An initial study on feasible treatment of Serbian lignite through utilization of low-rank coal upgrading technologies

Despite benefiting from vast fuel reserves, combustion of low-rank coals is commonly characterized by low thermal efficiency and high pollutant emissions, partly due to high moisture content of the coals in question. Thus, removal of moisture from low-rank coals is deemed an important quality upgrading method. The paper provides an overview of the current status of low-rank coal upgrading technologies, particularly with respect to utilization of drying and dewatering procedures. In order to examine the influence of relevant parameters on the moisture removal process, a model of convective coal drying in a packed, as well as in a fluid bed combustion arrangement, is developed and presented. Product-specific data (intraparticle mass transfer, gas–solid moisture equilibrium) related to the coal variety addressed herein (lignite) are obtained through preliminary investigations. Effective thermal conductivity of the packed bed as defined by Zehner/Bauer/Schlünder is used to define heat transfer mechanisms occurring in the packed bed. Similar two-phase fluidization model has been validated for different types of biomaterials.     

Nanocrystalline Nd2O3: Preparation,phase evolution,and kinetics of thermal decomposition of precursor

Nd₂O₃ was synthesized by calcining Nd₂(C₂O₄)₃·10H₂O in air. The precursor and its calcined products were characterized by thermogravimetry and differential scanning calorimetry, Fourier transform infrared spectroscopy, X-ray powder diffraction, and scanning electron microscopy. The results showed that high-crystallized Nd₂O₃ with hexagonal structure was obtained when the precursor was calcined at 1223 K in air for 2 h. The crystallite size of Nd₂O₃ synthesized at 1223 K for 2 h was about 48 nm. The thermal decomposition of the precursor in air experienced three steps, which are first, the dehydration of 10 crystal water molecules; then, the decomposition of Nd₂(C₂O₄)₃ into Nd₂O₂CO₃; and last, the decomposition of Nd₂O₂CO₃ into hexagonal Nd₂O₃. Based on the KAS equation, the values of the activation energies associated with the thermal decomposition of Nd₂(C₂O₄)₃?10H₂O were determined.     

Synthesis and characteristics of fly ash and bottom ash based geopolymers–A comparative study

The research was carried out to develop geopolymers mortars and concrete from fly ash and bottom ash and compare the characteristics deriving from either of these products. The mortars were produced by mixing the ashes with sodium silicate and sodium hydroxide as activator solution. After curing and drying, the bulk density, apparent density and porosity, of geopolymer samples were evaluated. The microstructure, phase composition and thermal behavior of geopolymer samples were characterized by scanning electron microscopy, XRD and TGA-DTA analysis respectively. FTIR analysis revealed higher degree of reaction in bottom ash based geopolymer. Mechanical characterization shows, geopolymer processed from fly ash having a compressive strength 61.4 MPa and Young's modulus of 2.9 GPa, whereas bottom ash geopolymer shows a compressive strength up to 55.2 MPa and Young's modulus of 2.8 GPa. The mechanical characterization depicts that bottom ash geopolymers are almost equally viable as fly ash geopolymer. Thermal conductivity analysis reveals that fly ash geopolymer shows lower thermal conductivity of 0.58 W/mK compared to bottom ash geopolymer 0.85 W/mK.     

Evaluation of hot corrosion behavior of CSZ,CSZ/micro Al2O3 and CSZ/nano Al2O3 plasma sprayed thermal barrier coatings

Hot corrosion is one of the main destructive factors in thermal barrier coatings (TBCs) which come as a result of molten salt effect on the coating–gas interface. Hot corrosion behavior of three types of plasma sprayed TBCs was evaluated: usual CSZ, layer composite of CSZ/Micro Al₂O₃ and layer composite of CSZ/Nano Al₂O₃ in which Al₂O₃ was as a topcoat on CSZ layer. Hot corrosion studies of plasma sprayed thermal barrier coatings (TBCs) were conducted in 45 wt% Na₂SO₄+55 wt% V₂O₅ molten salt at 1050 °C for 40 h. The graded microstructure of the coatings was examined using scanning electron microscope (SEM) and X-ray diffractometer (XRD) before and after hot corrosion test. The results showed that no damage and hot corrosion products was found on the surface of CSZ/Nano Al₂O₃ coating and monoclinic ZrO₂ fraction was lower in CSZ/Micro Al₂O₃ coating in comparison with usual CSZ. reaction of molten salts with stabilizers of zirconia (Y₂O₃ and CeO₂) that accompanied by formation of monoclinic zirconia, irregular shape crystals of YVO₄, CeVO₄ and semi-cubic crystals of CeO₂ as hot corrosion products, caused the degradation of CSZ coating in usual CSZ and CSZ/Micro Al₂O₃ coating.     

