A review: production of activated carbon from agricultural byproducts via conventional and microwave heating

Production of activated carbon (AC) from agricultural byproducts is a research field that has gained increased interest in recent years because of its potential for the disposal of agro‐residues. At the same time, a beneficial byproduct that can be used in a number of environmental applications is produced. This paper surveys the developments in the production processes of AC from agricultural byproducts in the past 7 years from 2005 to 2012 via conventional and microwave heating. Emphasis is placed on the applied methodology and the influences of activating conditions, such as carbonization temperature, retention time, and impregnation ratio. From the review of AC production processes, agricultural wastes produced by a chemical method with microwave heating can be a source of AC with relatively higher surface area than that produced via conventional heating. © 2013 Society of Chemical Industry     

Mechanical and physical properties of ultraoriented polyoxymethylene produced by microwave heating drawing

The properties of ultra-oriented polyoxymethylene tubes produced by drawing under microwave heating have been assessed by mechanical testing, optical microscopy, scanning electron microscopy, X-ray analysis, birefringence and differential scanning calorimetry. The highest Young's modulus of 58 GPa was obtained at room temperature (77 GPa at ?150°C) at a draw ratio of 33. The maximum tensile strength was 1.7 GPa at a draw ratio of 26. The nonuniformity of Young's modulus in a radial direction has been compared with the nonuniformity of the birefringence and heat of fusion.     

Hydrophobicity and catalytic properties of Ti-MFI zeolites synthesized by microwave and conventional heating

Ti-incorporated MFI zeolite (Ti-MFI-MW) has been synthesized with microwave heating. Their physicochemical properties such as surface hydrophobicity, and adsorption and catalytic properties have been compared with those of Ti-incorporated MFI zeolite (Ti-MFI-CH) prepared by conventional hydrothermal method. Competitive adsorption measurements with toluene and water revealed that the hydrophobicity index of Ti-MFI-MW (8.0) is higher than that of Ti-MFI-CH (6.2). IR spectra showed that Ti-MFI-MW also has a lower content of surface hydroxyl groups and adsorbs a larger amount of 1-hexene than Ti-MFI-CH. These results pointed out that Ti-MFI-MW is more hydrophobic than Ti-MFI-CH. Epoxidation reactions of 1-hexene and styrene with hydrogen peroxide have been conducted to investigate catalytic properties of the Ti-MFI zeolites according to the synthesis method. The conversions and epoxide selectivities over Ti-MFI-MW are higher than those over Ti-MFI-CH due to the enhanced surface hydrophobicity.     

Effect of industrial microwave irradiation on the physicochemical properties and pyrolysis characteristics of lignite

The surface functional groups and pyrolysis characteristics of lignite irradiated by microwave were comparatively studied to evaluate the feasibility of using industrial 915 MHz for lignite drying. The drying kinetics, micro structure, chemical functional groups, re-adsorption properties, and pyrolysis characteristics of the dried coal were respectively analyzed. Results indicated that for typical Chinese lignite studied in this paper, 915 MHz microwave drying was 7.8 times faster than that of the hot air drying. After industrial microwave drying, the sample possessed much higher total specific surface area and specific pore volume than that of air dried sample. The oxygen functional groups and re-adsorption ratio of microwave irradiated coal decreased, showing weakened hydrophilicity. Moreover, during the pyrolysis of the coal dried by hot air and microwave, the yield of tar largely increased from 1.3% to 8.5% and the gas production increased correspondingly. The composition of the tar was also furtherly analyzed, results indicated that Miscellaneous hydrocarbons (HCs) were the main component of the tar, and microwave irradiation can reduce the fraction of polycyclic aromatic hydrocarbons (PAHs) from 26.4% to 22.7%.     

