Effects on the crystallite growth behavior of LaPO4 nanopowders and its mechanical properties

The effects of pH of the reaction solution and the concentration of phosphoric acid on the crystal growth behavior of LaPO₄ crystallites were investigated and the mechanical properties of rare-earth phosphates were compared. As a result, the concentration of phosphoric acid of 10% was beneficial to the crystal growth of LaPO₄ nanocrystalline. When the pH value of the reaction solution was 2, the size of LaPO₄ crystallites increased gradually with the increasing reaction temperature, and the smallest crystallite size of 43.27 nm was obtained after heat-treatment at 1000 °C. Simultaneously, the activation energy for crystal growth of LaPO₄ nanocrystalline was relatively lower (26.82 kJ mol⁻¹). With the decreasing radii of rare-earth ions, the hardness, Young's modulus and fracture toughness of the bulk rare-earth phosphates exhibited a reduced tendency, resulted from the increase of porosity under the same preparation process.     

Heat capacity of nanocrystalline Yb2O3

A study of the structural, Raman scattering and thermodynamic properties of a nanocrystalline ceramic rare earth oxide Yb₂O₃ and its bulk counterpart is reported. The nanosized sample was obtained by mechanical milling of bulk Yb₂O₃ (99.998%, high purity powder). The Rietveld analysis of the X-ray diffraction data indicates the presence of nanoparticles with a mean grain size of 12 ± 1 nm after 75 h of milling time. The crystallographic structure within nanoparticles is cubic Ia-3 and the lattice parameter a = 10.455 ± 0.002 Å. The nanocrystalline structure is confirmed by the evaluation of transmission electron microscopy images, showing a size distribution with a mean size D_TEM = 8 ± 2 nm. Measurements of the specific heat (2 K–300 K) reveal an excess contribution respect to the unmilled (bulk) compound in the high temperature region above 70 K. At lower temperatures the results are consistent with a drastic change of the antiferromagnetic contribution (ordered below T_N = 2.2 K) as a result of the magnetic disorder arising from the size reduction process. The specific heat above T_N for the bulk and nanocrystalline samples are explained by the interplay among the phonon contribution, crystalline field and the presence of anharmonic effects. In the nanocrystalline state, broadening and shifts of the contribution of phonon modes to the Raman spectra, and a further reinforcement of the anharmonic contribution are found.     

Zinc oxide obtained by the solvothermal method with high sensitivity and selectivity to nitrogen dioxide

The effect of heat treatment using [Zn(H₂O)(O₂C₅H₇)₂] precursor ethylene glycol solution (synthesis temperature 125–185 °C, holding time 2–6 h) on the characteristics of the obtained nanocrystalline zinc oxide powder was studied. Under all synthesis conditions crystalline ZnO was formed with a wurtzite structure and an average crystallite size of 8–37 nm. The thermal behaviour and microstructure of the nanostructured ZnO powder were studied. The sensory properties of the obtained films in terms of the detection of hydrogen, methane, carbon monoxide and nitrogen dioxide were studied. The high sensitivity and selectivity of a thick ZnO film (synthesis temperature 145°С, holding time 6 h) when detecting NO₂ was established. It was found, that humidity had almost no effect on response value for NO₂ detection.     

Synthesis of zinc oxide nanocrystalline powders for cosmetic applications

The synthesis of zinc oxide (ZnO) nanocrystalline powders for cosmetic applications by a coprecipitation process has been investigated. When the Zn(OH)₂ precipitates are calcined at 373 K for 10 min, the crystalline phases comprise the major phase of Zn(OH)₂ and the minor phase of ZnO. XRD pattern shows that only ZnO is present and no other phase is detected when the Zn(OH)₂ precipitates calcined at 413 K for 10 min. The nanocrystallite size of ZnO increases slightly from 32.3 to 44.3 nm when the calcination temperature increases from 413 to 873 K. The activation energy of ZnO nanocrystallite growth is 2.02 kJ/mol, which reveals that the nanocrystalline ZnO is easily grown at low temperature. The UV transmission of ZnO nanocrystallites in the wavelength range from 290 to 375 nm is about 35%, indicating that the ZnO nanocrystallites have an excellent UV-absorbing capability.     

