Synthesis of triclinic and hexagonal SmBO3 powders by mechanically activated annealing of Sm2O3 and B2O3 blends

Samarium borate (SmBO₃) powders were fabricated from oxide raw materials by a two-step solid-state synthesis method including mechanical activation and annealing. Blends containing stoichiometric amounts of samarium oxide (Sm₂O₃) and boron oxide (B₂O₃) were mechanically activated in a high-energy ball mill and subsequently annealed in air. Afterwards, mechanically activated and annealed powders were washed with distilled water in order to remove probable unreacted B₂O₃ phase. The effects of mechanical activation duration (15 min, 1 h, 3 h and 9 h) and annealing temperature (700–1250 °C) on the resultant powders were investigated. Compositional, microstructural, physical, thermal and optical properties of the powders obtained throughout the different process steps were characterized by using an X-ray diffractometry (XRD), particle size analysis (PSA), stereomicroscopy (SM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), gas pycnometry, differential scanning calorimetry (DSC), heating stage microscopy (HSM), atomic absorption spectrometry (AAS), Fourier transform infrared (FTIR) spectrometry and ultraviolet-visible spectrophotometry (UV–vis) techniques. Fine-grained and pure SmBO₃ powders were successfully synthesized via a simple, feasible and scalable route, yielding both triclinic and hexagonal crystal structures. Triclinic SmBO₃ powders were synthesized after mechanical activation for 1 h and annealing at 700 °C for 2 h. The polymorphic transformation temperature of SmBO₃ powders from triclinic to hexagonal is about 1080 °C. Due to the effect of mechanical activation, the synthesis of triclinic SmBO₃ phase and its transformation to hexagonal form were found to take place at ∼50–100 °C lower temperatures than those reported in other methods. Mainly hexagonal SmBO₃ powders were obtained after annealing at 1150 °C in the presence of a very small amount of triclinic SmBO₃. The resultant powders showed intense UV absorptions in the range between 1025 and 1150 nm with minimum reflectivity of 0.57% (triclinic SmBO₃ phase) and 0.68% (hexagonal SmBO₃ phase) depending on their crystal structures.     

Effects of manganese oxide addition and reductive atmosphere annealing on the phase stability and microstructure of yttria stabilized zirconia

The effects of Mn₃O₄ addition and reductive atmosphere (N₂:H₂ = 97:3) annealing on the microstructure and phase stability of yttria stabilized zirconia (YSZ) ceramics during sintering at 1500 °C for 3 h in air and subsequent annealing in a reductive atmosphere were investigated. Mn₃O₄ added 6 mol% YSZ (6YSZ) and 10 mol% YSZ (10YSZ) ceramics were prepared via the conventional solid-state reaction processes. The X-ray diffraction results showed that a single cubic phase of ZrO₂ was obtained in 1 mol% Mn₃O₄ added 6YSZ ceramic at a sintering temperature of 1500 °C for 3 h. A trace amount of monoclinic ZrO₂ phases were observed for 1 mol% Mn₃O₄ added 6YSZ ceramics after annealing at 1300 °C for 60 cycles in a reductive atmosphere by transmission electron microscopy. Furthermore, a single cubic ZrO₂ phase existed stably as Mn₃O₄ added 10YSZ ceramics was annealed at 1300 °C for 60 cycles in reductive atmosphere.     

Effects of heat treatment conditions on the structural and magnetic properties of MgCuZn nano ferrite

