A modified method to synthesize single-phase forsterite nanoparticles at low temperature

In this paper forsterite (Mg₂SiO₄) nanopowder with particle size in the range of 33 and 112 nm was synthesized by a combination of sol–gel and ball milling methods. Magnesium nitrate and silica were used as the sources of magnesium and silicon in the forsterite nanopowder. Thermogravimetry (TG) analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR) and dynamic light scattering (DLS) techniques were utilized to characterize the synthesized powders. Single-phase nanocrystalline forsterite powder with mean crystallite size of about 16 nm was obtained from sol–gel method with subsequent ball milling for 5 h and heat treatment at 750 °C for 1 h. A combination of sol–gel and mechanical activation led to the formation of more homogeneous powder and subsequently lower sintering temperature to produce forsterite powder. In vitro biological studies were performed by immersing the forsterite samples in simulated body fluid (SBF). The results showed that nanostructure forsterite is bioactive and possessed apatite formation ability.     

Effect of sintering temperature on the microstructure and transparency of Nd,Y:CaF2 ceramics

1 at% Nd, 3 at% Y doped CaF₂ transparent ceramics were obtained by hot pressing at the sintering temperature varing from 500 to 800 °C under vacuum environment with co-precipitated CaF₂ nanopowders. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis showed that the obtained nanoparticles were single fluorite phase with grain size around 26 nm. Scanning electron microscopy (SEM) observations of the Nd, Y: CaF₂ ceramics indicated that the mean grain size of the ceramic sintered at 800 °C was about 748 nm. The influence of the temperature on the grain size, microstructure and optical transmittance was investigated. For the ceramic sintered at 800 °C, the transmittance was 85.49% at the wavelength of 1200 nm. The room temperature emission spectra of Nd: CaF₂ and Nd, Y: CaF₂ ceramics were measured and discussed.     

Enhancing the magnetic and antibacterial properties of ZnO nanopowders through Mn+Co doping

Undoped and doubly (Mn+Co) doped ZnO nanopowders were synthesized with different doping levels of Co (1, 2, 3, 4 and 5 at%) and constant Mn doping level (10 at%) using a simple soft chemical route. XRD profiles confirmed that the synthesized material is nanocrystalline ZnO with hexagonal wurtzite structure. No peaks other than the characteristic ZnO peaks were observed in the XRD pattern confirming the absence of any secondary phase. Antibacterial activities of synthesized ZnO nanopowders were tested against Staphylococcus aureus bacteria using agar well diffusion method. It was found that the antibacterial efficiency of the doubly doped ZnO nanopowders was remarkably high when the Co doping level was 5 at%. The obtained PL, SEM and TEM results are corroborated well with the antibacterial activity. Magnetic measurements showed that undoped ZnO sample exhibits diamagnetic behavior and as the Co doping level increases, the nanopowder behaves as a ferromagnetic material.     

Synthesis of (Ni,Mn,Co)O4 nanopowder with single cubic spinel phase via combustion method

(Ni,Mn,Co)O₄ nanopowders with single cubic phase were successfully synthesized using combustion methods. Particle size of the as-burnt nanopowders after combustion was about 20 nm. Crystallization behavior of the NMC was investigated using various techniques such as X-ray diffraction (XRD), thermogravimetric (TG), Fourier transform infrared (FT-IR) spectroscopy, and transmission electron microscopy (TEM). Calcination at different temperature from 400 °C to 700 °C provides the powders with increased crystallinity and grain size. However, further increasing temperature above 800 °C for calcination, cubic spinel phase of NMC partly transformed to tetragonal spinel phase, which implies that cubic spinel phase of NMC nanopowder synthesized by combustion method becomes unstable above 800 °C.     

Physico-chemical and biological studies on three-dimensional porous silk/spray-dried mesoporous bioactive glass scaffolds