Effects of microwave heating on changes in chemical and thermal properties of vegetable oils

The effects of microwave heating on the cooling profiles of two vegetable oils (corn oil and soybean oil) were studied using differential scanning calorimetry (DSC) and compared to changes in chemical parameters. These oils were exposed for several periods of time to three controlled treatments: low-, medium-, and high-power settings, respectively. The DSC results were derived from the cooling curve of oils at a scanning rate of 5°C/min. The chemical analyses of the oils included peroxide value, anisidine value, free fatty acid content, iodine value, and C18∶2/C16∶0 peak area ratio. A statistical comparison was carried out between DSC and the chemical parameters. In general, correlations were good between these parameters. Likewise, the experimental data showed that, for a given microwave power setting, a good correlation existed between DSC curve parameters and heating periods. These results indicate that DSC can be used as an objective nonchemical, instrumental technique to monitor lipid oxidation in both traditionally heated and microwave-heated oils.     

Experimental study on the heating effects of microwave discharge caused by metals

The heating effects of discharge triggered at the appearance of metals are important under microwave irradiation but special studies have rarely been carried out. In our experiments, the heat generation was successfully isolated by embedding metal in quartz sand and measured through an indirect calorimetric method. All the experiments and analyses indicate that the discharge triggered by metals under microwave irradiation has remarkable heating effects. The microwave power, irradiation time, metal amount, and atmosphere are all important factors to influence the discharge intensity and then the heating effects. The understanding of the effects of metal discharge will help us to further research on the treatment of solid waste, such as the microwave pyrolysis of electrical waste. It may also be a good idea to use the discharge phenomena and their heating effects by embedding some metals to the microwave field in some certain applications. © 2012 American Institute of Chemical Engineers AIChE J, 2012     

Effect of microwave heating on compressive strength of beech wood (Fagus sylvatica L.) parallel to grain

This article deals with the changes of strength of beech wood (Fagus sylvatica L.) when compression load is applied parallel to grain and the wood is heated using microwave (MW) energy for a time period determined by two variants of drying. The elasticity modulus and compressive strength parallel to grain were measured using a universal testing machine in accordance with a relevant technical standard. Regression equations describing the dependence of compressive strength on wood density were determined for both variants of heating with a given load. Furthermore, a three-factor analysis of variance was performed to verify the significance of possible factors influencing changes of wood strength and elasticity. Possible hypotheses of changes caused by MW heating and the significance of discovered factors are also discussed. The results showed reduction of average values of wood compressive strength parallel to grain. The values were reduced by 35% in variant 1 and by 41% in variant 2 as opposed to wood to which no MW energy was applied. The difference in strength between the variants was nearly 9%.     

微波法提取生姜总黄酮及光热稳定性研究

采用微波法,用乙醇提取生姜总黄酮,并对其光热稳定性进行了研究。结果表明,用1∶40(g/mL)的50%的乙醇水溶液,微波功率为464 W,萃取时间为60 s提取效果较好,此条件下总黄酮得率为1.59%;生姜黄酮在避光和加热条件下比较稳定,自然光、短时间紫外光和日光照射对生姜黄酮的影响不大。     

Effects of the doping concentration of boron on physicochemical,mechanical, and biological properties of hydroxyapatite

Ion doping is an approach to modify properties of materials, like hydroxyapatite (HA), that contributes to designing biomaterials with desired characteristics applicable in bone defect treatments. Recently, boron (B) has been noticed in biomaterial fields due to its beneficial effects on formation, growth, and quality of bone. In this study, B-doped HA nanoparticles with different molar concentrations of B (0.05, 0.1, 0.25, and 0.5) were synthesized through microwave-assisted wet precipitation. The effects of B content on various properties of HA were evaluated. The results demonstrated that the size of HA particles reduced from 106 nm to 89-85 nm in B doped materials. Meanwhile, the crystallinity degree of B doped HA (BHA) samples was between 89.90% and 93.77%, compared to 95.19% of HA. Diametral tensile strength of samples was measured in the ranges of 2.51 and 3.61 with no significant difference among groups. The micro-hardness of HA was 0.88 GPa, whilst doped ones had hardness values of 0.5 GPa–0.68 GPa. Biodegradability of samples increased from less than 1% to approximately 4% after 28 days, while B-doping did not make any change in the degradation rate. Doping dosages were appropriate in terms of bioactivity and cell viability, and B doping caused higher bioactivity and cell proliferation. All changed properties were dose-dependent and more effective in doped groups with a higher amount of B. Despite proliferative effect, 260 μg/l and 770 μg/l of B release in two groups with the highest dopant concentrations did not positively influence the osteogenic activity of cells. Our results demonstrated that doping concentrations that resulted in B release ≤260 μg/l seem more appropriate dosage, especially for bone tissue engineering and substitute applications due to promoted bioactivity and proliferation, as well as no obstructive effects on mechanical properties and osteogenic activities of HA.     