Mg_0.5Cu_0.05Zn_0.45Fe₂O₄ nanoparticles were prepared through sol–gel method using polyvinyl alcohol as a chelating agent. The as prepared sample was annealed at three different temperatures (500 °C, 700 °C and 900 °C). The phase formation, morphology and magnetic properties with respect to annealing temperature were studied using the characterisation techniques like X-ray diffraction (XRD) as well as Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM) and vibrating sample magnetometer (VSM), respectively. The crystallite size and magnetisation showed increasing trend with annealing temperature. The coercivity increased up to a particular annealing temperature and decreased thereafter, indicating transition from single domain to multi domain state with increasing annealing temperature. Further, to know the suitability of the material, as a ferrite core, in multilayer chip inductors, the powder sample annealed at 500 °C was compacted in the form of torroids and sintered at three different temperatures (800 °C, 900 °C and 950 °C). The permeability showed increasing trend with the increase of sintering temperature since the permeability depends on microstructure. The frequency dispersion of permeability, for the sintered samples, demonstrated high frequency stability as well as high operating frequency. The cut-off frequency for the sintered samples 800 °C, 900 °C and 950 °C is 32 MHz, 30.8 MHz and 30.4 MHz, respectively.     

Carbothermal production of ZrB2–ZrO2 ceramic powders from ZrO2–B2O3/B system by high-energy ball milling and annealing assisted process

Zirconium diboride (ZrB₂)-zirconium dioxide (ZrO₂) ceramic powders were prepared by comparing two different boron sources as boron oxide (B₂O₃) and elemental boron (B). The production method was high-energy ball milling and subsequent annealing of powder blends containing stoichiometric amounts of ZrO₂, B₂O₃/B powders in the presence of graphite as a reductant. The effects of milling duration (0, 2 and 6 h), annealing duration (6 and 12 h) and annealing temperature (1200–1400 °C) on the formation and microstructure of ceramic powders were investigated. Phase, thermal and microstructural characterizations of the milled and annealed powders were performed by X-ray diffractometer (XRD), differential scanning calorimeter (DSC) and transmission electron microscope (TEM). The formation of ZrB₂ starts after milling for 2 h and annealing at 1300 °C if B₂O₃ is used as boron source and after milling for 2 h and annealing at 1200 °C if B is used as boron source.     

Morphological and magnetic properties of BaFe12O19 nanoferrite: A promising microwave absorbing material

In this work, ultrapure hexagonal BaFe₁₂O₁₉ nanoferrite was synthesized by a facile co-precipitation method. Formation of single phase was analyzed by using wide angle X-ray diffraction. Crystallite size was found to increase from 50 nm to 78 nm when annealing temperature increased from 800 °C to 1000 °C, respectively. Ferrimagnetic behavior with moderate value of saturation magnetization and coercivity were studied at room temperature with the help of vibrating sample magnetometer (VSM). The electromagnetic radiation (EMR) absorption properties were studied in the frequency range of 2–18 GHz by using Vector Network analyzer (VNA). The maximum EMR absorption of −26.52 dB was observed at a frequency of 5.79 GHz. The FTIR spectra confirm tetrahedral and octahedral sites in BaFe₁₂O₁₉ structure. The surface morphology was analyzed by scanning electron microscopy (SEM), which reveals that the particles are agglomerated into irregular shapes. Particle size was measured with transmission electron microscopy (TEM), which was in correlation with the already calculated size from x-ray diffraction (XRD) spectra.     

Phase evolution and crystallization in Si–B–C–N ceramics derived from a polyborosilazane precursor: microstructural characterization

Amorphous Si–B–C–N ceramic powder samples obtained by thermolysis of polyborosilazane {B[C₂H₄Si(H)NH]₃}_n were isothermally annealed at different temperatures (1400–1800 °C) and hold-times (3, 10, 30, 100 h). Scanning electron microscopy (SEM) of annealed powders as well as polished cross sections of large powder particles from selected samples were carried out to study surface morphology, crystallization and associated microstructural changes. Microstructural and phase evolution were additionally investigated using high-resolution transmission electron microscopy (HRTEM) and energy filtering TEM (EFTEM). Higher surface areas of the powders were found to promote vapor-phase decomposition reactions resulting in SiC whisker growth. In coarser powders the influence of surface is manifested as a skin-core effect, where the ‘skin’ has undergone a higher degree of decomposition accompanied by an increased SiC crystal growth, compared to the ‘core.’ Crystallization of SiC occurs already at 1400 °C, although Si₃N₄ crystallization occurs only at 1700 °C, after more than 3 h of annealing.     

