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.     

Fabrication and characterization of nanocrystalline Mg-substituted fluorapatite by high energy ball milling

The aim of this work was preparation and characterization of Mg-substituted nanostructured FA powders. Mg-substituted nanostructured FA powders were synthesized with a chemical composition of Ca_10?xMg_x(PO₄)₆F₂, with x=0, 0.5, 1, 1.5 and 2 by mechanical alloying method. Successful substitution of Ca²⁺ with Mg ions in the fluorapatite lattice was investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Fourier transform infrared spectroscopy (FTIR). The results showed that after 12 h of milling, pure nanocrystalline Mg-substituted FA powders with different Mg contents were synthesized. The incorporation of Mg ions into the fluorapatite caused the decrease of the lattice parameters. With increasing Mg content, the crystallinity of powder decreased while the degree of agglomeration of powder increased. SEM and TEM analysis showed that the powder was agglomerated and composed of nanocrystalline particles with the average particle size of less than 100 nm.     

A study of the mechanochemical synthesis of NaNbO3

We have studied the mechanochemical synthesis of NaNbO₃, prepared from a powder mixture of Na₂CO₃ and Nb₂O₅. The formation of NaNbO₃ during milling was followed using thermal analysis and X-ray diffraction. According to the thermogravimetric analysis, after 20 and 40 h of milling there was still some residual carbonate, while the X-ray diffraction shows NaNbO₃ as the major crystalline phase present in the mixture. Based on the quantitative XRD phase analysis, the residual reactants were amorphous and as such undetectable with the X-ray diffraction. Furthermore, an X-ray line-broadening analysis was used to determine the NaNbO₃ crystallite size and the microstrain. A decrease in the NaNbO₃ crystallite size coupled with an increase in the amount of microstrains was found from 10 to 40 h of mechanochemical treatment. Finally, the TEM analysis confirmed the NaNbO₃ crystallite size determined by the X-ray line-broadening analysis.     

Reaction mechanisms of synthesis and decomposition of fluorapatite–zirconia composite nanopowders

The reaction mechanisms of formation and decomposition of fluorapatite?zirconia composite nanopowders were investigated after the mechanochemical process and subsequent thermal treatment. Experimental results indicated that formation of fluorapatite?zirconia composite nanopowders proceeded in several steps. In the first stage, phosphoric acid formed immediately upon addition of phosphorous pentoxide to the reaction mixture. Afterwards, anhydrous dicalcium phosphate was generated as a result of reaction between reagents with phosphoric acid. The synthesis progressed by the formation of the stoichiometrically deficient hydroxyfluorapatite?zirconia composite at milling times between 5 and 15 min. Ultimately, the fluorapatite?zirconia composite nanopowder was obtained after 300 min of milling. Results revealed that the annealing process led to a decomposition of fluorapatite to tricalcium phosphate and calcium fluoride, and to the transformation of monoclinic zirconia to the tetragonal form. Field emission scanning electron microscope observations showed that the milled sample was composed of fine particles with a mean particle size of about 45 nm after 300 min of milling. Besides, the mean particle size increased progressively due to crystal growth in the temperature range above 900 °C. According to the gained data, reaction mechanism steps were proposed to clarify the reactions occurring during the above-mentioned solid state process.     