The incorporation of a bioactive inorganic phase in polymeric scaffolds is a good strategy for the improvement of the bioactivity and the mechanical properties, which represent crucial features in the field of bone tissue engineering. In this study, spray-dried mesoporous bioactive glass particles (SD-MBG), belonging to the binary system of SiO₂-CaO (80:20 mol%), were used to prepare composite scaffolds by freeze-drying technique, using a silk fibroin matrix. The physico-chemical and biological properties of the scaffolds were extensively studied. The scaffolds showed a highly interconnected porosity with a mean pore size in the range of 150 µm for both pure silk and silk/SD-MBG scaffolds. The elastic moduli of the silk and silk/SD-MBG scaffolds were 1.1±0.2 MPa and 6.9±1.0 MPa and compressive strength were 0.5±0.05 MPa and 0.9±0.2 MPa, respectively, showing a noticeable increase of the mechanical properties of the composite scaffolds compared to the silk ones. The contact angle value decreased from 105.3° to 71.2° with the incorporation of SD-MBG particles. Moreover, the SD-MBG incorporation countered the lack of bioactivity of the silk scaffolds inducing the precipitation of hydroxyapatite layer on their surface already after 1 day of incubation in simulated body fluid. The composite scaffolds showed good biocompatibility and a good alkaline phosphatase activity toward human mesenchymal stromal cells, showing the ability for their use as three-dimensional constructs for bone tissue engineering.     

Synthesis and characterization of Ba1−xSrxTiO3 nanopowders by citric acid gel method

Stoichiometric and monophasic Ba_1−xSr_xTiO₃ (x = 0.3) nanopowders were successfully prepared by the citric acid gel method using barium nitrate, strontium nitrate and tetra-n-butyl titanate as Ba, Sr, Ti sources and citric acid as complexing reagent. Thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), infrared (IR) spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize the thermal decomposition behavior, the crystallization process and the particle size and morphology of the calcined powders. The results indicated that single-phase and well-crystallized Ba_1−xSr_xTiO₃ (x = 0.3) nanopowders with particle size around 80 nm could be obtained after calcining the dried gel at 950 °C for 2 h.     

Two step sintering of a novel calcium magnesium silicate bioceramic: Sintering parameters and mechanical characterization

Two-step sintering (TSS) was applied to control the grain growth during sintering of a novel calcium magnesium silicate (Ca₃MgSi₂O₈ – Merwinite) bioceramic. Sol–gel derived nanopowders with the mean particle size of about 90 nm were sintered under different TSS regimes to investigate the effect of sintering parameters on densification behavior and grain growth suppression. Results showed that sintering of merwinite nanopowder under optimum TSS condition (T₁ = 1300 °C, T₂ = 1250 °C) yielded fully dense bodies with finest microstructure. Merwinite compacts held at T₂ = 1250 °C for 20 h had the average grain size of 633 nm while the relative density of about 98% was achieved. Mechanical testing was performed to investigate the effect of grain growth suppression on the hardness and fracture toughness. Comparison of mechanical data for samples sintered under two sintering regimes, including TSS and normal sintering (NS), showed TSS process resulted in significant enhancement of fracture toughness from 1.77 to 2.68 MPa m^1/2.     

Synthesis and characterization of high-density B2O3-added forsterite ceramics

Boria (B₂O₃)-added forsterite (Mg₂SiO₄) ceramics were synthesized and their properties were characterized. The addition of B₂O₃ was aimed to produce high density forsterite ceramics at a low sintering temperature. The raw materials were purified silica sand and commercial magnesia powders. Fosterite powder was produced from a solid reaction between the raw powders at 1100 °C prior to addition of B₂O₃, uniaxial pressing and sintering at 1200 °C. The amount of added B₂O₃ varied between 0%, 4%, and 8% by weight. Elemental analysis was performed by X-ray fluorescence (XRF) spectroscopy on the purified silica powder, whereas phase analyses were obtained from X-ray diffraction (XRD) data. Characterization of the ceramics included diameter shrinkage, density-porosity, thermal expansion, Vickers hardness, and dielectric constant. The results showed that the silica powder contained 98.7 at% Si with minor impurities, including 0.5 at% Ti, but the only identified crystalline phase was quartz. Further phase analysis of the ceramics showed that the addition of B₂O₃ reduced the amount of formed forsterite and increased the amount of cristobalite, proto- and clino-enstatite (MgSiO₃) as well as suanite (Mg₂B₂O₅). The highest forsterite content was found in B₂O₃-free ceramics, approximately 88.1 wt%. Moreover, the addition of B₂O₃ also reduced the diameter of the sample by more than 21%, resulting in a very dense ceramic with an apparent porosity of only 0.3%. The Vickers hardness significantly increased from 0.3 GPa for the B₂O₃-free ceramic to 10.9 GPa for the 4% B₂O₃ sample. The dielectric constant of the B₂O₃-added forsterite ceramics was improved by approximately 2–6 times that of the B₂O₃-free ceramic, which was primarily attributed to the loss of porosity in the samples.     