Synthesis of nanocrystalline cerium oxide by high energy ball milling

We have synthesized pure nanocrystalline CeO₂ powders of nearly spherical shape using high-energy attritor ball mill. Milling parameters such as the milling speed of 400 rpm, ball to powder ratio (40:1), milling time (30 h) and water cooled media were determined to be suitable for synthesizing nanosize (～10 nm) powders of CeO₂. The powders after milling for various durations (up-to 50 h) were characterized by X-ray Diffraction, Scanning Electron Microscopy, Energy-dispersive X-ray Spectrometry and Transmission Electron Microscopy. An average particle size of 10 nm was obtained at 30 h milling, after which the particle agglomeration started, and a mixture of nanocrystalline and amorphous phase was observed after 50 h milling.     

The influence of reactive milling on the structure and magnetic properties of nanocrystalline MnFe2O4

Nanocrystalline manganese ferrites (MnFe₂O₄) have been synthesized by direct milling of metallic manganese (Mn) and iron (Fe) powders in distilled water (H₂O). In order to overcome the limitation of wet milling, dry milling procedure has also been utilized to reduce crystallite size. The effects of milling time on the formation and crystallite size of wet milled MnFe₂O₄ nanoparticles have been investigated. It has been observed that single phase 18.4 nm nanocrystalline MnFe₂O₄ is obtained after 24 h milling at 400 rpm. Further milling caused deformation of the structure as well as increased crystallite size. With the aim of reducing the crystallite size of 18.4 nm, MnFe₂O₄ sample dry milling has been implemented for 2 and 4 h at 300 rpm. As a result, the crystallite size has been reduced to 12.4 and 8.7 nm, respectively. Effects of the crystalline sizes on magnetic properties were also investigated. Magnetization results clearly demonstrated that crystallite size has much more effect on the magnetic properties than average particle size.     

Synthesis, characterization and optical properties of Zn1−xTixO nanoparticles prepared via a high-energy ball milling technique

Zn_1−xTi_xO (x = 0, 0.01, 0.03 and 0.05) nanoparticles were prepared by high-energy ball milling at 400 rpm. The milled powders were characterized by X-ray diffractometer (XRD) and the results exhibited that Ti-doped ZnO nanoparticles consisted of single phase with hexagonal structure when the mixtures of ZnO and TiO₂ powders were milled for 20 h. The crystallite size reduced as a function of the doping content and milling time from 1 to 10 h then increased after milling for 20 h and when the annealing temperature increased. The strain changed inversely to the crystallite size. A wider band-gap was obtained by increasing the doping content and annealing temperature because of a reduction in defect concentration. Both ZnO- and Ti-doped ZnO nanoparticles caused damage to S. aureus, E. coli, P. mirabilis, S. typhi and P. aeruginosa.     

Structural and optical properties of MgМоO4 prepared by mechanochemical technique

MgMoO₄ with hexagonal particles were prepared by combining high energy ball milling with heat treatment technique. The influence of the mechanochemical activation/heat-treatment on the phase, structural and morphology transformation were investigated by X-ray powder diffraction analysis (XRD), infrared spectroscopy (IR), differential scanning calorimetry (DSC), particle size distribution (PSD) and scanning electron microscope (SEM). Optical properties of the final product were studied by UV–Vis and photoluminescence (PL) measurements. Mechanochemical activation of the initial oxides for 10 h ball milling leads to a full amorphization of MoO_3, only. The heat-treatment at different temperatures after 10 h milling time results to the phase formation of MgMoO₄. The reaction started at 400 °C for 5 h and completed at 800 °C for 5 h. The calculated energy band gap value as prepared MgMoO₄ is 2.03 eV and exhibits photoluminescence emission above 600 nm. The CIE chromaticity coordinates (x = 0.53 and y = 0.43) were found to lie in the orange region.     