Radiation Stability of Spark‐Plasma‐Sintered Lead Vanadate Iodoapatite

Spark‐plasma‐sintered lead vanadate iodoapatite Pb_9.85(VO₄)₆I_1.7, a promising nuclear waste form for the immobilization of I‐129, was irradiated with energetic ions, electrons, and gamma rays, to investigate its radiation stability. In situ TEM observation of the 1 MeV Kr²⁺ irradiation shows that lead vanadate iodoapatite generally exhibits higher tolerance against ion irradiation‐induced amorphization than lead vanadate fluorapatite, and the spark plasma sintering can further enhance its radiation stability attributed to the enhanced crystallinity, reduced defect concentration, and denser microstructure. The critical amorphization dose and critical temperature for the SPS‐densified iodoapatite at 700°C are determined to be 0.25 dpa at room temperature and 230°C, respectively. No significant phase transformation or microstructural damage occurred under energetic electron and gamma irradiations. Raman spectra of gamma‐ray‐irradiated iodoapatite indicate improved V–O bond order at 500 kGy dose. Generally, the spark‐plasma‐sintered iodoapatite exhibits excellent radiation stability for nuclear waste form applications. The significantly enhanced radiation stability of the SPS‐densified iodoapatite suggests that SPS holds great promise for fabricating iodoapatite waste form with minimum iodine loss and optimized radiation tolerance for effective management of highly volatile I‐129.     

Electrical and thermophysical properties of mechanochemically obtained lanthanide hafnates

This contribution presents the synthesis and thermophysical characterization of seven lanthanide hafnates Ln₂Hf₂O₇ (Ln=Sm³⁺, Eu³⁺, Gd³⁺, Dy³⁺, Y³⁺, Ho³⁺, Yb³⁺); the title samples were prepared at room temperature by mechanically milling stoichiometric mixtures of the corresponding elemental oxides. Irrespective of the lanthanide ion involved, milling promotes the formation of highly disordered fluoritelike materials. Postmilling thermal treatments facilitate the formation of the fluorite ordered derivative, the pyrochlore structure, but only for the larger lanthanides (Sm³⁺, Eu³⁺, Gd³⁺). Impedance spectroscopy measurements revealed that these materials show a moderate‐to‐good oxygen ion conductivity at high temperatures; furthermore, those adopting the pyrochlore structure give higher σ_dc and lower E_dc than their fluorite analogues (σ_dc at 750°C>10^?3 S·cm^?1 vs <5·10^?4 S·cm^?1, respectively). The same trend also holds for the thermal resistivity at high temperatures; the highest thermal resistivity and thus, lowest κ was obtained for Eu₂Hf₂O₇ (κ~1.3·W·m^?1·K^?1 at 800°C). Therefore, Ln₂Hf₂O₇ phases might be attractive component materials for electrochemical devices and thermal insulating coatings.     

In Situ Synthesis of a ZrB2‐Based Composite Powder Using a Mechanochemical Reaction for the Zircon/Magnesium/Boron Oxide/Graphite System

A ZrSiO₄/B₂O₃/Mg/C system was used to synthesize a ZrB₂‐based composite through a high‐energy ball milling process. As a result of the milling process, a mechanically induced self‐sustaining reaction (MSR) was achieved in this system. A composite powder of ZrB₂–SiC–ZrC was prepared in situ by a magnesiothermic reduction with an ignition time of approximately 6 min. The mechanism for the formation of the product was investigated by studying the relevant subreactions, the stoichiometric amount of B₂O₃, and thermal analysis.     

Preparation of Fe3Si‐Al2O3 Nanocomposite Powders by Mechanochemical Reaction of Fe3O4‐Si‐Al Powder Mixtures

Phase evolution and morphology of Fe₃O₄‐Si‐Al powder mixtures during ball milling from 30 min to 20 h were investigated. A 3‐h critical milling was necessary for the occurrence of mechanically activated combustion reaction. The reaction results in the formation of Fe (Si), Fe₃Si, and α‐Al₂O₃. During ball milling from 3 to 20 h, Fe (Si) and Fe₃Si were combined into disordered Fe₃Si intermetallic and Fe₃Si‐Al₂O₃ composite powder was formed. The presence of in situ formed alumina leads to a decrease in crystallite and particle sizes. The Fe₃Si‐Al₂O₃ particles after milling for 20 h had a crystalline size of 10~12 nm.     