Influence of microwave heating on the kinetic of acrylic acid polymerization and crosslinking

Kinetics of isothermal formation of poly (acrylic acid) (PAA) hydrogels through polymerization of acrylic acid and crosslinking of the PAA formed in a conventionally heated reaction system and in a microwave heated reaction system were investigated. It was found that in the microwave heated system the reaction rate constant of PAA hydrogel formation significantly increased (from 32 to 43 times) when compared with the conventionally heated system. The isothermal kinetics of the PAA hydrogel formation during the microwave process could be described by the so‐called first‐order chemical reaction kinetics model. In contrast, the so‐called second‐order chemical reaction rate model could best describe the isothermal kinetics of the PAA hydrogel formation during the conventionally heated process. Also, in the microwave heated system, the reaction kinetics of the PAA hydrogel formation and its kinetic parameters changed, that is, the activation energy (E_a) decreased by about 19% and the pre‐exponential factor (lnA) decreased by 2.2 times. The decrease in activation energy, change in entropy of activation energy, and decrease in the pre‐exponential value of PAA hydrogel formation under microwave heating are caused with increased energy of the reactive species when compared with their energy in thermal activation. Increased energy of the reactive species is a consequence of rapid transfer and absorption of the energy of microwave field to the existing reactive species. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Effect of the temperature and sintering time on the thermal,structural, morphological,and vibrational properties of hydroxyapatite derived from pig bone

This paper focuses in the study of the effect of the temperature and sintering time on structural, morphological, thermal, and vibrational properties of hydroxyapatite obtained from pig bone (BHA). A three-step process was used to get BHA: hydrothermal cleaning, calcination, and cooling. Samples were calcined and cooled inside the furnace under atmosphere air. The samples were calcined at 600 and 1000 °C and sintered at 1, 7, 20, 50 h and studied accordingly. XRD and Raman studies showed an improvement in the crystalline quality as a function of the sintering time, while the samples calcined at 1000 °C did not exhibit structural changes as a function of the sintering time. The presence of magnesium oxide (MgO) was confirmed by XRD and micro EDS analysis as a result of the temperature treatment because this compound was not found in the samples calcined at 600 °C. The crystalline quality of these samples was studied using XRD and Raman spectroscopy.     

Effect of fluoride additive on the mechanical properties of hydroxyapatite/alumina composites

The effect of fluoride additives on the mechanical properties of hydroxyapatite/alumina composites was investigated. When MgF₂ (5 vol%) was added to hydroxyapatite/alumina composites, the decomposition of hydroxyapatite was suppressed due to the substitution of F⁻ for OH⁻ in the crystal structure. Comparing two additives, such as MgF₂ and CaF₂, MgF₂ showed much more effective for the suppression of phase decomposition in the hydroxyapatite/alumina composites due to the enhanced substitution of F⁻ for OH⁻. In the case of MgF₂ addition, a relatively high-mechanical properties (flexural strength: ∼170 MPa; Vickers hardness: ∼7 GPa) was obtained compared to MgF₂-free composites.     

The effect of microwave heating on the microstructure and the mechanical properties of reaction-bonded boron carbide

Reaction-bonded boron carbide composites were fabricated by both microwave (under Ar/10% H₂) and conventional heating (under vacuum or Ar/10% H₂). Silicon carbide (SiC) formation occurred in all cases and was slightly favored in the case of microwave heating under Ar/H₂. The resulting microstructures were influenced by the heating process and atmosphere; the SiC existed in the form of needles with conventional heating under vacuum. SiC small polygonal grains were present after microwave heating under Ar/H₂. Both the atmosphere and the electromagnetic field influence the SiC morphology. Despite this difference, the hardness and toughness of composites obtained by both heating techniques were similar.     

