Effect of crystallite size and calcination temperature on the thermal stability of single nanocrystalline chromium oxide: expressed by novel correlation

   The thermal reaction of chromium acetylacetonate in various organic solvents at 300 °C for 2 h yielded an amorphous product. Single nanocrystalline chromium oxide was obtained after being calcined at 300 °C for 1 h. The crystallite size of product is in the range of 16–26 nm. In this work, the thermal stability of product was given by BET/BET₀. It was found that the crystals of large crystallite size show higher thermal stability than the crystals of small crystallite size. Thermal stability of chromium oxide can be presented by the correlation of the BET surface area after calcination, crystallite size of as-synthesized product and calcination temperature (500–900 °C) as shown below.

Effect of crystallite size of zinc oxide on the mechanical,thermal and flow properties of polypropylene/zinc oxide nanocomposites

ZnO nanoparticles were prepared using zinc chloride and sodium hydroxide in chitosan medium. Prepared ZnO (NZO) and commercial ZnO (CZO) was characterized by scanning electron microscopic and X-ray diffraction studies. PP/ZnO nanocomposites were prepared using 0–5 wt% of zinc oxide by melt mixing. It was then compression moulded into films. Transparency of the composite films were improved by reducing the crystallite size of ZnO. Melt flow index studies revealed that NZO increased the flow characteristics of PP while CZO decreased. X-ray diffraction studies indicated α-form of isotactic polypropylene. An increase in mechanical properties, dynamic mechanical properties and thermal stability of the composites were observed by the addition of ZnO. Uniform dispersion of the ZnO was observed in the scanning electron micrographs of the tensile fractured surface of composites.     

Effect of size of copper nanoparticles on its catalytic behaviour in Ullman reaction

The condensation of iodobenzene to biphenyl is an industrially important reaction due to its significant role in organic synthesis as drug intermediates. The reaction takes place in the presence of copper powder as catalyst. We have shown in this paper that the size of the copper nanoparticles as well as its exposed surface area is responsible for the yield of chemical reaction. The uncapped copper powder showed a 43% conversion of iodobenzene to biphenyl in 5 h under our experimental conditions. Same amount of copper nanoparticles (size, ∼66 nm diameter) prepared by citrate capping showed 88% conversion of iodobenzene to biphenyl, which increased to about 95% when 8 nm diameter capped copper nanoparticles are used. Surprisingly, 5 nm size copper nanoparticles showed no change in the yield of about 95%.     

Effect of calcining temperature on particle size of hydroxyapatite synthesized by solid-state reaction at room temperature

Using diammonium phosphate, calcium nitrate tetrahydrate and sodium bicarbonate as raw materials, hydroxyapatite (HAP) was facilely synthesized by solid-state reaction at room temperature. The crystallinity, phase, morphology and particle size of the products were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FT-IR), thermogravimetry/differential thermal analysis (TG–DTA) and particle size analyzer. The influence of calcining temperature on the crystallinity and composition of HAP phase was also investigated.     

Influence of reaction parameters on size and shape of silica nanoparticles

It was investigated that the effect of various synthesis parameters such as, types of silicon alkoxides and alcohols, ammonia (catalyst) concentration, and reaction temperature on the particle size and shape of silica nanosphere by Sol-Gel method. When different silicon alkoxides were used, the silica particles except tetramethylorthosilicate (TMOS), all were spherical, and after reaching a maximum, the sizes were decreased with carbon chain length due to its steric effect. As the alcohol chain length increased from methanol (MeOH) to n-butanol (BuOH), the average particle size was increased from 30 nm to 800 nm. In general, all are having narrow particle size except BuOH sample were distributed bimodally (150 and 800 nm). When ammonia concentration increases, the particles also increased, however on increasing the reaction temperature, the particle sizes were reduced.     