Low temperature synthesis of ultrafine non vermicular α-alumina from aerosol decomposition of aluminum nitrates salts

In this paper, spherical, smooth and unagglomerated ultrafine amorphous powder particles were prepared by ultrasonic spray pyrolysis (USP) of easy-handling aqueous aluminum nitrate salts increasing the precursor solute concentration to 0.5 mol L^?1 and reducing the pyrolysis temperature to 700 °C. The transformation of the USP alumina powders into α-Al₂O₃ was studied using combination of X-ray diffraction, electron microscopy, infrared spectroscopy, BET surface area, thermogravimetry and differential thermal analysis. A downward shift of the onset temperature of α-phase transformation to 900 °C has been detected using a larger precursor solution concentration and performing a milling before calcination due to an increase in the surface density of defects, in surface area and in anisotropic particle shape. Additional post-milling of the low calcined powders allowed the preparation of agglomerate-free pure ultrafine α-Al₂O₃ powder particles (～100 nm, 28 m² g^?1), free of vermicular microstructures.     

Direct Combustion Synthesis of Silicon Carbide Nanopowder from the Elements

Direct synthesis of silicon carbide (SiC) nanopowders (size 50–200 nm, BET ~20 m²/g) in Si–C system is conducted in an inert atmosphere (argon) using a self‐propagating high‐temperature synthesis (SHS) approach. A preliminary short‐term (e.g., minutes) high‐energy ball milling (HEBM) of the initial mixture, which involves pure Si and C powders, is used to enhance system reactivity. Two conditions of HEBM with different force fields (17G and 90G) are applied and the results are compared. The influence of HEBM's conditions on the microstructure of mechanically treated mixtures and combustion products is also investigated and discussed. Obtained results suggest that by changing the intensity of mechanical treatment one may prepare a completely amorphous reactive mixture containing carbon and silicon, or gradually change the ratio of (Si/C)–SiC phases and finally produce pure silicon carbide powder during the milling process. The influence of HEBM on the combustibility of the Si/C mixture possesses a critical character: the self‐sustained reaction becomes feasible only after a critical time of ball milling (i.e., 10 min for 90G; 30 min for 17G). Comparison of the microstructures for as‐milled and as‐synthesized powders reveals that for all investigated conditions the morphologies of the as‐milled reactive Si/C media are essentially the same as that for SiC combustion products. The mechanism for direct synthesis of SiC by combustion reaction is also proposed.     

Effects of Na+ substitution Cs+ on the microstructure and thermal expansion behavior of ceramic derived from geopolymer

As part of a series of studies, effects of Na⁺ substitution on the thermal evolution of cesium‐based geopolymers on heating were studied. A series of sodium‐substituted cesium‐based geopolymers, Cs_(1?x)Na_xGPs (where x=0, 0.1, 0.2, 0.3, and 0.4), were prepared and treated at 1300°C for 2 hours to obtain the corresponding ceramic products. The thermal evolution process was disclosed by virtue of a variety of technical, including TG‐DTA, thermal shrinkage, XRD analysis, SEM, and TEM investigation. The results indicated that unheated Cs_(1?x)Na_xGPs was not completely amorphous after the substitution of Na⁺ and the crystallinity of Cs_(1?x)Na_xGPs gradually increased with the rise of sodium content. Meanwhile, the average particle sizes of Cs_(1?x)Na_xGPs also increased evidently with increases in sodium substitution. The final product after heat treatment mainly consisted of pollucite (CsAlSi₂O₆) and amorphous glass phase. The particle size of pollucite grain gradually decreased as more Cs⁺ were replaced maybe owing to the role of Na⁺ in the nucleation process of pollucite. Two forms of Na⁺ present in the final products: A small portion was present in the pollucite grains due to Na⁺ partial occupied the crystallographic sites of Cs⁺; and the rest were present in the amorphous glass phase among the pollucite grains. The average coefficient of thermal expansion (CTE) of resulting Cs_(1?x)Na_xGPs ceramics increased from 4.80×10^‐6 K^?1 (x=0) to 7.26×10^?6 K^?1 (x=0.4) with increases in sodium substitution, which could be due to the amorphous glass phase had a relatively higher CTE than that of pollucite.     