Nano-explosion synthesis of multi-component ceramic nano-composites

Here we demonstrate a unique processing technique which is based on engineering multi-component ceramic nanopowders and composites with precise morphology by nano-explosive deagglomeration/calcination. Multiple nano-explosions of impregnated cyclotrimethylene trinitramine deagglomerate the nanopowder due to the highly energetic impacts of the blast waves, while the solid-solubility of one component into the other is enhanced by the extremely high local temperature generated during the nano-explosions. We applied this technique to produce nanosize agglomerate-free ceria–gadolinia solid solution powder with uniform morphology and an average aggregate size of 32 nm, and 8 mol% yttria-doped zirconia aggregates with an average size of 53 nm impregnated with platinum (2–14 nm).     

Spark plasma sintering of transparent Y2O3 ceramic using hydrothermal synthesized nanopowders

Y₂O₃ nanopowders were synthesized by the hydrothermal treatment of Y(NO₃)₃·6H₂O and citric acid (CA) as Y⁺³ and the capping agent, respectively. The effect of different CA:Y⁺³ mol ratios, heat treatment time, and calcination temperature was investigated in order to determine their influence on the morphology, particle size and phase of Y₂O₃ nanopowders. The narrow size distribution of particles was obtained with CA:Y⁺³ mol ratio=1.6, heat treatment time of 6 h, and a calcination temperature at 900 °C for 90 min. Then, the synthesized Y₂O₃ nanopowder was consolidated by the spark plasma sintering technique at 1500 °C with a heating rate of 100 °C/min and held for 8 min before turning off the power. As a result, the ceramic prepared with 3 mm thickness got the highest transmission of 80% at 2.5–6 µm wavelength. The highest density and the grain size of yttria ceramic were 99.58% and 1–1.2 µm at 1500 °C, respectively.     

Effect of sintering temperature and dopant concentration on microstructure and electrical conductivity of ultra-fine-grained ZrO2–Sc2O3 ceramics

Phase transformations in ZrO₂ + xSc₂O₃ solid solutions (6.5 < x < 11 mol%) at sintering of ceramics obtained from nanopowders produced by laser evaporation of the ceramic targets have been studied. The Sc₂O₃ concentration increasing from 6.5 to 11 mol% is accompanied by the sintering temperature decreasing and the average grain size growth from 130 nm to 760 nm. At concentration of about 7 mol% Sc₂O₃ an abrupt increase of the average grain size and electric conductivity is observed. The sinterability of the ZrO₂ − хSc₂O₃ ceramics is affected by the prehistory of nanopowders preparation. The characteristics of ceramics obtained from nanopowders evaporated from the targets based on (ZrO₂ + xmol% Sc₂O₃) mixture and on the (ZrO₂ − 11mol% Sc₂O₃) solid solution significantly differ, namely, in the latter the sintering temperature is markedly lower and the shrinkage rate is higher. Besides, its average grain size is substantially lower and the conductivity is higher.     

Influence of Er doping on the structural,optical and luminescence properties of pulsed laser deposited Er: BaZrO3 thin films

Pure and erbium doped (1, 2, 3 and 5 at%) Barium zirconate (BZE) thin films have been deposited on Si (0 0 1) substrate via pulsed laser deposition using 100 mJ Nd: YAG laser operated at second harmonics (532 nm). Er doping significantly affects the surface morphology, microstructure and optical properties of grown thin films. All the films exhibit cubic BaZrO₃ structure and are polycrystalline in nature as extracted from XRD data. The optical band gap energies (3.75–3.63 eV) of doped (1, 2, 3, and 5 at%) BZE thin films are found to be less than that of pure BZO film (4.03 eV). PL spectra, excited at 328 nm, mainly consist of violet-blue (412 nm) and green (523–543 nm) emissions for all the doped films. The green emission increases with the increase in Er doping upto 3 at% and then concentration quenching effect appears at 5 at%. It is noted that the relative intensity of PL emission and the optical band gap can be tuned by varying Er concentration to alter the properties of the phosphor. The emission peaks in photoluminescence spectra makes the Er: BZO films potential candidates to be used in optoelectronic devices such as light emitting diodes (LEDs).     