Effect of Fe2O3 crystallite size on its mechanochemical reaction with La2O3 to form LaFeO3

Fe₂O₃ powders with different crystallite sizes prepared by heating FeOOH at various temperatures were ground with La₂O₃ powder using a planetary ball mill to investigate the effect of crystallite size on mechanochemical synthesis of LaFeO₃. Fe₂O₃ powder with smaller crystallite size obtained by heating at lower temperature reacts more easily with La₂O₃ than that with larger size. The mechanochemical reaction proceeds with an increase in grinding time. Specific surface area of the LaFeO₃ powder synthesized has a large value of over 11 m²/g. The mechanochemical process can be also applied to synthesize other iron complex oxides with rare earth elements such as Pr, Nd and Sm.     

Effect of temperature schedule on the particle size of barium titanate during solid-state reaction

Nano-sized BaTiO₃ particles were synthesized by a solid-state reaction in air using a 2-step heat treatment process. The dependence of the particle size and tetragonality (=c/a) on the temperature schedule was investigated. The temperature was held for 10 h at an intermediate temperature before heating to the target temperature of 1000°C for 1 h. Although more heat was consumed, the powders synthesized by the 2-step heat treatment method showed a much smaller particle size with higher tetragonality than those produced by direct heating. For example, the mean particle size was <100 nm at a holding temperature ranging from 500-700 °C, while it was 246 nm for direct heating to the same target temperature of 1000 °C. This was explained by the enhanced nucleation rate during temperature holding stage based on the TG/DTA and high temperature XRD results.     

Formation of zinc oxide nanoparticles by mechanochemical reaction

Abstract

The synthesis of zinc oxide (ZnO) nanocrystallites by mechanochemical reaction of ZnCl₂ and Na₂CO₃ with NaCl as diluent and following thermal treatment was investigated using X-ray diffraction (XRD) and transmission electron microscopy (TEM). Calcination of the as milled powder at 600°C in air and removal of NaCl through washing formed ZnO nanocrystallites with an average crystal size of ~ 21 nm, which increased with increasing thermal treatment temperature. Milling time and NaCl/ZnCl₂ molar ratio exerted prominent effects on the crystal size of the ZnO nanoparticles. The mechanism of nanocrystallite growth is discussed.     

Fabrication of iron oxide nanoparticles: magnetic and electrochemical sensing property

In this paper, we report a simple synthesis of Fe₂O₃ nanoparticles using hydrothermal method. The formation of the sample was confirmed by X-ray diffraction analysis, X-ray photoelectron spectroscopy, Fourier transformed infrared spectroscopy, Raman spectroscopy and UV–Visible absorption spectroscopy. The average crystallite size of the synthesized Fe₂O₃ nanoparticles was estimated to be 61 nm and the particles were of good crystalline nature. Field emission-scanning electron microscopy study of the sample revealed that the Fe₂O₃ powder has rod-like morphology which is composed of nanoparticles. The vibrating sample magnetometer measurement shows that the nanoparticles possess ferromagnetic property. The synthesized Fe₂O₃ nanoparticles were used to modify glassy carbon electrode (Fe₂O₃/GCE) and the modified electrode was used to detect pyrocatechol (PC) in a pH 7.4 phosphate buffer solutions by cyclic voltammetry and chronoamperometry. At the Fe₂O₃/GCE, PC is oxidized at less positive potential with larger current response than the bare GCE. The proposed sensor exhibits great potential in the field of electrochemical sensing of PC.     

Electrical and gas sensing properties of self-aligned copper-doped zinc oxide nanoparticles

Electrical and gas sensing properties of nanocrystalline ZnO:Cu, having Cu X wt% (X = 0.0, 0.5, 1.0, and 1.5) in ZnO, in the form of pellet were investigated. Copper chloride and zinc acetate were used as precursors along with oxalic acid as a precipitating reagent in methanol. Material characterization was done by X-ray diffraction (XRD), scanning electron microscopy (SEM), field emission scanning electron microscopy (FE-SEM) and inductive coupled plasma with optical emission spectrometry (ICP-OES). FE-SEM showed the self-aligned Cu-doped ZnO nano-clusters with particles in the range of 40-45 nm. The doping of 0.5% of copper changes the electrical conductivity by an order of magnitude whereas the temperature coefficient of resistance (TCR) reduces with increase in copper wt% in ZnO. The material has shown an excellent sensitivity for the H₂, LPG and CO gases with limited temperature selectivity through the optimized operating temperature of 130, 190 and 220 °C for H₂, LPG and CO gases, respectively at 625 ppm gas concentration. The %SF was observed to be 1460 for H₂ at 1% Cu doping whereas the 0.5% Cu doping offered %SF of 950 and 520 for CO and LPG, respectively. The response and recovery time was found to be 6 to 8 s and 16 s, respectively.     