Influence of precursor deficiency sites for borate incorporation on the structural and biological properties of boronated hydroxyapatite

The biological properties of hydroxyapatite (HA) are significantly influenced by its compositional characteristics especially doping elements and/or Ca/P ratio, which can be altered by precursor chemistry. In this study, a group of boronated (B-incorporated) hydroxyapatite (BHA) was synthesized using a precipitation method by setting the Ca/P ratio to the stoichiometric value of HA (1.67), while altering the precursor chemistry by adjusting either (Ca + B)/P (Ca-deficient precursor, BC) or Ca/(P + B) (P-deficient precursor, BP). After heat-treatment, the partial decomposition of the BC was observed, forming tricalcium phosphate as the byproduct, however, the BP showed phase stability at all temperatures. The B-ionic species in the form of (BO₂)^? and (BO₃)^3? were incorporated into the HA structure at the (PO₄)^3? and (OH)^? positions, respectively. The incorporation of the B species also facilitated the incorporation of (CO₃)^2? groups specifically in the BPs. This is the first finding on BHA reporting that preferential (CO₃)^2? incorporation depends on the precursor chemistry used. As a result, osteoblast adhesion was superior on the BPs compared to pure HA owing to the carbonated structure, increasing cell spreading area. As such, this in vitro study highlighted that the present P-deficient precursor approach for synthesizing BHA improved biocompatibility properties and should, thus, be further considered for the next-generation of improved orthopedic applications.     

Effect of Fe3+ substitution on the structural modification and band structure modulated UV absorption of hydroxyapatite

The effect of Fe³⁺ ionic substitution in hydroxyapatite (Ca_10-xFe_x(PO₄)₆(OH)₂) was studied using structural modifications, resulting in an improvement in UV absorption through a tailored optical band structure. Ca²⁺ of HA being larger compared to Fe³⁺ contributes to the shrinkage of the lattice. Undoped HA has a peak at 1085 cm⁻¹ (ʋ₃ PO₄³⁻) which is shifted to 1033 cm⁻¹ for Fe-HA, because of the perturbation in HA structure. An improvement of UV absorption in the entire UVA and UVB range with an increase in Fe content because of a decrease in bandgap from 5.9 eV to 2.1 eV with undoped and doped HA. Theoretically obtained band gap and optical behaviour of the systems are well correlated with the experimental findings. Moreover, the use of marine biowaste from cuttlefish bone, as the source of HA; low cost and promising UV absorption can have a potential application as UV protective sunscreen filters.     

Effects of strontium - erbium co-doping on the structural properties of hydroxyapatite: An Experimental and theoretical study

Five different samples of Sr-based Er-doped hydroxyapatites (HAps) in the dissimilar quantities like 0, 0.35, 0.70, 1.05 and 1.40 at% were produced via a wet chemical process. The prepared samples were investigated experimentally by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray (EDX), Fourier transform infrared (FTIR) spectroscopy, differential thermal analysis (DTA), and in vitro biocompatibility tests. In addition, the density of states (DOS) and band structures were investigated theoretically. It was found that the presence of Er as a dopant affected the lattice parameters, while the EDX measurements confirmed that the presence of Er at various concentrations caused a Ca-deficiency because the addition of Er decreased the Calcium/Phosphorus molar ratio from 1.67 to 1.61. For all samples, the single-phase distribution of HAp was observed. The crystallinity percentage of the samples was found to be 89% or more according to two different methods. The calculations with respect to Scherrer and Williamson-Hall methods showed that the crystallite sizes of the samples were found to be in the ranges of 29–34 nm and 25-42 nm, respectively. DTA investigations revealed that all samples exhibited thermal stability in the temperature range of 25 °C - 1000 °C. No remarkable morphological alterations were observed. Furthermore, the theoretical studies confirmed that the band structures narrowed with an increase in Er concentration.     

