Catalytic effect of potassium carbonate on carbothermic production of hexagonal boron nitride

Effect of potassium carbonate addition on the carbothermic formation of hexagonal boron nitride (hBN) was investigated by keeping the K₂CO₃ added B₂O₃+C mixtures in nitrogen atmosphere at 1400 °C for 40–160 min. K₂CO₃ amount was varied in the range of 10–60 wt% of the B₂O₃+C mixture. Products were subjected to XRD and quantitative analyses, SEM and TEM observations, and particle size measurement. Amount of hBN increased considerably with K₂CO₃ addition; also particle size and crystallinity improved. Catalytic role of K₂CO₃ was suggested as forming a potassium borate melt in which hBN particles form, in addition to carbothermic formation reaction. Effect of K₂CO₃ on increasing the hBN amount decreased when it was used over 40%. This was attributed to the rapid evaporation of the formed potassium borate liquid.     

Catalytic effect of alkaline earth oxides on carbothermic formation of hexagonal boron nitride

Effect of MgO, CaO and BaO on carbothermic formation of hexagonal boron nitride (h-BN) was investigated. B₂O₃–C mixtures containing alkaline earth oxide additives were reacted at 1500 °C for 30–120 min in nitrogen atmosphere. Formed phases in the reaction products were determined by powder-XRD analyses, and amounts of the constituents were determined by chemical analyses. Particle size and morphology of the formed h-BN powders were examined by FESEM and particle size distributions were determined by particle size analyzer. Addition of alkaline earth oxides was found to increase the amount and grain size of h-BN significantly and to decrease the amount of B₄C formed in the system. Investigated alkaline earth oxides presented similar catalytic effects according to chemical analyses and FESEM observations. While the average particle size of h-BN powder obtained from plain mixture was 149 nm, those obtained from MgO, CaO and BaO containing mixtures were 297, 367 and 429 nm, respectively.     

On the synthesis of lithium boron nitride (Li3BN2)

Li₃BN₂ is a known lithium ion conductor and has been predicted lately to be a high capacity cathode for Li-ion batteries (Németh, 2014) [4]. In this study, we have investigated the synthesis of Li₃BN₂ through reactions between Li₃N and h-BN powders. Effects of the reaction temperature, holding time and reaction atmosphere on the formation of Li₃BN₂ have been evaluated. Rietveld refinement of X-ray diffraction patterns of all powders has been performed to quantify the percentage of each phase. The results show that both the reaction temperature and holding time affect strongly the percentage of Li₃BN₂ in the reaction product. Furthermore, a trace amount of oxygen in the reaction atmosphere can negatively impact the purity of the reaction product because of the high affinity of Li₃BN₂ towards oxidation.     

Nanocrystalline BaAl2O4 powders prepared by aqueous combustion synthesis

In order to achieve BaAl₂O₄ formation via combustion synthesis, two types of recipes were designed: single fuel recipes (urea, glycine, β-alanine or hexamethylenetetramine) and innovative fuel mixture recipes (urea+glycine, urea+β-alanine and urea+hexamethylenetetramine respectively). No combustion reactions were noticed in the case of single fuel recipes based on urea or hexamethylenetetramine. The only crystalline phase present in the case of the powders obtained in such a way was unreacted Ba(NO₃)₂. Glycine and β-alanine generated smoldering reactions, leading to the formation of black powders, which consist of Ba(NO₃)₂, BaCO₃ and traces of BaAl₂O₄ (in the case of β-alanine). Fuel mixture recipes proved to be better than single fuel recipes, yielding BaAl₂O₄ as main crystalline phase. The specific surface area of the resulted powders ranges between 2.0 and 3.9 m²/g. The urea and glycine fuel mixture was the most efficient yielding BaAl₂O₄ with an average crystallite size of 60 nm.     

Study on the synthesis of Al2W2MoO12 by a simple stearic acid route and its negative thermal expansion property

The Sc₂W₃O₁₂-type negative thermal expansion material attracts an extensive interest because of its thermodynamic stability and great chemical flexibility. In this paper, nanostructured Sc₂W₃O₁₂-type negative thermal expansion material Al₂W₂MoO₁₂ was synthesized by a convenient method, the stearic acid method. The thermodynamic property of the precursor was studied by thermogravimetric and differential scanning calorimetric methods. The structure and the size of the resulted products were characterized by powder X-ray diffraction and transmission electron microscopy, respectively. The negative thermal expansion property was studied by high temperature powder X-ray diffraction. Results show that nanostructured Al₂W₂MoO₁₂ powders with average size about 10 nm were first synthesized here by the stearic acid method and the nanoparticles would be rearranged to form large rods by prolonging the heat-treatment time. The obtained nanoparticles exhibited negative thermal expansion property in the temperature range of 20–600 °C.     

