Smaller Average Crystallite Size Research Articles

Synthesis of submicrometer zirconium carbide formed from inorganic–organic hybrid precursor pyrolysis

Zirconium carbide (ZrC) was synthesized from inorganic–organic hybrid precursor’s pyrolysis by solution-based processing. Zirconium-containing complexes, which were obtained by chelation of oxide bidentate ligands to zirconium, were used to combine with phenolic resin to form precursors for ZrC. The precursors using specific ligands including acetylacetone, ethyleneglycol, and salicylic acid transformed into pure ZrC at a relatively low temperature (1,550 °C) in addition to that using lactic acid. As a comparison, synthesis of ZrC only using zirconium oxychloride octahydrate (ZrOCl2·8H2O) and phenolic resin was also conducted. The synthesized powders had a small average crystallite size (~300 nm), and a low oxygen content (~2.5 at.%). The conversions from as-synthesized preceramic precursors to ceramics were studied by means of FTIR, SEM, EDS, XRD, and XPS. The oxidation behavior of the synthesized ZrC in air was studied by DSC-TG analysis.

Structural and magnetic properties of Pr2Co7 powders

The Pr2Co7 intermetallic compounds are known to crystallize in two polymorphic forms. A hexagonal of the Ce2Ni7 type structure and a rhombohedral of the Gd2Co7 type structure were obtained by mechanical milling and subsequent annealing in high vacuum. The highest intrinsic coercivity value 18 kOe at room temperature and magnetization ratio (Mr/Mmax = 0.64) were determined for alloys milled during 5 h and annealed at 800°C for 30 min. The enhanced remanence (Mr/Mmax > 0.50) suggests strong exchange interaction between the adjacent crystallites. This is in good agreement with the small average crystallite size (D ≈ 50 nm). The high magnetic properties observed in these nanostructured Pr2Co7 intermetallic alloys have their origin in its relatively high uniaxial magnetocrystalline anisotropy field, and in the homogeneous nanostructure developed by mechanical milling process and subsequent annealing.

Growth of Wafer Size Graphene on SiC Substrates

Graphene, as strict two-dimensional material, exhibits exceptionally good electronic properties. In this paper, graphene was prepared on SiC substrates at different temperature based on two types of pre-treated surface. The surface morphology was characterized by atomic force microscopy (AFM) and scanning electronic microscopy (SEM). The results on SiC surface pre-treatment showed that chemical mechanical polishing (CMP) was an effective surface treatment method for reproducible and controlled growth of graphene. Images of the Si-surface revealed that the thickness of graphitic layers increased with annealing temperature. Meanwhile, a mesh-like network of wrinkles tended to tent-like features with the increase of temperature. The residual stresses, average crystallite size and number of graphene layers were analyzed by Raman spectroscopy. Little shift of 2D-band indicated the presence of certain stresses. Results among four samples showed that graphene layers grown on MP C-surface substrates had the thickest layers，contained the smallest average crystallite size La and exhibited no stresses. While graphene layers grown on Si-surface under 1600°C built upon compressive stresses, exhibited largest La and least number of graphene layers, indicating perfect quality.

The effect of deposition energy on the microstructure and mechanical properties of high speed steel films prepared using a filtered cathodic vacuum arc

Energetic deposition of high speed steel (HSS) films has been performed using a filtered cathodic vacuum arc system fitted with a AISI M2 HSS cathode. The mechanical properties and microstructure of the films were investigated as a function of the deposition energy. The stoichiometry and microstructure of the deposited films were different to that of the cathode and a dependence on the deposition energy was found. Films deposited at low deposition energies (< 100 eV) exhibited reduced concentrations of Mo and W and a smaller average crystallite size than the M2 HSS cathode. These films were found to be significantly harder than the M2 HSS cathode, possibly due to their smaller average crystallite size. A HSS film consisting of an amorphous phase with isolated crystals of bcc Fe was produced using high deposition energy and the deposition rate was found to decrease, likely due to sputtering.