Performance evaluation of Sm0.5Sr0.5CoO3−δ fibers with embedded Sm0.2Ce0.8O1.9 particles as a solid oxide fuel cell composite cathode

Sm_0.2Ce_0.8O_1.9 (SDC)–embedded Sm_0.5Sr_0.5CoO_3?δ (SSC) composite fibers were successfully fabricated by electrospinning using commercial SDC nanopowders and an SSC precursor gel containing polyvinyl alcohol (PVA) and hydrated metal nitrate. After calcination of the composite fibers at 800 °C, the fibers of 300 ± 80 nm in diameter with a well-developed SSC cubic-perovskite structure and fluorite SDC were successfully obtained. An anode-supported single cell composed of NiO–Gd_0.2Ce_0.8O_1.9 (GDC)/GDC/SSC–SDC fibers was fabricated, and its electrochemical performance was evaluated. The maximum power densities were 1250 and 360 mW/cm² at 700 and 550 °C, respectively, which we ascribe to the excellent properties of the SSC fibers with embedded SDC particles such as a highly porous and continuous structure promoting mass transport and a charge transfer reaction.     

Preparation of Bi4Ti3O12 nanopower by azeotropic co-precipitation and dielectric properties of the sintered ceramic

Bi₄Ti₃O₁₂ nanopowders were prepared by an azeotropic co-precipitation method and the phase evolution process, microstructure and sintering behavior were investigated. The results indicate that well dispersed and agglomerate-free nanocrystalline Bi₄Ti₃O₁₂ with average particle size of 21 nm can be obtained by calcinating the precursor at 750 °C, which is 50 °C lower than traditional solid reaction. The relative density of the ceramic reaches 96% at 1000 °C and shows no evident decrease until 1100 °C. The broadened sintering temperature range and the lower loss tangent of the ceramic show good sintering activity of the nanopowders.     

Fabrication and properties of transparent Tb:YAG fluorescent ceramics with different doping concentrations

Terbium doped yttrium aluminum garnet (Tb:YAG) transparent ceramics with different doping concentrations were fabricated by the solid-state reaction method using commercial Y₂O₃, α-Al₂O₃ and Tb₄O₇ powders as raw materials. Samples sintered at 1750 °C for 20 h were utilized to observe the optical transmittance, microstructure and fluorescence characteristics. It is found that all the Tb: YAG ceramics with different doping concentrations exhibit homogeneous structures with grain size distributions around 22–29 µm. For the 5 at% Tb:YAG transparent ceramics, the grain boundaries are clean with no secondary phases. The photoluminescence spectra show that Tb:YAG ceramics emit predominantly at 544 nm originated from the energy levels transition of ⁵D₄→⁷F₅ of Tb³⁺ ions, and the intensity of the emission peak reaches a maximum value when the Tb³⁺ concentration is 5 at%. The in-line transmittance of the 5 at% Tb:YAG ceramics is 73.4% at the wavelength of 544 nm, which needs to be further enhanced by optimizing the fabrication process. We think that Tb:YAG transparent ceramics may have potential applications in the high-power white LEDs.     

The effect of acid and base catalysts on phase purity and dissolution behavior of sol–gel derived in situ silica coated apatite composite nanopowders

Synthesis conditions of biomaterials have a key role on their final properties, which affect their performance in the human body. So, the aim of this study was to evaluate the effect of acid and base catalysts used in processing of silica synthesis, on phase purity and dissolution behavior of the in situ silica coated apatite composite nanopowders prepared via the sol–gel method. Composite nanopowders were synthesized in the presence of 0.1 N HNO₃ and acetic acid as the acid catalyst (A-S-AP composite nanopowder) and ammonium hydroxide as the base catalyst (B-S-CP composite nanopowder). The prepared composite nanopowders were characterized by XRD, FTIR, TEM, and BET techniques, acid dissolution test, and AAS spectroscopy. Results indicated that the base catalyst, rather than the acid catalyst, induced a nano-scale phase separation, which paved the way for the creation of a new crystalline phase (whitelockite) along with HA. In addition, the results of dissolution behavior evaluations in physiological saline solution showed that A-S-AP composite nanopowder, rather than B-S-CP composite nanopowder, had more calcium ions release and faster in vitro biomineralization, which are very important for bone repair and regeneration.     

Fabrication of transparent yttria ceramics by alcoholic slip-casting

Y₂O₃ transparent ceramics were prepared from alcoholic slurries of Y₂O₃ nanopowders via a slip-casting method to avoid the hydrolysis issue. Polyvinyl pyrrolidone (PVP), polyethylene glycol (PEG) and polyethylenimine (PEI) were used as dispersants to improve the rheological properties of the slurries. It was found that PEI is the most effective dispersant in ethanol. The adsorbed amount of PEI was evaluated by infrared absorption and rheology measurements. Y₂O₃ slurry with a solid loading of 20.8 vol% and a viscosity of <0.1 Pa s at the shear rate of 10 s⁻¹ was obtained using 1.5 wt% PEI. The slurry yielded a homogeneous green body, and finally resulted in a high-quality Y₂O₃ ceramic with the in-line transmittance of 80% at 800 nm.     