Fabrication of Yb-doped Lu2O3-Y2O3-La2O3 solid solutions transparent ceramics by self-propagating high-temperature synthesis and vacuum sintering

A comparative analysis of sintering and grain growth processes of lutetium oxide and lutetium-yttrium-lanthanum oxides solid solutions, as well as optical properties, luminescence and laser generation of (Lu_xY_0.9-xLa_0.05Yb_0.05)₂O₃ transparent ceramics are reported. Fabrication of highly dispersed initial powders of these compounds was performed via nitrate-glycine self-propagating high-temperature synthesis (SHS) method. The powders were compacted at 300?MPa and vacuum sintered at temperatures up to 1750?°С. Optical ceramics of (Lu_0.65Y_0.25La_0.05Yb_0.05)₂O₃ elemental composition were shown to have the highest in-line transmittance, which achieved 78% at the wavelength of 800?nm. Generation of laser radiation at a wavelength of 1032?nm with the differential efficiency of 20% was demonstrated in the (Lu_0.65Y_0.25La_0.05Yb_0.05)₂O₃ ceramics.     

Synthesis of LaAlO3 powder using triethanolamine

LaAlO₃ powders were successfully synthesized by pyrolysis of complex compounds of lanthanum and aluminum with triethanolamine (TEA). The precursors and the derived powders were characterized by simultaneous thermogravimetry analysis (TG) and differential scanning calorimetry analysis (DSC), X-ray diffractometry (XRD), specific surface area measurements, and transmission electron microscopy (TEM). Pure LaAlO₃ phase was obtained at 775 °C for 2 h or 750 °C for 4 h, without formation of any intermediate phase. Pores were found from TEM images of LaAlO₃ powders prepared at 800 °C for 2 h.     

Modification of grain boundary structure of SrTiO3 using hydroxyl additives

Strontium titanate (SrTiO₃) is a typical perovskite material due to its crucial properties such as a good dielectric constant and favorable conductivity. Early studies reported that hydroxyl (OH) groups can manipulate the shape of the SrTiO₃ surface and control the crystal growth by changing the surface energy. This concept may be used to increase the population of (111) surfaces to form more Σ3 grain boundaries, which have lower energy for high ionic conductivity. In this study, two common OH additives, glycerol and 1,2-propanediol, were used for preparation of SrTiO₃ powders. SrTiO₃ powders were synthesized by spray pyrolysis and then sintered to form polycrystalline SrTiO₃. The results showed that the additive-treated SrTiO₃ powders had a higher population of (111) planes and exhibited higher conductivity than did un-treated powder. In addition, the highest population of Σ3 grain boundaries was generated by 1,2-propanediol treatment (11.0 ± 0.8%) of SrTiO₃, followed by glycerol treatment (10.0 ± 0.3%) and no treatment (2.5 ± 0.2%), respectively.     

The effect of carbon and nickel additions on the precursor synthesis of Cr3C2-Ni nanopowder

Decreasing crystal size to nanoscale is a proven method to enhance material properties. In this study, nanosize Cr₃C₂ and Cr₃C₂-Ni were synthetized and the reaction sequence was studied. Aqueous precursors using only water-soluble raw materials with varying carbon contents and a nickel addition were spray-dried. Glycine was used as a carbon source and chromium acetate hydroxide as a chromium source in the precursor solutions. Nickel nitrate hexahydrate was introduced as a nickel source to yield a metallic binder into the carbide nanopowder.Resulting powders were heat-treating to identify an applicable precursor composition producing the targeted Cr₃C₂ phase with crystal size of tens of nanometers. Thermal synthesis tests of the precursor powders to yield Cr₃C₂ took place at a temperature between 900 and 1300?°C under an Argon atmosphere. The synthesis of nanosize Cr₃C₂-Ni powder was successful at 1000?°C in 30?min, in a case of the best precursor. In order to produce the carbide phase with no residual oxide traces, relative carbon load has to be 48?wt%, while the stoichiometric amount of carbon in Cr₃C₂ is 13?wt%. When also introducing the nickel source into the precursor, an even higher carbon load was required. The carbon surplus needed to enable the Cr₃C₂ synthesis attributes to the non-homogeneity of the precursor composition.The chemical synthesis starting from water-soluble raw materials is a promising way of preparing nanosize Cr₃C₂-Ni with the targeted phase configuration.     