Effect of air-pressure on room temperature hydrogen sensing characteristics of nanocrystalline doped tin oxide MEMS-based sensor

Nanocrystalline indium oxide (In2O3)-doped tin oxide (SnO2) thin film sensor has been sol-gel dip-coated on a microelectrochemical system (MEMS) device using a sol-gel dip-coating technique. Hydrogen (H2) at ppm-level has been successfully detected at room temperature using the present MEMS-based sensor. The room temperature H2 sensing characteristics (sensitivity, response and recovery time, and recovery rate) of the present MEMS-based sensor has been investigated as a function of air-pressure (50-600 Torr) with and without the ultraviolet (UV) radiation exposure. It has been demonstrated that, the concentration of the surface-adsorbed oxygen-ions (which is related to the sensor-resistance in air), the ppm-level H2, and the oxygen (O2) partial pressure are the three major factors, which determine the variation in the room temperature H2 sensing characteristics of the present MEMS-based sensor as a function of air-pressure.     

Room temperature CO and H2 sensing with carbon nanoparticles

We report on a shell-shaped carbon nanoparticle (SCNP)-based gas sensor that reversibly detects reducing gas molecules such as CO and H(2) at room temperature both in air and inert atmosphere. Crystalline SCNPs were synthesized by laser-assisted reactions in pure acetylene gas flow, chemically treated to obtain well-dispersed SCNPs and then patterned on a substrate by the ion-induced focusing method. Our chemically functionalized SCNP-based gas sensor works for low concentrations of CO and H(2) at room temperature even without Pd or Pt catalysts commonly used for splitting H(2) molecules into reactive H atoms, while metal oxide gas sensors and bare carbon-nanotube-based gas sensors for sensing CO and H(2) molecules can operate only at elevated temperatures. A pristine SCNP-based gas sensor was also examined to prove the role of functional groups formed on the surface of functionalized SCNPs. A pristine SCNP gas sensor showed no response to reducing gases at room temperature but a significant response at elevated temperature, indicating a different sensing mechanism from a chemically functionalized SCNP sensor.     

Effect of temperature on rheological properties of copper oxide nanoparticles dispersed in propylene glycol and water mixture

This paper reports on experimental investigation of the rheological behavior of copper oxide nanoparticles dispersed in a 60:40 propylene glycol and water mixture. Nanofluids of a particle volume concentration from 0 to 6% have been tested in this study. The experiments were conducted over a temperature range of -35 degrees C to 50 degrees C to establish their behavior for use as a heat transfer fluid in cold climates. The experiments reveal that this nanofluid in the range of particle volume percentage tested exhibits a Newtonian behavior. A new exponential correlation has been developed from the experimental data, which expresses the viscosity as a function of particle volume percent and the temperature of the nanofluid. The slope of relative viscosity curve was found to be higher at lower temperatures.     

Synthesis of iron oxide nanoparticles of narrow size distribution on polysaccharide templates

We report here the preparation of nanoparticles of iron oxide in the presence of polysaccharide templates. Interaction between iron (II) sulfate and template has been carried out in aqueous phase, followed by the selective and controlled removal of the template to achieve narrow distribution of particle size. Particles of iron oxide obtained have been characterized for their stability in solvent media, size, size distribution and crystallinity and found that when the negative value of the zeta potential increases, particle size decreases. A narrow particle size distribution with D ₁₀₀ = 275 nm was obtained with chitosan and starch templates. SEM measurements further confirm the particle size measurement. Diffuse reflectance UV-vis spectra values show that the template is completely removed from the final iron oxide particles and powder XRD measurements show that the peaks of the diffractogram are in agreement with the theoretical data of hematite. The salient observations of our study shows that there occurs a direct correlation between zeta potential, polydispersity index, bandgap energy and particle size. The crystallite size of the particles was found to be 30–35 nm. A large negative zeta potential was found to be advantageous for achieving lower particle sizes, owing to the particles remaining discrete without agglomeration.     