Preparation,characterization and photocatalytic activity evaluation of micro- and mesoporous TiO2/spherical activated carbon

The influences of heat-treatment temperature and activation time on the properties of TiO₂ supported on spherical activated carbon (TiO₂/SAC) were investigated. Nano-sized TiO₂ was dispersed on the spherical activated carbon with the size of 10–30 nm. Some anatase phase of TiO₂ was transformed to rutile phase of TiO₂ with an increase of heat-treatment temperature. All of the TiO₂/SAC photocatalysts had microporous structure, with the mesopore volume increasing over an activation time of 6 h. The TiO₂/SAC photocatalysts obtained at activation times of 6 h and 9 h were observed synergistic effects between adsorption and photocatalysis in the removal of humic acid.     

Self-propagating high-temperature synthesis of barium titanate and subsequent densification by spark plasma sintering (SPS)

This paper describes the self-propagating high-temperature synthesis (SHS) of perovskitic oxides, specifically BaTiO₃, and their subsequent densification by spark plasma sintering. With the final goal of obtaining dense nanostructured materials, SHS products were mechanically treated at different milling time conditions, before densification. It was found that the grain size of ball milled powders decreases with increasing milling time, this effect being more evident at early stages of milling. Depending upon the ball milling (BM) conditions adopted, crystallite size in the range 15–70 nm was obtained. After milling for 5 h, the resulting powders (20–30 nm) were sintered by SPS, at 700 A, for different periods of time. By properly varying sintering time in the interval 70–140 s, it is possible to obtain products with relative density in the range 66–99%, respectively. In particular, grain growth during sintering was found to be limited (below 50 nm) if the electric current is applied for time intervals equal to or less than 100 s. The observed dielectric properties are typical of a nanocrystalline BaTiO₃ ceramic.     

The role of the diluent phase in the mechanochemical preparation of TiO2 nanoparticles

Anatase and rutile TiO₂ nanoparticles were synthesized via mechanochemical reaction and subsequent annealing of the products. TiO₂ nanoparticles were prepared by the use of planetary ball milling of titanyl sulphate (TiOSO₄.xH₂O.yH₂SO₄) and NaCl powders as the reactant and diluent phases, respectively. In this paper, the effect of volume fraction of diluent phase (NaCl) on the particle size distribution and agglomeration of TiO₂ nanoparticles is studied. Final products were obtained by annealing the milled powders at 700 °C for half an hour and subsequent washing out the water-soluble NaCl. X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) investigations showed that the increase in NaCl:TiOSO₄ weight ratio (NTR) leads to the formation of nanocrystalline anatase and rutile particles with more uniform size distribution, lower weight ratio of rutile phase and lower agglomeration of particles.     

Mechanosynthesis of nanostructured magnetic Ni-Zn ferrite

Nanocrystalline Ni-Zn ferrite (NiZnFe₂O₄) was directly produced by high energy ball milling of stoichiometric mixture of ZnO, NiO, Fe₂O₃ powders. X-ray powder diffractometry (XRD), scanning electron microscopy (SEM), simultaneous thermal analysing (STA), Fourier transform infrared spectroscopy (FTIR) and vibrating sample magnetometer (VSM) were carried out to characterize the structural, chemical and magnetic aspects of NiZnFe₂O₄ compound. The formation of NiZnFe₂O₄ phase appeared to involve two stages; development of Zn ferrite by diffusion of ZnO in Fe₂O₃ followed by diffusion of NiO in Zn ferrite to form Ni-Zn ferrite. The crystallite size of final product after 60 h of ball milling time was estimated to be 18 nm which increased to 45 nm after annealing at 800 °C for 4 h. After annealing of ball milled powders, the saturation magnetization was increased and coercivity was decreased as lattice defects and internal strain reduced.     

Influence of high energy ball milling on structural,microstructural and optical properties of TiO2 nanoparticles

The influence of high-energy ball milling on structural, microstructural, and optical properties of TiO₂ by modifying the nanoparticle size was studied. Five samples were extracted at different milling times (0, 2, 4, 8, and 13 h). The average particle sizes estimated by dynamic light scattering (DLS) were 205, 155.8, 116.8, 82.9, and 82.7 nm at 0, 2, 4, 8, and 13 h, respectively. X-ray diffraction analysis confirmed progressive broadening of the peaks as the milling time elapsed. Besides, a correlation was found between d spacing and the average crystal size. The UV–Vis diffuse reflectance spectra of TiO₂ revealed a decrease in reflectance due to particle size reduction. Similarly, an alteration of the bandgap transition energy was presented, whose values gradually decreased from 2.966 eV to 2.861 eV for the sample without and with the maximum duration milling performed (13 h), respectively. Likewise, the SEM analysis showed a distribution in nanoparticle size that became more homogeneous and smaller average grain size as the milling duration was longer.     