Synthesis of zirconium diboride platelets from mechanically activated ZrCl4 and B powder mixture

ZrB₂ platelets were prepared by mechanochemical processing a zirconium (IV) chloride–boron mixture with subsequent annealing from 800 °C to 1200 °C. The phases present were identified by X-ray diffraction. The size and morphology of the synthesized ZrB₂ powders were characterized by scanning electron and transmission electron microscopy. At 800 °C, ZrO₂ was detected in absence of ZrB₂. At or above 1000 °C, ZrCl₄–B converted to ZrB₂. Moreover, at 1200 °C, ZrCl₄–B completely converted to ZrB₂ without trace quantities of residual ZrO₂. The synthesized ZrB₂ consisted of platelets with a diameter of 0.1–2.1 μm and a thickness of 40–200 nm.     

Synthesis,characterization, and microwave dielectric properties of ZnTiTa2O8 ceramics with ixiolite structure obtained through the aqueous sol–gel process

Nano-sized ZnTiTa₂O₈ powders with ixiolite structure, with particle sizes ranging from 10 nm to 30 nm, were synthesized by thermal decomposition at 950 °C. The precursors were obtained by aqueous sol–gel and the compacted and sintered ceramics with nearly full density were obtained through subsequent heat treatment. The microstructure and electrical performance were characterized by field emission scanning electron microscopy, x-ray diffraction, and microwave dielectric measurements. All the samples prepared in the range 950–1150 °C exhibit single ixiolite phase and relative density between ~87% and ~94%. The variation of permittivity and Q·ƒ value agreed with that of the relative density. Pure ZnTiTa₂O₈ ceramic sintered at 1050 °C for 4 h exhibited good microwave dielectric properties with a permittivity of 35.7, Q·ƒ value of 57,550 GHz, and the temperature coefficient of resonant frequency of about −24.7 ppm/°C. The relatively low sintering temperature and excellent dielectric properties in the microwave range would make these ceramics promising for applications in electronics.     

Synthesis and characterization of nanometric gadolinia powders by room temperature solid-state displacement reaction and low temperature calcination

Nanometric-sized gadolinia (Gd₂O₃) powders were obtained by applying solid-state displacement reaction at room temperature and low temperature calcination. The XRD analysis revealed that the room temperature product was gadolinium hydroxide, Gd(OH)₃. In order to induce crystallization of Gd₂O₃, the subsequent calcination at 600 ∼ 1200 °C of the room temperature reaction products was studied. Calculation of average crystallite size (D) as well as separation of the effect of crystallite size and strain of nanocrystals was performed on the basic of Williamson-Hall plots. The morphologies of powders calcined at different temperatures were followed by scanning electron microscopy. The pure cubic Gd₂O₃ phase was made at 600 °C which converted to monoclinic Gd₂O₃ phase between 1400° and 1600 °C. High-density (96% of theoretical density) ceramic pellet free of any additives was obtained after pressureless sintering at 1600 °C for 4 h in air, using calcined powder at 600 °C.     

Structural,optical, and electrical properties of conducting p-type transparent Cu–Cr–O thin films

Cu–Cr–O films were prepared by DC magnetron co-sputtering using Cu and Cr targets on quartz substrates. The films were then annealed at temperatures ranging from 400 °C to 900 °C for 2 h under a controlled Ar atmosphere. The as-deposited and 400 °C-annealed films were amorphous, semi-transparent, and insulated. After annealing at 500 °C, the Cu–Cr–O films contained a mixture of monoclinic CuO and spinel CuCr₂O₄ phases. Annealing at 600 °C led to the formation of delafossite CuCrO₂ phases. When the annealing was further increased to temperatures above 700 °C, the films exhibited a pure delafossite CuCrO₂ phase. The crystallinity and grain size also increased with the annealing temperature. The formation of the delafossite CuCrO₂ phase during post-annealing processing was in good agreement with thermodynamics. The optimum conductivity and transparency were achieved for the film annealed at approximately 700 °C with a figure of merit of 1.51×10⁻⁸ Ω⁻¹ (i.e., electrical resistivity of up to 5.13 Ω-cm and visible light transmittance of up to 58.3%). The lower formation temperature and superior properties of CuCrO₂ found in this study indicated the higher potential of this material for practical applications compared to CuAlO₂.     