FTIR spectroscopic study of the effect of microwave heating on the transformation of cellulose I into cellulose II during mercerization

Fourier transform infrared (FTIR) spectroscopy techniques were used in the study of the effect of microwave (MW) heating on the structural properties of cotton fibers and on the mercerization mechanism of these fibers. Samples of the fibers were microwave heated for different times and different microwave powers. Also, mixtures of cotton fibers and aqueous solution of NaOH with different concentrations were exposed to microwave radiation for different times and different powers. It was found that microwave heating of cotton fibers under these experimental conditions causes no observable changes in their spectral features apart from slight changes in the intensities of the absorption bands. The determined values of the absorbances ratio A1375 cm^?1/A2900 cm^?1 revealed that microwave heating for short periods and at low powers decreases the crystallinity of cotton as the result of the drying effect of microwave heating, while microwave heating for longer periods and at higher power results in recristallyzation of the fibers. The analysis of the experimentally obtained data revealed that microwave heating that causes molecular motions by migration of ions and rotations of the dipoles produces no considerable effects on the mechanism of mercerization but only reduces the concentration of NaOH in the solution and the time of treatment which are needed for the complete transformation of cellulose lattice type I into cellulose lattice type II without any heating. Also it was found that the magnitude of reductions depends on the applied power. Moreover the results proved that the deconvolution and the second derivatives of the FTIR spectra of cotton fibers can be used as a useful tool for distinguishing cellulose lattice type II from cellulose lattice type I. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Effects of microwave heating on dielectric and piezoelectric properties of PZT ceramic tapes

Commercial lead zirconate titanate (PZT) perovskite powders were used to fabricate ceramic tape and then sintered by microwave and conventional methods. Both dielectric and piezoelectric properties of PZT ceramic tapes were studied in terms of sintering process. X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM) show the PZT perovskite phase with smaller grain size and dense microstructure can be obtained at a lower sintering temperature by microwave process. It was also observed that shrinkage ratio and bulk density of the tapes sintered at 800 °C were obtained about 19% and 7.46 g/cm³ by the microwave heating method, respectively, that is corresponding to those values of sintered PZT tapes at 950 °C by conventional process. Moreover, the dielectric constant and maximum permittivity are increased about 30% as compared with conventional processing method. The experimental results demonstrated that the characteristics of the PZT tapes could be significantly improved by microwave heating method. These results demonstrate that such a simple approach can upswing the piezoelectric and dielectric properties of these tapes by using microwave process with a short heating time.     

Influence of materials dielectric properties on the petroleum oil removal from waste under microwave irradiation

The effects of dielectric materials on the petroleum oil removal from solid waste under microwave heating were fully detailed within this research. The experiments clearly show that although water has high dielectric properties, it is not efficient to remove heavy oils due to its volatility. Salt water having high loss factor contributes to the oil removal at low concentration, whereas high concentration results in less. Activated carbon was proved to be more efficient for either light or heavy petroleum oils. The simulation results yielded good estimations of power density throughout the sample. The simulated temperature distribution clearly explains the experimental results and can be used to predict the efficiency of organic removal from solid wastes. © 2011 Canadian Society for Chemical Engineering     

Effect of grain size and density of spray-pyrolyzed hydroxyapatite particles on the sinterability of hydroxyapatite disk

The effect of grain size and density of hydroxyapatite particles, which were prepared by different spray-pyrolysis temperatures, on the sinterability of hydroxyapatite disk was investigated. Calcium phosphate solution (Ca/P ratio of 1.67 and 0.1 M concentration) was prepared by reacting calcium nitrate tetrahydrate and diammonium hydrogen phosphate solutions, and adding nitric acid. Spray-pyrolysis was carried out at 900 °C, 1200 °C, and 1500 °C at a carrier gas flowing rate of 10 L/min. The particles synthesized at 900 °C were large, hollow spheres with a hole at the outer surface, a broad size distribution, but had small grain sizes. Conversely, the particles synthesized at 1500 °C were small, solid spheres with a narrow size distribution, but had large grain sizes. The particles synthesized at 1200 °C had intermediate properties. A sinterability test conducted at 1100 °C for 1 h demonstrated that small and dense particles with large grain sizes showed a higher relative sintered density compared with large and hollow particles with small grain sizes. The results were explained in terms of the grain size and density of a particle, which were inversely and proportionally affected to sinterability. The practical implication of these results is that highly sinterable hydroxyapatite powders can be synthesized through spray-pyrolysis at a high temperature under a fixed initial concentration of calcium phosphate solution and flow rate of carrier gas.