Multiple paths to nanocrystalline high silica beta zeolite

Several principles from nanozeolite synthesis lore were investigated in order to prepare discrete pure silica nanocrystalline BEA zeolite. It was discovered that lowering reaction temperature to 100 °C results in a significant reduction in nano-BEA crystal size, that lowering synthesis mixture water content to the lowest possible level for a ‘pourable’ gel provides nano-BEA crystals, that tetraethylorthosilicate (TEOS) more consistently provides smaller crystals than fumed silica as a raw material, that increasing the level of structure-directing agent resulted in more discrete and smaller crystals (25–30 nm), and that ‘surfactant’ addition could generate even smaller nano-BEA crystals or nanocrystalline domains, but mild to severe aggregation was still a problem. Nearly pure silica nanocrystalline BEA zeolites were prepared from Al-containing products by a calcination followed by acid extraction procedure. The calcination step is critical for more complete Al removal. Defects generated by acid extraction could be healed by treatment of resultant powder with a hexamethyldisilazane (HMDS). Another key finding was that Al-containing synthesis mixtures could provide nanocrystalline BEA zeolite with 4,4′-trimethylenebis(N-methyl, N-benzyl-piperidinium) dihydroxide (TMP(OH)₂) as organic structure-directing agent. Interestingly, adjusting synthesis parameters in TMP and Al-containing synthesis mixtures provided some differences to pure Si BEA products. For example, stirring or lowering reaction temperature did not decrease crystal size, but did shorten crystallization time. The smallest crystals from TMP-based, Al-containing reaction mixtures were prepared when a freeze–dried colloidal silica was utilized as silica source.     

Structural,magnetic and magnetocaloric properties of nanostructured Pr0.5Sr0.5MnO3 manganite synthesized by mechanical alloying

Nanostructure Pr_0.5Sr_0.5MnO₃ compounds were elaborated by mechanical milling in a planetary high energy ball mill at various milling times followed by high temperature sintering under air at 1400 °C for 20 h. The phase structure, the morphology, the magnetic and magnetocaloric properties of the powders were characterized by X-ray diffractometry, scanning electron microscopy and a vibrating sample magnetometer. Rietveld refinement of the X-ray diffraction patterns shows that all specimens crystallize in the tetragonal system with I4/mcm space group. Increasing the milling time up to 16 h, the average crystallites size decreases to the nanoscale (∼36 nm). During the intermediate stage of milling, significant changes occur in the morphology of the powder particles, due to the severe plastic deformation. Significant refinement in particle size was found evident at the final stage of milling (∼2 µm). From magnetic measurements, it was found that all samples present two magnetic transitions as a function of temperature. The Curie temperature T_C decreases with increasing milling time. Moreover, it was revealed that the antiferromagnetic domains fractions highly dependent on crystallites sizes. A large magnetocaloric effect and an important value of the relative cooling power around Neel temperature was observed in all samples. These characteristics may be related to the first-ordered nature of this transition. Moreover, the magnetic entropy change and the relative cooling power were increased with decreasing crystallites sizes.     

Mesoporous nanocrystalline zirconium oxide: novel preparation and photoluminescence property

A novel strategy involving the combination of soft-templating and solid–liquid method (CSSL) is presented to synthesize mesoporous nanocrystalline zirconia with high specific surface area, that is, the mesostructured zirconia hybrid is firstly synthesized via cooperative assembly between zirconium sulphate as inorganic precursor and 1-hexadecyl-3-methylimidazolium bromide (C₁₆mim⁺Br⁻) as the structure-directing agent, and subsequently ground with solid magnesium nitrate salt followed by heat-treatment in air. The resulting zirconia material after calcination at 600 °C possesses a wormlike arrangement of mesopores surrounded by tetragonal ZrO₂ nanocrystallites of ca. 2.3 nm. The BET surface area is 255 m²/g and the pore size is ca. 4.3 nm. However, no mesoporous structure exists in the obtained zirconia material via the simple soft-templating method at the same calcination temperature. Photoluminescence (PL) spectra of the obtained mesoporous nanocrystalline ZrO₂ show a strong emission peak at ca. 394 nm under UV excitation of 250 nm wavelength.     