Effect of alumina incorporation on restricting grain growth of nanocrystalline tin(IV) oxide

Abstract In this project, nanocrystalline SnO2 powders were successfully prepared by (a) citrate sol-gel and (b) direct precipitation methods. Powders were characterized using thermal analysis techniques (DTA-TG-DSC), X-ray powder Diffraction (XRD), surface area (BET) and electrical conductivity measurements. XRD patterns showed the presence of the cassiterite structure. SnO2 particles, prepared through sol-gel method exhibit crystallite sizes in the range from 3.1 to 22.3 nm when the gel is heat treated at different temperatures up to 900°C. SnO2 nanocrystallites prepared by the precipitation method are comparatively larger in size. The higher specific surface area was obtained for the powder prepared using sol-gel method and the obtained average grain size (d) is relatively large compared with that of the average crystallite size. The powders show a semiconducting behavior with increasing temperature. The higher conductivity obtained for SnO2 prepared by sol-gel method can be attributed to their smaller average crystallite size. XRD of alumina doped powder exhibits finer particles than pure SnO2. TEM images showed that the particles are spherical in shape and consist of a core of SnO2 surrounded by a coating of alumina. The calculated surface area was found to decrease with temperature increases. Due to the effective role of Al2O3 additive as a grain growth inhibitor for the matrix grains, the observed surface area for the coated materials are predominantly higher than for the uncoated materials.

Synthesis and characterization of nitrogen-doped TiO 2/AC composite for the adsorption–photocatalytic degradation of aqueous bisphenol-A using solar light

A photocatalytic composite, namely nitrogen-doped titanium dioxide supported on activated carbon, or N-TiO 2/AC, was synthesized using the sol–gel method. This composite was characterized using X-ray diffraction (XRD), N 2 adsorption–desorption isotherm, UV–vis spectrophotometry, X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The titania was found to consist predominantly of anatase. The AC-supported N-TiO 2 had a smaller average crystallite size (ca. 5.0 nm) as compared to the unsupported N-TiO 2 (ca. 5.4 nm). XPS revealed that interstitial nitrogen doping occurred in N-TiO 2 and N-TiO 2/AC. Observations under TEM revealed that N-TiO 2 nanocrystals were anchored on the surface of the AC. The dual-functionality properties of N-TiO 2/AC were evaluated using bisphenol-A (BPA) as the target pollutant in the aqueous phase. The effect of pH and influence of excitation wavelengths were investigated. Inhibition of BPA adsorption occurred at pH 11.0, thus resulted in slower kinetics of BPA photodegradation. N-TiO 2/AC was found to be photoresponsive under visible-light (420–630 nm) illumination.

Pulsed-Laser-Induced Simple Synthetic Route for Tb3Al5O12:Ce3+Colloidal Nanocrystals and Their Luminescent Properties

Cerium-doped Tb3Al5O12(TAG:Ce3+) colloidal nanocrystals were synthesized by pulsed laser ablation (PLA) in de-ionized water and lauryl dimethylaminoacetic acid betain (LDA) aqueous solution for luminescent bio-labeling application. The influence of LDA molecules on the crystallinity, crystal morphology, crystallite size, and luminescent properties of the prepared TAG:Ce3+colloidal nanocrystals was investigated in detail. When the LDA solution was used, smaller average crystallite size, narrower size distribution, and enhanced luminescence were observed. These characteristics were explained by the effective role of occupying the oxygen defects on the surface of TAG:Ce3+colloidal nanocrystal because the amphoteric LDA molecules were attached by positively charged TAG:Ce3+colloidal nanocrystals. The blue-shifted phenomena found in luminescent spectra of the TAG:Ce3+colloidal nanocrystals could not be explained by previous crystal field theory. We discuss the 5d energy level of Ce3+with decreased crystal size with a phenomenological model that explains the relationship between bond distance with 5d energy level of Ce3+based on the concept of crystal field theory modified by covalency contribution.