One-pot hydrothermal synthesis of MoS2 nanosheets/C hybrid microspheres

Uniform MoS₂ nanosheets/C hybrid microspheres with mean diameter of 320 nm have been successfully synthesized via a facile one-pot hydrothermal route by sodium molybdate reacting with sulfocarbamide in d-glucose solutions. The products were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). XRD patterns showed that the MoS₂ was kept as a two-dimensional nanosheet crystal and C was retained as amorphous even after their annealing treatment at 800 °C. TEM and SEM images indicated that the MoS₂ nanosheets were uniformly dispersed in the amorphous carbon. The experiment results also revealed that the appropriate amount of d-glucose had an obvious effect on the formation of uniform MoS₂ nanosheets/C hybrid microspheres. A possible formation process of MoS₂ nanosheets/C hybrid microspheres was preliminarily presented.     

IBLC effect leading to colossal dielectric constant in layered structured Eu2CuO4 ceramic

Dielectric (ε_r′) studies of phase pure T′-type Eu₂CuO₄ ceramics of two markedly different grain sizes (D), prepared by (i) conventional powder mixing and (ii) citrate complexation-Pechini process, have been carried out in the frequency range 0.1 Hz to 1 MHz, and at temperatures −100 °C to 150 °C. ε_r′ is found to be highly grain size dependent. For the sample with coarse bar-like grains (D²~17×6 μm²) ε_r′ is >10³, and for the finer grain size sample with bimodal distribution (D₁~1 μm, D₂~3 μm) ε_r′ is ~10⁵; for both the samples, high ε_r′ value is nearly frequency independent over 500 Hz≤f<100 kHz and T≥30 °C. The impedance spectroscopy (IS) study has clearly shown that both, the coarse- and the fine-grained samples consist of semiconducting grains and insulating grain boundaries that primarily lead to an internal barrier layer capacitance (IBLC) effect. And thus, manifest colossal dielectric constant (ε_r′>10³) in Eu₂CuO₄ ceramics. The smaller grain size (Pechini) sample, with over an order higher number of grains and grain boundary network, showing over an order higher ε_r′ (~10⁵) compared to the coarse grained one, further endorses the IBLC effect.     

Novel hydroxide precursors to prepare NaNbO3 and KNbO3

Firstly, fresh niobium hydroxide was precipitated from NbF₅ solution using an aqueous ammonium hydroxide under basic conditions. Then a simple procedure of mixing potassium (or sodium) and niobium hydroxides together and heating at a low temperature (<400 °C) produced KNbO₃ (KN) or NaNbO₃ (NN) powders. This is the lowest temperature so far reported for the formation of these phases. The phase content and lattice parameters are determined by X-ray diffraction (XRD). The average particle size and morphology were studied by scanning electron microscopy (SEM).     

Effect of propellant on the combustion synthesized Sr-doped LaMnO3 powders

Lanthanum strontium manganite (LSM) powders of composition La_0.7Sr_0.3MnO₃ are good candidates for cathode application in solid oxide fuel cells. This paper reports the synthesis of LSM powders from nitrate precursors by the combustion method, using two different propellants (urea and glycine) and varying the propellant/nitrate ratio. Thermogravimetric analysis (TGA) revealed two or three decomposition stages of the as-synthesized samples, with complete burn out of organics at about 850–900 °C. X-ray diffraction (XRD) patterns showed formation of only LSM phase for the sample synthesized with excess of urea, whereas SrCO₃ and MnCO₃ phases were also found for the samples prepared from glycine. The powder is better crystallized when a homogeneous gel is formed before burning. The crystallite size calculated using the Scherrer equation is in the range of 15–20 nm. Scanning electron microscopy (SEM) revealed the presence of agglomerates, formed by fine particles of different shapes.     

Synthesis of pure phase BiFeO3 powders in molten alkali metal nitrates

Pure phase BiFeO₃ powders were successfully synthesized in molten alkali metal nitrates (KNO₃–NaNO₃) at 500 °C. The as-prepared BiFeO₃ had a rhombohedral structure which was studied using X-ray diffraction. The plate-like morphologies were investigated through scanning electron microscopy and transmission electron microscopy. The average length and width of BiFeO₃ plates were 400 and 200 nm, respectively. Furthermore, the mechanism of formation of BiFeO₃ was also discussed through X-ray diffraction, thermogravimetry, differential thermal analysis and mass spectrometry.     