Effect of firing temperature on microstructure and dielectric properties of chromium oxide based glass composite thick films on stainless steel substrate

We report the influence of firing temperature on Al₂O₃–chromium oxide based (Cr₂O₃–Bi₂O₃–B₂O₃–SiO₂–Al₂O₃) glass composite (named as GC-1 composite) thick films of thickness (27?±?3) µm deposited onto 0.6 mm thick austenitic grade stainless steel (DIN 1.4301/AISI 304) substrate by screen printing technique, which can be used as a substitute to alumina substrate. Prior to formulation of glass composite, the chromium oxide based glass (named as GC-1) phase was prepared separately by melt-quench technique. X-ray diffraction analysis confirmed amorphous nature of the GC-1 glass. The thermo gravimetric analysis and differential scanning calorimetry of the GC-1 glass shows thermal stability over the temperature range of 20–1000 °C. We observed that the firing temperature significantly influences microstructural and dielectric properties of the GC-1 composite film. The deposited GC-1 composite films onto stainless steel base were fired at temperatures between the range of 550–750 °C, showed the surface resistivity in the range of (1.0–6.9?±?0.2) × 10¹² ohms per square. The microstructure of these composite films recorded using scanning electron microscopy and electrical properties recorded using LCR meter were correlated with each other. The study revealed that the film fired at 600 °C were found to be superior among the samples under investigation in terms of microstructure, stable relative permittivity [36 (±?1)] and low loss tangent [0.02 (±?0.002)] in frequency range of 1–200 kHz, and surface resistivity (~?5.1?×?10¹² ohms per square).     

Synthesis and sintering of tin-doped indium oxide nanoparticles with uniform size

The urea-based hydrothermal (UBH) method can synthesize indium tin oxide (ITO) nanopowders with good monodispersity and size uniformity. However, the resulting formation of high pressure CO₂ gas by the hydrolysis of urea during the hydrothermal process is unsafe. The pressure generated by the UBH method can be lowered by connecting the hydrothermal reactor to a vessel containing sodium hydroxide solution to quickly absorb CO₂ gas. ITO nanoparticles with particle sizes of 90 ± 3 nm and 40 ± 3 nm can be produced. The size of the as-prepared nanoparticles is readily controlled by adjusting the precursor concentration. Using properly mixed nanoparticles with a volume ratio of V_40 nm:V_90 nm = 30:70 as the raw materials, ITO can be sintered to a high and consistent density of 99.3–99.5% of the theoretical density.     

Effects of reaction temperature on size and optical properties of CdSe nanocrystals

We report experimental results on the reaction temperature dependence of luminescence properties in size-controlled CdSe nanocrystals. Such reaction temperature dependent property is also sizedependent. The diameter of the CdSe nanocrystals is tuned from 4–11.0 nm by varying the reaction temperatures. The growth process and characterization of CdSe nanocrystals are determined by photoluminescence (PL) spectroscopy, ultraviolet-visible (UV-Vis) spectroscopy, X-ray photoelectron spectrometry (XPS), X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The influence of reaction conditions on the growth of CdSe nanocrystals demonstrates that low reaction temperature is favourable for the formation of high quality CdSe nanocrystals.     

The effect of temperature on microstrains and crystallite growth in alumina

The effects of temperature on the microstrains and crystallite growth in milled alumina have been examined, using a modern method of X-ray diffraction profile analysis. The microstrains may be relieved by annealing at temperatures between 600 and 1000° C, and this relief is accompanied by crystallite size growth. Isotropy was observed with regard to the residual stresses and cleavage properties, in the milled crystals.