Phase transformation and morphological evolution of electrospun zirconia nanofibers during thermal annealing

Pure zirconia nanofibers were fabricated by electrospinning zirconia-polymer precursor and subsequent annealing. Fiber properties such as polymer decomposition, crystallization formation, phase transformation, surface morphologies, etc., were investigated by various techniques, including thermogravimetric analysis (TGA) and differential thermal analysis (DTA), high temperature differential scanning calorimeter (HTDSC), powder X-ray diffractometer (XRD), field emission scanning electron microscopy (FESEM), etc. It was found that the crystallization of as-spun fibers started at 450 °C and the initial crystallized zirconia phase was tetragonal (t), which began transforming to monoclinic (m) phase at 650 °C as evidenced by XRD; HTDSC showed at different thermal circles, the m-to-t transformation temperatures remained virtually unchanged while the reverse t-to-m temperatures systematically shifted from 924.9 to 978.6 °C as the progress of thermal circles; FESEM examinations revealed that fibers calcined to 1000 °C went through thermal grooving due to surface diffusion during heat treatment; fibers heated to 1370 °C formed the so-called “bamboo wires”, where volume diffusion was the dominant driving force.     

Thermal stability and mechanical properties of HVOF/PVD duplex ceramic coatings produced by HVOF and cathodic vacuum arc

Duplex ceramic coatings, consisting of an inner NiCr-Cr₃C₂-based coating and an outmost AlCrN film, were produced on the steel substrate in succession by velocity oxygen-fuel spraying (HVOF) and cathodic vacuum arc methods, and then isochronally annealed at annealing temperatures below 900 °C for 2 h. The thermal stability and mechanical properties of the annealed samples were systematically studied by means of X-ray diffraction, Optical microscope and transmission electron microscope, in association with mechanical property measurements. The results show that the microstructure, phase evolution and mechanical properties of duplex ceramic coatings are significantly dependent on the annealing temperature. Metastable fcc-AlCrN solid solution in AlCrN film first decomposes to rich-Al and rich-Cr domains by spinodal decomposition at 700 °C, leading to a notable increase in hardness due to its smaller grain size and high elastic strain field, and then to equiaxed hcp-AlN and Cr₂N by the nucleation and growth at 900 °C, leading to a notable decrease in hardness due to the recrystallization and the formation of hcp-AlN. Meanwhile, the both decarburization of Cr₃C₂ to Cr₇C₃ occurs at 800 °C, but becomes more intensive at 900 °C, leading to a notable loss in hardness. In addition, the dissolution of Cr₃C₂ produces high density of porosity, which also reduces the hardness. The hardness tests show the following ordering of load-bearing capacity for the duplex ceramic coatings: 700 °C>As-deposited >800 °C>900 °C. Tribological property measurements demonstrate that the wear resistance of the tested duplex ceramic coatings obeys the following ordering: 700 °C>As-deposited >800 °C>900 °C. The improved wear resistance is due to high surface hardness, load-bearing capacity and thermal stability. In addition, the wear mechanisms are shown.     

Mechanochemical-molten salt synthesis of α-Al2O3 platelets

The polyaluminium chloride (PACl) precursor was used for a simple and scaled-up mechanochemical-molten salt synthesis of α-Al₂O₃ platelets. PACl, as a low temperature α-Al₂O₃ precursor, was firstly mechanically activated by high-energy ball milling for 5 min, followed by a next 5 min ball milling in the presence of a NaCl–KCl salt mixture. The starting formation temperature of the α-Al₂O₃ phase was 600 °C. In the subsequent annealing in the temperature range of 660–1000 °C, the α-Al₂O₃ phase with a well developed plate-like morphology was obtained. The products were characterized by X-ray powder diffractometry, scanning electron microscopy (SEM), and thermal analysis (DTA, TG) and solution ²⁷Al NMR spectroscopy.     