Microstructure and electrical conductivity of Mo/TiN composite powder for alkali metal thermal to electric converter electrodes

A Mo/TiN composite powder has been synthesized by a sol–gel method to improve the electrical performance and microstructural stability of the alkali metal thermal to electric converter electrode. The core (TiN)–shell (Mo) structure of the composite powder is confirmed by energy-dispersive X-ray spectroscopy and scanning electron microscopy. The composite powder is primarily composed of submicron (400–800 nm) particles that are coated on a core (>3–5 μm) particle. The Mo/TiN composite electrode exhibits high electrical conductivities of 1000 Scm⁻¹ at 300 °C and 260 Scm⁻¹ at 700  °C in an Ar atmosphere. The electrode exhibits excellent tolerance against grain growth during thermal cycling tests (R.T.↔800 °C), where the average growth rate of Mo grains in the Mo/TiN composite electrodes is controlled less than 0.5%/time (0.62→0.65 μm), while the growth rate in Mo electrodes is 306.7%/time (0.24→3.92 μm). It can be concluded that the Mo/TiN composite powder will suppress the degradation of the electrode and enhance the performance and durability of the unit cell at elevated temperatures.     

Synthesis and thermal behavior of Cu/Y2W3O12 composite

Cu metal matrix composite with Y₂W₃O₁₂ as a thermal expansion compensator was fabricated by high energy ball milling followed by compaction and sintering, and its thermal properties were explored for the potential applications as heat sinks in electronic industries, high precision optics, and space structures. The volume fraction of reinforcement was varied from 40% to 70% in order to tailor the composite for the simultaneous accomplishment of low thermal expansion and high thermal conductivity. The synthesis technique was optimized by varying the parameters like milling time from 1 to 20 h and sintering temperature from 600 to 1000 °C in order to achieve densified composites. The relative density of the composites is found to be around 90% for the 10 h milled powders followed by compaction at a pressure of 700 MPa and sintering at a temperature of 1000 °C. The thermal expansion of the composites exhibits linear behavior in the temperature range 200 to 800 °C and the low coefficient of thermal expansion (CTE) is found to be for Cu–70%Y₂W₃O₁₂ composite whose value, 4.32±0.75×10⁻⁶/°C, matches with that of Si substrate. The thermal conductivities are found to increase with a decrease in the volume fraction of the reinforcement and decrease with an increase in the temperature for all the samples. The experimentally determined CTE and thermal conductivity values are found to be comparable to those predicted by the thermal expansion based Kerner and Turner model and the thermal conductivity based Maxwell model, respectively.     

Hybrid dual-curable cyanate ester/boron phosphate composites via sequential thiol-ene photopolymerization and thermal polymerization

The aim of this study was to improve hybrid dual-curable cyanate ester/boron phosphate composites via sequential thiol-ene photopolymerization and thermal polymerization for high performance applications such as aerospace and electronic devices. A novel 2,2′-diallylbisphenol A dicyanate ester (DA-BADCy) which is the allyl group containing cyanate ester was synthesized and characterized. DA-BADCy, silicon containing monofunctional thiol compound, trifunctional thiol compound and boron phosphate were cured using both ultraviolet (UV) and thermal methods. Using thiol-ene system, cyanate ester formulations, which are normally prepared at high temperatures, were prepared at room temperature. This study maintains ease of application for cyanate esters. Thermal stability, flammability and thermal conductivity of the samples were evaluated by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), limiting oxygen index (LOI) and laser flash method, respectively. The samples were characterized with the following analysis; gel content, water absorption capacity and stress–strain test. Hydrophobicity of the samples was determined by the contact angle measurements. Moreover, the surface morphology of the samples was investigated by a scanning electron microscopy (SEM–EDS). The obtained results prove that the composites have good thermal and mechanical properties and with the help of easier preparation techniques, they can be used in many applications such as aerospace, electronic devices, materials engineering.     