Structural and luminescence properties of silica powders and transparent glass‐ceramics containing LaF3:Eu3+ nanocrystals

In this work, silica powders and transparent glass‐ceramic materials containing LaF₃:Eu³⁺ nanocrystals were synthesized using the low‐temperature sol‐gel technique. Prepared samples were characterized by TG/DSC analysis as well as X‐ray diffraction and IR spectroscopy. The transformation from liquid sols toward bulk powders and xerogels was also examined and analyzed. The optical behavior of prepared Eu³⁺‐doped sol‐gel samples were evaluated based on photoluminescence excitation (PLE: λ_em = 611 nm) and emission (PL: λ_exc = 393 nm, λ_exc = 397 nm) spectra as well as luminescence decay analysis. The series of luminescence lines located within reddish‐orange spectral scope were registered and identified as the intra‐configurational 4f⁶‐4f⁶ transitions originated from Eu³⁺ optically active ions (⁵D₀ → ⁷F_J, J = 0‐4). Moreover, the R/O‐ratio was also calculated to estimate the symmetry in local framework around Eu³⁺ ions. The luminescence spectra and double‐exponential character of decay curves recorded for fabricated nanocrystalline sol‐gel samples (τ₁(⁵D₀) = 2.07 ms, τ₂(⁵D₀) = 8.07 ms and τ₁(⁵D₀) = 0.79 ms, τ₂(⁵D₀) = 9.76 ms for powders and glass‐ceramics, respectively) indicated the successful migration of optically active Eu³⁺ ions from amorphous silica framework to low phonon energy LaF₃ nanocrystal phase.     

Investigation on mechanochemical combustion behavior of Mg–V2O5–Co3O4-C reactive system to synthesize VC–Co nanocomposite powder

VC–Co nanocomposite powders were obtained by mechanochemical combustion synthesis from a mixture of V₂O₅, Co₃O₄, C and Mg powders. The synthesized powders were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). VC–Co nanocomposite was directly produced after 10 min milling through a mechanically induced self-sustaining reaction without further heat treatment. TEM analysis showed that a nanostructured powder with a mean particle size of 100 nm was procured in the sample milled for 10 min.     

Thermal,Structural, and Enhanced Photoluminescence Properties of Eu3+‐doped Transparent Willemite Glass–Ceramic Nanocomposites

The precursor glass in the ZnO–Al₂O₃–B₂O₃–SiO₂ (ZABS) system doped with Eu₂O₃ was prepared by the melt‐quench technique. The transparent willemite, Zn₂SiO₄ (ZS) glass–ceramic nanocomposites were derived from this precursor glass by a controlled crystallization process. The formation of willemite crystal phase, size, and morphology with increase in heat‐treatment time was examined by X‐ray diffraction (XRD) and field‐emission scanning electron microscopy (FESEM) techniques. The average calculated crystallite size obtained from XRD is found to be in the range 18–70 nm whereas the grain size observed in FESEM is 50–250 nm. The refractive index value is decreased with increase in heat‐treatment time which is caused by the partial replacement of ZnO₄ units of ZS nanocrystals by AlO₄ units due to generation of vacancies. Fourier transform infrared (FTIR) reflection spectroscopy was used to evaluate its structural evolution. Vickers hardness study indicates marked improvement of hardness in the resultant glass‐ceramics compared with its precursor glass. The photoluminescence spectra of Eu³⁺ ions exhibit emission transitions of ⁵D₀→⁷F_j (j = 0, 1, 2, 3, and 4) and its excitation spectra show an intense absorption band at 395 nm. These spectra reveal that the luminescence performance of the glass–ceramic nanocomposites is enhanced up to 17‐fold with the process of heat treatment. This enhancement is caused by partitioning of Eu³⁺ ions into glassy phase instead of into the willemite crystals with progress of heat treatment. Such luminescent glass–ceramic nanocomposites are expected to find potential applications in solid‐state red lasers, phosphors, and optical display systems.     