Characterization of strontium aluminate phosphors prepared from milled SrCO3

SrAl₂O₄:Eu²⁺,Dy³⁺ phosphors were prepared by solid-state reaction from milled SrCO₃. The effect of milling treatment of SrCO₃ on the formation and physical properties of SrAl₂O₄ phosphors was investigated by DTA, XRD, BET, SEM and PL. The results indicate that small crystallite size and large specific surface area of the milled SrCO₃ were able to increase the contact points between the reactants and to reduce the average transport distance for materials diffusion. Therefore, the solid-state reaction can be accelerated and the formation of SrAl₂O₄ was facilitated. On the other hand, the number of nucleation sites was also suggested to be increased that leads to a decrease in SrAl₂O₄ crystallite size and an increase in specific surface area. The increased specific surface area was proposed to increase the emission intensity and afterglow decay.     

Correlations between microstructure and dielectric properties of hexagonal boron nitride

Hexagonal boron nitride is a material with a unique combination of mechanical, chemical, and electrical properties and therefore of considerable technical and commercial interest. Nevertheless, there exists only very limited knowledge concerning the correlation of microstructure and electrical and dielectrical properties of such materials. In this work, the microstructure, dielectric breakdown resistance (dielectric strength), and low permittivity of different BN ceramics and composites were investigated. Besides exhibiting a very high specific electrical resistivity of 10¹³–10¹⁵ Ω cm, the materials had excellent dielectric strengths (up to 53 kV/mm) and low electrical permeability (4.1). The dielectric strength depended strongly on the porosity and to a lesser extent on the content of secondary phases, whereas the permittivity was influenced by the secondary phases. The aging of the materials in humid air did not significantly alter these values. The permittivity was found to be independent of frequency between 0.1 MHz and 10 MHz and temperature up to 300 °C.     

Synthesis of CuGa2O4 nanoparticles by precursor and self-propagating combustion methods

Copper gallate spinels, CuGa₂O₄, have been synthesized by two wet chemical routes: precursor method and self-propagating combustion involving a glycine-nitrate system. All complex precursors have been characterized by chemical analysis, infrared spectroscopy (IR), ultraviolet visible spectroscopy (UV–vis), electron paramagnetic resonance spectroscopy (EPR), thermal analysis and scanning electron microscopy (SEM). The copper gallate spinel oxides have been further investigated by X-ray diffraction (XRD), SEM, IR, UV–vis, magnetic measurements and EPR. The crystallite size of the copper gallate was found about 280 Å.     

Highly textured (Na1/2Bi1/2)0.94Ba0.06TiO3 ceramics prepared by the screen-printing multilayer grain growth technique

The screen-printing multilayer grain growth (MLGG) technique was successfully applied to perovskite-structured lead-free piezoelectric ceramics. Highly textured (Na_1/2Bi_1/2)_0.94Ba_0.06TiO₃ ceramics with (1 0 0) orientation were firstly fabricated by MLGG method with (or without) template particles. The MLGG approach using anisotropic Bi₄Ti₃O₁₂ templates resulted in >90% grain orientation, whereas the same approach without template particles resulted in high orientation degree. The grain orientation mechanism of MLGG using screen-printing was different form that of tape-casting and extrusion in templated grain growth (TGG) and reactive templated grain growth (RTGG) techniques. The interface between adjacent layers, which were formed by screen-printing, was the main mechanism for the texture development in MLGG technique. Compared with other grain orientation techniques, screen-printing was a simple, inexpensive and effective method to fabricate grain oriented lead-free piezoelectric ceramics.     

Low-temperature synthesis of ZnNb2O6 powders via hydrothermal method

ZnNb₂O₆ powder was successfully synthesized via hydrothermal method with Nb₂O₅ and Zn(NO₃)₂·6H₂O as raw materials and cyclohexane as solvent. Phase composition, morphology, and chemical composition were determined via a combination of XRD, SEM, TEM and EDS techniques. The effects of synthesis temperature and reaction time on phase composition and particle morphology were investigated in this paper. The results showed that fine ZnNb₂O₆ powders could be obtained at a hydrothermal temperature of 190 °C or above under different reaction time.     

Effect of ammonium chloride on the morphology of hexagonal boron nitride prepared by magnesium thermal reduction

Hexagonal boron nitride (h-BN) crystals with cylindrical and nanoplate shape were prepared by the magnesium thermal reduction method. A simple and novel strategy successfully controlled the preferred growth direction of h-BN crystal by adding a small amount of ammonium chloride to the reaction system, realized the transformation of the h-BN crystal growth mode from cylindrical to nanoplate, and the thickness of h-BN crystal nanoplates rapidly decreased from 2 μm to 40 nm. XRD results showed that the ammonium chloride content also significantly promoted the crystallinity of the synthesized h-BN nanoplates, and the G.I. index decreased from 2.4 to 1.9. Raman and TEM results showed that the crystallinity of synthesized h-BN was close to the properties of the single crystal.