Nanocrystalline PrCo 5 alloy obtained by mechanical milling

Nanostructured PrCo 5 powders were obtained by mechanical milling, annealing in high vacuum in sealed ampoules and subsequent quenching in water. The highest intrinsic coercivity value ( i H C = 1.385 MA/m) and magnetization ratio ( σ r/ σ max = 0.66) were determined in powders milled for 240 min and annealed at 1073 K for 1 min. The enhanced remanence ( σ r/ σ max > 0.5) suggests strong exchange interactions between the adjacent crystallites. This is in good agreement with the small average crystallite size, 〈 D〉 = 15 nm. The high magnetic properties observed in these nanostructured PrCo 5 intermetallic alloys have their origin in its relatively high uniaxial magnetocrystalline anisotropy field, and in the homogeneous nanostructure developed by mechanical milling process and subsequent annealing.

Synthesis of Monodispersed Fine Hafnium Diboride Powders Using Carbo/Borothermal Reduction of Hafnium Dioxide

Fine hafnium diboride (HfB2) powders have been prepared by modified carbothermal/borothermal reduction of hafnium dioxide (HfO2) at relatively low temperatures (1500°–1600°C) for 1–2 h. The XRD patterns could be indexed as hexagonal HfB2 and no evidence of HfC, HfO2, or other impurities was observed. Glow discharge mass spectrometer analysis indicates that the synthesized HfB2 powders had high purity. The synthesized HfB2 powders had small average crystallite size (around 1 μm) and low oxygen content (&lt;0.30 wt%). Scanning electron microscopy observation of the as‐prepared powders demonstrated quasi‐column morphology and laser particle size analysis showed monodispersity (polydispersity 0.005).

Elaboration of boehmite nano-powders by spray-pyrolysis

Boehmite (γ-AlOOH) synthesis have been investigated using a spray pyrolysis (SP) device starting from a stable sol of Al-tri- sec-butoxide peptized by nitric acid. Free spherical particles from 100 to 500 nm have been elaborated. Particles sub-structure is made of nano-crystalline boehmite with very small average crystallite size (one crystal cell along the b axis). The nano-crystalline boehmite synthesized by SP at low temperature (200 °C) is spontaneously dispersible in water without any surface treatment. Boehmite powder may be transformed to transition γ-alumina by heat post-treatment. Powders of sub-micrometric and spherical γ-alumina particles may also be synthesized by SP at 700 °C.

In Situ Synthesis of Ultrafine ZrB2–SiC Composite Powders and the Pressureless Sintering Behaviors

Ultrafine ZrB2–SiC composite powders have been synthesized in situ using carbothermal reduction reactions via the sol–gel method at 1500°C for 1 h. The powders synthesized had a relatively smaller average crystallite size (&lt;200 nm), a larger specific surface area (∼20 m2/g), and a lower oxygen content (∼1.0 wt %). Composites of ZrB2+20 wt% SiC were pressureless sintered to ∼96.6% theoretical density at 2250°C for 2 h under an argon atmosphere using B4C and Mo as sintering aids. Vickers hardness and flexural strength of the sintered ceramic composites were 13.9±0.3 GPa and 294±14 MPa, respectively. The microstructure of the composites revealed that elongated SiC grain dispersed uniformly in the ZrB2 matrix. Oxidation from 1100° to 1600°C for 30 min showed no decrease in strength below 1400°C but considerable decrease in strength with a rapid weight increment was observed above 1500°C. The formation of a protective borosilicate glassy coating appeared at 1400°C and was gradually destroyed in the form of bubble at higher temperatures.