Mechanochemical synthesis of MoSi2–SiC nanocomposite powder

MoSi₂–25 wt.%SiC nanocomposite powder was successfully synthesized by ball milling Mo, Si and graphite powders. The effect of milling time and annealing temperature were investigated. Changes in the crystal structure and powder morphology were monitored by XRD and SEM, respectively. The microstructure of powders was further studied by peak profile analysis and TEM. MoSi₂ and SiC were synthesized after 10 h of milling. Both high and low temperature polymorphs (LTP and HTP) of MoSi₂ were observed at the short milling times. Further milling led to the transformation of LTP to HTP. On the other hands, an inverse HTP to LTP transformation took place during annealing of 20 h milled powder at 900 °C. Results of peak profile analysis showed that the mean grain size and strain of the 20 h milled powder are 31.8 nm and 1.19% that is in consistent with TEM image.     

Microwave assisted low temperature synthesis of MnZn ferrite nanoparticles

MnZnFe₂O₄ ferrite nanoparticles were prepared by co-precipitation method using a microwave heating system at temperature of 100 °C. X-ray diffraction reveals the samples as prepared are pure ferrite nanocrystalline phase, transmission electron microscopy image analysis shows particles are in agglomeration state with an average size of about 10 nm, furthermore, crystal size of samples are increased with longer microwave heating.     

Nanocrystalline Barium Strontium Titanate Ceramics Synthesized via the “Organosol” Route and Spark Plasma Sintering

Dense nanocrystalline barium strontium titanate Ba_0.6Sr_0.4TiO₃ (BST) ceramics with an average grain size around 40 nm and very small dispersion were obtained by spark plasma sintering at 950°C and 1050°C starting from nonagglomerated nanopowders (~20 nm). The powders were synthesized by a modified “Organosol” process. X‐ray diffraction (XRD) and dielectric measurements in the temperature range 173–313 K were used to investigate the evolution of crystal structure and the ferroelectric to paraelectric phase transformation behavior for the sintered BST ceramics with different grain sizes. The Curie temperature T_C decreases, whereas the phase transition becomes diffuse for the particle size decreasing from about 190 to 40 nm with matching XRD and permittivity data. Even the ceramics with an average grain size as small as 40 nm show the transition into the ferroelectric state. The dielectric permittivity ε shows relatively good thermal stability over a wide temperature range. The dielectric losses are smaller than 2%–4% in the frequency range of 100 Hz–1 MHz and temperature interval 160–320 K. A decrease in the dielectric permittivity in nanocrystalline ceramics was observed compared to submicrometer‐sized ceramics.     

High-efficiency mechanochemical synthesis of Strontium carbonate nanopowder from Celestite

In this study, the conversion of Celestite to SrCO₃ was studied by wet mechanochemical synthesis in a high-energy ball mill and treatment with Na₂CO₃. For this purpose, solid strontium carbonate and soluble Na₂SO₄ were obtained after wet milling of Celestite powder and sodium carbonate. The solid phase was washed with water at room temperature by filter pressing. X-Ray diffraction patterns showed that the SrCO₃ nanopowder was synthesized and conversion boosted with increasing the milling time up to 8 hours Also, Rietveld refinement analysis was used to calculate the fraction of SrCO₃ as well as structural properties of synthesized samples. It was found that initial Celestite could be converted to strontium carbonate with a purity more than 98% using high-energy milling without simultaneous heating. The optimum milling time was determined as 4 hours resulting in formation of nanopowders with an average particle size around 90 nm. Field Emission Scanning Electron Microscopy (FE-SEM), clearly showed the nanoscale structure of the synthesized powders.