found to increase with increasing zirconia content. Damping factors measured for porous AZ composites with porosities 25–71%, prepared with corn starch as a pore former, have been found to depend only slightly on porosity, unless the porosities are extremely high (>70%). At these porosities, however, where the Young's moduli approach zero, the damping factors exhibit a steep increase.     

Direct and facile synthesis of Li–Sr–Zn ferrites via low temperature solution combustion route

Li–Sr–Zn nanoferrites i.e. Li_0.25Sr_0.5–xZn_xFe_2.25O₄, where ‘x’ varies from 0–0.5, have been synthesized using a low temperature solution combustion method which proves to be an efficient and economical technique for synthesizing these type of nanoferrites. The as-synthesized nanoferrites have cubic spinel structure as characterized by X-ray powder diffraction. Powder XRD and TEM (Transmission Electron Microscopy) characterization also evidence that the crystallite and particle size are in close agreement to each other. Mössbauer spectroscopy studies demonstrate that there is a gradual transition from ferrimagnetic to superparamagnetic character, which is also supported by the saturation magnetization and coercivity values. At room temperature, the nanoferrites were found to be superparamagnetic with negligible coercivities approaching towards zero while saturation magnetization values were found to be in the range 6.87–30.10 emu g⁻¹. The frequency dependent dielectric constant and loss values are in accordance with Koop׳s model. These nanoferrites show great potential in high density recordings, magnetic nanodevices and biomagnetic applications.     

NaCl–H2O-assisted preparation of SrTiO3 nanoparticles by solid state reaction at low temperature

An environmentally friendly NaCl–H₂O system was developed to synthesize monodisperse strontium titanate (SrTiO₃) nanoparticles from commercially available raw materials (SrCO₃ and rutile) by solid state reactions. The formation rate of SrTiO₃ was accelerated by the addition of NaCl and water vapor. Single phase SrTiO₃ was obtained by calcination at 700 °C for 2 h in water vapor (H₂O flow rate of 2.0 mL/min) by the addition of 50 wt% NaCl, although 900 °C and 750 °C for 2 h were required to complete the reaction by calcinations in air and air by the addition of 50 wt% NaCl, respectively. The results demonstrate that both NaCl and H₂O played vital roles to accelerate the formation of SrTiO₃ nanoparticles at relatively low temperature. On the basis of experiments and analysis, a rational growth mechanism has been proposed and discussed.     

Fischer–Tropsch synthesis with ethene co-feeding: Experimental evidence of the CO-insertion mechanism at low temperature

Experiments were performed at both normal and rather extreme Fischer–Tropsch Synthesis (FTS) operating conditions over a typical cobalt-based catalyst, with the aim of exploring if aspects of the reaction mechanism could be elucidated. The results show that CO reacted when co-feeding C₂H₄ with syngas, while CO did not react with H₂ in absence of C₂H₄, under extremely low-temperature conditions (140°C). The adsorbed CO and C₂H₄ may behave as monomers and initiators, respectively, and react with each other to form long chain hydrocarbons. It suggests that the C C bond coupling precedes the C O bond dissociation, which is consistent with the CO-insertion mechanism. C_3–6 product distribution with a feed of H₂/CO/C₂H₄ at low temperature followed the same trends in terms of normal FTS product distribution. The observed FTS-type chain growth reaction that occurs at abnormally low temperatures (140°C) when co-feeding C₂H₄ may provide new insights into the chemistry of FTS.