The influence of concentration on the formation of BaTiO3 by direct reaction of TiCl4 with Ba(OH)2 in aqueous solution

The formation of fine BaTiO₃ particles by reaction between liquid TiCl₄ and Ba(OH)₂ in aqueous solution at 85 °C and pH⩾13 has been studied for 0.062⩽[Ba²⁺]⩽0.51 mol l⁻¹. The concentration of Ba²⁺ ions has a strong influence on reaction kinetics, particle size and crystallite size. When [Ba²⁺]>≈0.12 mol l⁻¹, the precipitate consists of nanosized (≈30 nm) to submicron (100–300 nm) particles of crystalline BaTiO₃. At lower concentrations, the final product is a mixture of crystalline BaTiO₃ and a Ti-rich amorphous phase even for very long reaction times. A two-steps precipitation mechanism is proposed. Initially, a Ti-rich amorphous precipitate is rapidly produced. Reaction between the amorphous phase and the Ba²⁺ ions left in solution then leads to crystallisation of BaTiO₃. In addition to nucleation and growth of nanocrystals, the final size and morphology of BaTiO₃ particles obtained at low concentration can be determined by aggregation of nanocrystals and heterogeneous nucleation on existing crystal surfaces.     

Hot Pressing of Infrared‐Transparent Y2O3–MgO Nanocomposites Using Sol–Gel Combustion Synthesized Powders

A sol–gel combustion method has been used to synthesize Y₂O₃–50 vol%MgO composite nanopowders. Solutions of the precursor nitrates were mixed with citric acid and ethylene glycol, heated from 200°C to a predetermined temperature gradually, giving nanocrystalline ceramic powders. The influence of the ratio of yttrium nitrate to the whole precursor mixture and the holding temperature on the properties of the composite nanopowder was investigated using a combination of thermal analysis, X‐ray diffraction, specific surface area analysis, and scanning electron microscopy techniques. When the ratio of yttrium nitrate to the whole precursor mixture reaches 22.5 mol%, the average particle size of synthesized composite nanopowder is 13 nm and the specific surface area is 45.9 m²/g. Then the synthesized Y₂O₃–MgO composite nanopowder was consolidated by the hot‐pressing technique at 1200°C with different dwell time. As a result, the nanocomposite ceramic prepared with a dwell time of 60 min got the highest transmittance of 75% at 5 μm wavelength. The cut‐off wavelength of Y₂O₃–MgO nanocomposite ceramic reaches 9.8 μm, which is superior to other mid‐IR transparent materials. In addition, the fabricated sample is more or less transparent in visible wavelengths and the transmittance at 0.8 μm is as high as 14.5%.     

Synthesis,characterization, metal sorption,and biological activity of poly(N‐heterocylic acrylamide)

N‐heterocyclic acrylamide monomers were prepared and then transferred to the corresponding polymers to be used as an efficient chelating agent. Polymers reacted with metal nitrate salts (Cu²⁺, Pb²⁺_, Mg²⁺, Cd²⁺, Ni²⁺, Co²⁺_, Fe²⁺) at 150°C to give metal‐polymer complexes. The selectivity of the metal ions using prepared polymers from an aqueous mixture containing different metal ion sreflected that the polymer having thiazolyl moiety more selective than that containing imidazolyl or pyridinyl moieties. Ion selectivity of poly[N‐(benzo[d]thiazol‐2‐yl)acrylamide] showed higher selectivity to many ions e.g. Fe³⁺, Pb²⁺, Cd²⁺, Ni²⁺, and Cu²⁺. While, that of poly[N‐(pyridin‐4‐yl)acrylamide] is found to be high selective to Fe³⁺ and Cu²⁺ only. Energy dispersive spectroscopy measurements, morphology of the polymers and their metallopolymer complexes, thermal analysis and antimicrobial activity were studied. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42712.     