Stabilization of nano-TiO2 aqueous dispersions with poly(ethylene glycol)-b-poly(4-vinyl pyridine) block copolymer and their incorporation in photocatalytic acrylic varnishes

In this work, TiO₂ nanoparticles were dispersed and stabilized in water using a novel type of dispersant based on tailor-made amphiphilic block copolymers of poly(ethylene glycol)-block-poly(4-vinyl pyridine) (mPEG-b-P4VP) prepared by atom transfer radical polymerization (ATRP). The performance of this new block copolymer as dispersant was compared to a polyelectrolyte dispersant commonly used for TiO₂, sodium salt of polyacrylic acid (Na-PAA). The effect of dispersion technique and type and amount of dispersant on deagglomeration and stability of the TiO₂ aqueous suspensions were studied. After incorporation in a standard waterborne acrylic varnish formulation, dry film transparency, photocatalytic activity, and nanoparticle cluster size were also evaluated. The results show that mPEG-b-P4VP copolymer with appropriate block lengths can have a better performance than Na-PAA in terms of aqueous dispersion stabilization and cluster size reduction in the acrylic matrix. This translates into higher film transparency and photocatalytic performance.     

Biodegradable and thermostable synthetic hyperbranched poly(urethane-urea)s as advanced surface coating materials

Aliphatic hyperbranched poly(urethane-urea)s with different weight percentages of branch generating moiety were synthesized by a one pot A₂ + BC₂ approach. Isophorone diisocyanate was used as the A₂ type monomer, while a tri-functional dihydroxyamine compound synthesized from ?-caprolactam and diethanol amine acted as the BC₂ monomer. Evidence supporting the hyperbranched structure of the synthesized poly(urethane-urea) was obtained from ¹H NMR spectra. FTIR study confirmed the nature and extent of hydrogen bonding present in this novel macromolecule. A Gaussian band fitting procedure of the IR band at amide-I region showed that the extent of hydrogen bonding increases with the increase of weight percentage of the tri-functional compound. The tensile strength, elongation at break, impact resistance, scratch hardness and gloss followed an increasing trend with the same. The thermal degradation of the hyperbranched poly(urethane-urea) was found to be dependent on the weight percentage of the BC₂ type moiety. The kinetics of thermal degradation studied by the Ozawa method showed that the activation energy required for thermal degradation of hyperbranched polymer is higher than its linear polyurethane analog. The synthesized polymer was found to be biodegradable by Pseudomonas aeruginosa bacteria. The study showed superiority of the hyperbranched structure over the linear one. Thus the results indicated the potential usage of the studied hyperbranched poly(urethane-urea) as an advanced surface coating material.     

Phosphorus based indole and imidazole functionalized hyperbranched polyester as antimicrobial surface coating materials

Third generation of hyperbranched polyester (HP) synthesized via melt condensation and end-capped with bis indole and imidazole phosphoryl chloride. The modified polyesters subjected to different characterization viz., FTIR, ¹H and ¹³C NMR, GPC and elemental analysis. NMR analysis was used for the determination of OH conversion that demonstrates linear units had a lower reactivity as compared to terminal OH groups in HP. The thermogravimetric analysis (TGA) and differential scanning calorimetric (DSC) studies indicated that the HP was stable up to 276 °C and the glass transition temperature was 79.6 °C. After modification of HP, thermal stability was increased for indole modified HP, but the glass transition temperature was decreased. The flame retardance of the hyperbranched polyester was also evaluated with the help of TGA analysis and showed higher value of limiting oxygen index (LOI) for modified polyesters. Morphology of the modified polyester showed more roughness than unmodified HP and this nature positive correlated with bacterial zone of inhibition value. Biological studies showed that the HP and modified HP are able to inhibit the reproduction of gram-positive and gram-negative bacteria. Indole modified HP is showed good activity compared to imidazole modified HP.