Antiferromagnetic resonance in nanocrystalline nickel oxide studied using fourier transform infrared spectroscopy

Antiferromagnetic resonance (AFMR) in nanocrystalline nickel oxide samples having different average crystallite sizes ranging from ∼ 2–3 nm to ∼ 16–17 nm is studied using Fourier transform infrared spectroscopy. The well defined absorption peak at ∼ 34 cm − 1 in the spectra of all the samples is attributed to the bulk AFMR. The low frequency absorption peak in the region 20–30 cm − 1 in the spectra of samples with smaller average crystallite sizes is attributed to a surface mode. The size dependent variation of the spectral features is discussed on the basis of the large volume fraction of the surface atoms and the modified exchange interaction at the surface.

Carbothermal synthesis of ultra-fine zirconium carbide powders using inorganic precursors via sol–gel method

Ultra-fine zirconium carbide (ZrC) powders have been synthesized by carbothermal reduction reactions using inorganic precursors zirconium oxychloride (ZrOCl2 · 8H2O) as sources of zirconium and phenolic resin as the carbon source. The reactions were substantially completed at relatively lower temperatures (∼1400 °C/1 h) and the synthesized powders had a small average crystallite size (<200 nm) and a large specific area (54 m2/g). The oxygen content of the powder synthesized at 1400 °C/1 h was less than 1.0 wt%. The thermodynamic change process in the ZrO2–C system and the synthesis mechanism were studied.

Carbothermal Preparation of Ultra-Fine TiB&lt;sub&gt;2&lt;/sub&gt; Powders Using Solution-Derived Precursors via Sol-Gel Method

Ultra-fine titanium diboride (TiB2) powders have been prepared by carbothermal reduction reactions in TiO2-B2O3-C system using tetrabutyl titanate, boric acid and phenolic resin as the solution-derived precursors. The reactions were substantially completed at relatively lower temperature (&lt;1400°C) and the resulting products had a smaller average crystallite size (&lt; 200 nm). However, below 1100°C, titanium carbide was the predominant phase and the relative content decreased with the rise of temperature. The thermodynamic change in TiO2-B2O3-C system was mainly studied by TG-DTA and the mechanism of synthesis of TiB2 was discussed. The crystallite size and morphology of the synthesized powders were characterized by SEM and TEM.

New Route to Synthesize Ultra‐Fine Zirconium Diboride Powders Using Inorganic–Organic Hybrid Precursors

Ultra‐fine zirconium diboride (ZrB2) powders have been synthesized using inorganic–organic hybrid precursors of zirconium oxychloride (ZrOCl2·8H2O), boric acid, and phenolic resin as sources of zirconia, boron oxide, and carbon, respectively. The reactions were substantially completed at a relatively low temperature (∼1500°C). The synthesized powders had a smaller average crystallite size (&lt;200 nm), a larger specific surface area (∼32 m2/g), and a lower oxygen content (&lt;1.0 wt%), which were superior to some commercially available ZrB2 powders. The thermodynamic change in the ZrO2–B2O3–C system was mainly studied by thermogravimetric and differential thermal analysis. The crystallite size and morphology of the synthesized powders were characterized by transmission electron microscopy and scanning electron microscopy.

Synthesis and Ceramic Conversion Reactions of Pinacolborane- and Diethylborazine-Modified Poly(vinylsiloxane)s. The Development of a Processable Single-Source Polymeric Precursor to Boron-Modified Silicon Carbide