Complex Effect of Sm3+/W6+ Codoping on α‐β Phase Transformation and Phonon Scattering of Oxygen‐Deficient La2Mo2O9

Lanthanum molybdate, La₂Mo₂O₉, has been attracted considerable attention owing to its high concentration of intrinsic oxygen vacancies, which could be reflected by enhanced phonon scattering and low thermal conductivity. A new series of La₂Mo₂O₉‐based oxides of the general formula La_2?xSm_xMo_2?xW_xO₉, where x ≤ 0.2, were synthesized by citric acid sol–gel process. The variation in thermal conductivity with Sm³⁺and W⁶⁺ fractions was analyzed based on structure information provided by X‐ray diffraction and Raman spectroscopy. The fully dense La_2?xSm_xMo_2?xW_xO₉ ceramics showed a minimum thermal conductivity value [κ = 0.84 W·(m·K)^?1,T = 1073 K] at the composition of La_1.8Sm_0.2Mo_1.8W_0.2O₉, which stems from the multiple enhanced phonon scatterings due to mass and strain fluctuations at the La³⁺ and Mo⁶⁺ sites as well as the high concentration of intrinsic oxygen vacancies embedded in the crystal lattice. The thermal conductivities present an abrupt decrease at the structural transition, which is due to the phase transformation from a low‐temperature ordered form (monoclinic α‐La₂Mo₂O₉) to a high‐temperature disordered form (cubic β‐La₂Mo₂O₉₎.     

Structural and Electrical Properties of Ni‐Substituted Mg‐Mn Nanoferrite by Coprecipitation Route

Ni²⁺ ions doped on Mg_0.40Mn_0.60‐xNi_xFe₂O₄ compositions with 0.00 ≤ x ≤ 0.60 have been synthesized by coprecipitation method and taken for the present work to study the dielectric properties and impedance characterization using the XRD and electrical measurements. The X‐ray diffraction and FT‐IR revealed that the ferrite has single‐phase cubic spinel structure. The calculated particle size from XRD data verified using SEM as well as AFM. These photographs show that the ferrites have crystalline size in the range of 20–50 nm. It was observed that the particle size decreased and Ni concentration increased. The dielectric constant and dielectric loss decreased with increase in nonmagnetic Ni²⁺ ions. Electrical properties indicate that synthesized nanoferrite particles have high resistivity.     

Identification of Substitution Mechanism in Group VIII Metal Oxides Doped Pb(Zn1/3Nb2/3)O3–PbZrO3–PbTiO3 Ceramics with High Energy Density and Mechanical Performance

Group VIII metal oxides, that is, Fe₂O₃, Co₂O₃, and NiO have been introduced to 0.2Pb(Zn_1/3Nb_2/3)O₃–0.8Pb(Zr_0.50Ti_0.50)O₃ (PZN–PZT) to deterministically identify the substitution mechanism and meantime to tailor mechanical and piezoelectric properties in obtaining energy harvesting materials. On the basis of the X‐ray diffraction and Raman analysis, it is clear that the group VIII metal oxides induce a phase transformation from the morphotropic phase boundary to the tetragonal phase side, and the corresponding grain size increases accordingly. It is reasonable to deduce that two types of substitution behaviors coexist in the group VIII metal oxides added PZN–PZT system. Due to the mixed valence of +2 and +3, the foreign doping ions prefer to enter the B site in the perovskite structure, not only substituting for Ti⁴⁺, Zr⁴⁺, and Nb⁵⁺ ions in the inequivalence replacement but also substituting for Zn²⁺ ions in the equivalence replacement. The proposed complex substitution mechanisms can give the full explanation about the grain growth phenomena and the variation in mechanical and electric properties in the modified PZN–PZT system. At the same doping level of 0.3 mol%, the maximum transduction coefficient (d₃₃·g₃₃ = 13120 × 10^?15 m²/N) and good fracture toughness (K_IC = 1.32 MPa m^1/2) are obtained in Co₂O₃ added 0.2PZN–0.8PZT ceramics, which shows great promise as practical materials for energy harvesting device applications.