Influence of annealing treatment on magnetic properties of Fe2O3/SiO2 and formation of ε-Fe2O3 phase

Magnetic properties of Fe₂O₃/SiO₂ samples were studied after being produced by sol-gel synthesis and formation of ε-Fe₂O₃ polymorph. Samples were thermally treated, using different annealing temperatures and annealing times. The size and morphological characteristics of the iron oxide nanoparticles were examined using a TEM microscope. We used the “ellipticity of shapes”, which is a measure of how much the shape of a nanoparticle differs from a perfect ellipse, in order to quantitatively describe morphological properties of nanoparticles. Coercivity measurements were used to identify and monitor the formation of the epsilon-iron oxide phase during the thermal treatments (annealing). Coercivity values were in the range from 1.2 to 15.4 kOe, which is in accordance with previous experience regarding the existence of ε-Fe₂O₃. We have determined the optimal formation conditions for the ε-Fe₂O₃ polymorph (t=1050 °C for 7 h, H_C=15.4 kOe), as well as the narrow temperature interval (1050–1060 °C) in which the polymorph abruptly vanished (H_C=2300 Oe), on the basis of results of the magnetic properties. The threshold temperature for the ε-Fe₂O₃ phase transformation was measured as 1060 °C. We found that different annealing temperatures and annealing times significantly affected magnetic properties of the examined samples.     

Preparation of barium hexaferrite coatings using atmospheric plasma spraying

Thick coatings of barium hexaferrite with the compositions BaFe₁₂O₁₉ and BaCoTiFe₁₀O₁₉ were prepared using atmospheric plasma spraying (APS) technology. The coatings were prepared from pre-reacted powders of the desired composition. The as-deposited coatings were poorly crystallized, but their crystallinity was improved with a subsequent annealing. The crystallization mechanism of the sprayed hexaferrites was studied during annealing up to 1300 °C, using X-ray powder diffraction combined with thermal analysis and with electron microscopy including microanalysis. Single-phase coatings were obtained after annealing treatments at 1100–1300 °C. Their magnetic properties showed that they would be suitable for absorbers at microwave and mm-wave frequencies, depending on the coating phase's composition, the crystallinity and the thicknesses.     

Synthesis and investigation of TiO2 nanotube arrays prepared by anodization and their photocatalytic activity

TiO₂ nanotube arrays were successfully prepared by anodic oxidation method in the electrolyte of ethylene glycol and deionized water mixed in 9:1 volumetric ration including 0.5 wt.% NH₄F. The microstructure and phase compositions of samples annealing from 0 °C to 800 °C were characterized by field-emission scanning electron microscope (FESEM) and X-ray diffraction (XRD). FESEM showed that the obtained nanotubes with diameter 80–100 nm and length 4.89 μm were highly ordered and perpendicular to Ti substrate. The tubular structure collapsed at 680 °C. The photocatalytic activity of samples annealing at different temperature were calculated by the degradation of a model dye, methyl orange (MO), under UV light illumination. The results indicated the phase composition and the morphology of TiO₂ nanotubes both played an important role in the degradation of MO. In addition, the effects of initial solution pH and dye concentration on degradation of MO had also been investigated. As a result, the optimum values of calcination temperature, initial solution pH and dye concentration were found to be 550 °C, 3, 10 mg/l, respectively. The best photodegradation of MO was 76% under illumination for 3 h.     

Synthesis and electrochemical properties of nickel oxide/carbon nanofiber composites

The electrospinning of polyacrylonitrile (PAN) with a polyaniline and graphene sol–gel mixture produced uniform, smooth fibers with an average diameter of 0.3 μm. These electrospun fibers were stabilized for 2 h at 200 °C and then carbonized at 800 °C for 5 h. Composites were prepared by depositing Ni(OH)₂ on the carbon nanofibers (CNFs) and calcining them at different temperatures. The composites were characterized with X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The effect of the calcination temperatures on the electrochemical properties was studied using cyclic voltammetry and electrochemical impedance spectroscopy. The specific capacitance (SC) was found to be highest (738 F g⁻¹) at a calcination temperature of 400 °C. The charge transfer resistance (R_p) decreased as the calcination temperature was increased. However, the electrical double layer capacitance (EDLC) increased with an increase in the calcination temperature. The EDLC increased from 0.144 F g⁻¹ at a calcination temperature of 100 °C to 485 F g⁻¹ at a calcination temperature of 500 °C.     