The design, synthesis, and properties of two new types of poly(borosiloxane) preceramic polymers, diethylborazine- (PVS−DEB) and pinacolborane-modified (PVS−PIN) poly(vinylsiloxane)s, are reported. The polymers are synthesized in excellent yields by the RhH(CO)(PPh3)3-catalyzed reactions of poly(vinylsiloxane) (PVS) with either diethylborazine (DEB−H) or pinacolborane (PIN−H). The spectroscopic data for the polymers, along with results of the RhH(CO)(PPh3)3-catalyzed hydroboration reactions of diethylborazine and pinacolborane with the model compound vinyltris(trimethylsiloxy)silane, are consistent with predominately anti-Markovnikov hydroboration of the polymer vinyl groups by the boranes. The degree of polymer hydroboration is simply controlled by varying the reactant ratios with the compositions of the PVS−DEB polymer ranging from (MeSiO1.5)0.42(PhSiO1.5)0.37(ViMe2SiO0.5)0.17(DEB(CH2)2Me2SiO0.5)0.04 to (MeSiO1.5)0.42(PhSiO1.5)0.37(DEB(CH2)2Me2SiO0.5)0.21 and those of the PVS−PIN polymers from (MeSiO1.5)0.42(PhSiO1.5)0.37(ViMe2SiO0.5)0.18(PIN(CH2)2Me2SiO0.5)0.03 to (MeSiO1.5)0.42(PhSiO1.5)0.37(PIN(CH2)2Me2SiO0.5)0.21. The borane-modified polymers are soluble and stable in ethers, acetone, methylene chloride, benzene, and pentane. Molecular weight analyses using SEC with refractive index, viscometry, and light-scattering triple detection show significant differences in hydrodynamic volume, molecular weight, radius of gyration, and conformation, which indicate that the PVS−PIN and PVS−DEB polymers adopt a more branched and spherical structure compared to the unmodified PVS. Bulk pyrolysis of the modified resins produced SiOCB ceramic chars with boron contents ranging from 0.3 to 0.7% depending upon the percentage of polymer modification. XRD and TEM studies of the 1800 °C chars of PVS and PVS−PIN showed significant differences in the degree of grain growth and distribution with the boron-modified char showing both a smaller average crystallite size and a more narrow size range. The PVS−PIN polymers were found to be excellent melt-processable, single-source precursors to small-diameter SiOCB fibers.

Preparation, reduction, and chemisorption behavior of niobia-supported nickel catalysts

Two niobia(Nb 2O 5)-supported nickel catalysts, containing 2 and 10 wt% nickel, were prepared by incipient wetness impregnation. Subsequent to reduction in hydrogen at 573 and 773 K for 1 h, these catalysts adsorbed a smaller amount of hydrogen at room temperature than silica-supported nickel catalysts similarly prepared. The suppression in hydrogen adsorption was more pronounced for the 2 wt% sample, which had a smaller average crystallite size as determined by X-ray line broadening measurement. Thermogravimetric analysis showed a facile reduction of the nickel precursor salt to metallic nickel. The chemisorption behavior was thus ascribed to strong metal-support interactions (SMSI). These results compared with similar data previously obtained for titania(TiO 2)-supported nickel catalysts showed that niobia was a more interacting support than titania for nickel, when parameters such as average crystallite size and reduction treatment were comparable. The extent of interaction appeared to correlate with the reducibility of the oxide support, although quantitative thermogravimetric measurements suggested that the amount of support being reduced was small.

Reactivity of lime and related oxides. XIII. Sulphating of hydrated lime

AbstractSamples of quicklime and hydrated lime have been sulphated by sulphuric acid and by sodium sulphate solutions at 22°. Rates of sulphating, changes in surface area and phase composition and crystallite size have been correlated with the surface properties of the initial limes. The products have been compared with calcium sulphate dihydrate directly precipitated from solution by double decomposition, and subsequently aged.In acid sulphating, the gypsum from the quicklime has approximately the same specific surface (and average crystallite size) as that directly precipitated from 0.5 M‐calcium nitrate and N‐sulphuric acid. The more active intermediate hydrated lime dissolves more readily and does not impede the sulphating so seriously as the less active initial hydrated lime samples. The latter give products with larger specific surface (smaller average crystallite size) from the more concentrated acid solutions, where the rapid separation of gypsum ultimately impedes the sulphating of the remaining lime.Sulphating with 0.5 M‐soldium sulphate produces increasing alkalinity until about 16% of both the quicklime and the hydrated lime have been converted to gypsum. The sulphate initially separating, probably as a gel, impedes the reaction. The surface decreases until the gypsum begins to crystallise, but its crystallinity remains inferior to that of the acid‐sulphated lime of much lower specific surface.