Facile synthesis of ZnO nanostructures and enhancement of their sinterability via short-time cryo-milling

Zinc oxide (ZnO), a wide band-gap semiconductor, has received a great interest due to its potential applications in various fields both as nanostructures and as sintered compacts. In this study, we report on the synthesis of the ZnO nanostructures and facilitation of their sintering for the production of fine-grained dense compacts. The facile synthesis of gram scale ZnO nanostructures was achieved by thermal decomposition of zinc acetate dihydrate (Zn(Ac)₂·2H₂O) or Zn(Ac)₂·2H₂O/graphite mixtures at 300 °C for 12 h. Thermal decomposition of Zn(Ac)₂ resulted in the formation of mostly ZnO nanoparticles with wurtzite structure along with ZnO nanorods, while the addition of graphite significantly promoted the growth of ZnO nanowires. Microstructural and phase properties of the obtained ZnO nanostructures were determined by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high resolution TEM (HRTEM) techniques, all of which revealed the successful synthesis of high quality ZnO nanostructures. In addition to synthesis and characterization of the ZnO nanostructures, we report on the enhancement of their sinterability by a subsequent cryogenic milling for a short duration of 5 min. As a result of the applied cryo-milling, fabrication of highly dense (96.2%) sintered compacts with fine grain sizes (572 nm) could be achieved after pressureless sintering at 1000 °C for 2 h.     

Comparison between microwave and conventional calcination techniques in regard to reactivity and morphology of co-precipitated BaTiO3 powder,and the electrical and energy storage properties of the sintered samples

Barium titanate (BaTiO₃) nanopowders were synthesized by an aqueous co-precipitation method followed by calcination. Either 2.45 GHz microwaves or conventional heating was used in order to investigate the impact of these techniques on the synthesis time, microstructure, and electrical properties of the materials. The heating temperatures ranged from 620 °C to 810 °C. X-ray diffraction (XRD) revealed pure BaTiO₃ formation by microwave heating in a noticeably shorter time (five minutes) compared to conventional heating (3 h). Field emission scanning electron microscopy (FESEM) results confirmed that the microwave process led to nanocube formation, whereas in the conventional procedure, the particles tended to form spherical shapes. To evaluate the electrical properties, the samples heated at 620 °C were conventionally sintered at 1280 °C, 1330 °C, and 1380 °C. Higher dielectric, piezoelectric, and ferroelectric properties and more energy-saving efficiency (ε_r=1012, tan δ=0.035 d₃₃=85 pC/N, p_r=6.2 µC/cm² and η=48% respectively) were achieved in the microwave-heated BaTiO₃ sintered at 1380 °C compared to the conventionally heated BaTiO₃ (ε_r=824, tan δ=0.030 d₃₃=75 pC/N, p_r=5 µC/cm² and η=27%) demonstrating that microwave calcination substantially affects the final electrical properties.     

Fully densified zircon co-doped with iron and aluminium prepared by sol–gel processing

A sol–gel technique has been used to prepare Fe and Al doped zircon. Structural properties have been studied by X-ray diffraction, nuclear magnetic resonance, scanning electron microscopy and transmission electron microscopy (TEM). Fully densified zircon was produced with high zircon yield and promising microstructures. The presence of Fe promotes zircon formation, while Al improves densification. The zircon phase starts to form at 1215 °C, with almost single phase zircon obtained at 1400 °C when heated for 1 h. Densification increases very significantly (to 99.7% of theoretical density) when the holding time was increased to 48 h from 1 h. TEM micrographs reveal a crystalline grain boundary phase containing some Fe and Al.