Synthesis,characterization and alcohol sensing property of Zn-doped SnO2 nanoparticles

The Zn-doped SnO₂ nanoparticles synthesized by the chemical co-precipitation route and having dopant concentration varying from 0 to 4 at%, were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM) for structural and morphological studies. XRD analyses reveal that all the samples are polycrystalline SnO₂ having tetragonal rutile structure with nanocrystallites in the range 10–25 nm. The TEM images show agglomeration of grains (cluster of primary crystallites). A corresponding selected area electron diffraction pattern reveals the different Debye rings of SnO₂, as analyzed in XRD. Alcohol sensing properties of all the Zn-doped samples were investigated for various concentrations of methanol, ethanol and propan-2-ol in air at different operating temperatures. Among all the samples examined, the 4 at% Zn-doped sample exhibits the best response to different alcohol vapors at the operating temperature of 250 °C. For a concentration of 50 ppm, the 4 at% Zn-doped sample shows the maximum response 85.6% to methanol, 87.5% to ethanol and 94.5% to propan-2-ol respectively at the operating temperature of 250 °C. A possible reaction mechanism of alcohol sensing has been proposed.     

Preparation of ultra-fine Sm0.2Ce0.8O1.9 powder by a novel solid state reaction and fabrication of dense Sm0.2Ce0.8O1.9 electrolyte film

A novel solid state reaction was adopted to prepare Sm_0.2Ce_0.8O_1.9 (SDC) powder. A mixed oxalate Sm_0.2Ce_0.8(C₂O₄)_1.5·2H₂O was synthesized by milling a mixture of cerium acetate hydrate, samarium acetate hydrate, and oxalic acid for 5 h at room temperature. An ultra-fine SDC powder with the primary particle size of 5.5 nm was obtained at 300 °C. The ultra-low temperature for the formation of SDC phase was due to the atomic level mixture of the Sm³⁺ and Ce⁴⁺ ions. The crystal sizes of SDC powders at 300 °C, 550 °C, 800 °C, and 1050 °C were 5.5 nm, 11.4 nm, 24.1 nm and 37.5 nm, respectively. The sintering curves showed that the powder calcined at lower temperature was easier to be sintered owning to its smaller particle size. A solid oxide electrolytic cell (SOEC), comprising porous La_0.8Sr_0.2Cu_0.1Fe_0.9O_3−δ (LSCF) for substrate, LSCF–SDC for active electrode, SDC for electrolyte, and LSCF–SDC for symmetric electrode, was fabricated by dip-coating and co-sintering techniques. An extremely dense SDC film with the thickness of 20 μm was obtained at only 1200 °C, which was about 100–300 °C lower than the literatures׳ reports. The designed SOEC was proved to work effectively for decomposing NO (3500 ppm, balanced in N₂), 80% NO can be decomposed at 600 °C.     

Dielectric characteristics of donor-acceptor modified BaTiO3 ceramics

Sm/Mn codoped BaTiO₃ ceramics were investigated for their microstructure and dielectric characteristics. The powders were prepared by the conventional solid state procedure. The concentration of Sm₂O₃ as a donor dopant has been kept from 0.1 up to 5.0 at%. The content of MnO₂ as acceptor was kept constant at 0.05 at% Mn in all samples. The specimens were sintered at 1290 °C, 1320 °C and 1350 °C in an air atmosphere for two hours.A mainly uniform and homogeneous microstructure with average grain size ranging from 0.3 µm to 2.0 µm was observed in low doped samples. In highly doped samples, apart from the fine grained matrix, the appearance of local area with secondary abnormal grains was observed.The dielectric properties were investigated as a function of frequency and temperature. The low doped samples exhibit the high value of dielectric permittivity at room temperature and the greatest change at the Curie temperature. The highest value of dielectric constant (ε_r=6800) was measured for 0.1Sm/BaTiO₃ samples sintered at 1350 °C. A nearly flat permittivity-temperature response and lower values of ε_r were obtained in specimens with 2.0 and 5.0 at% additive content. The dielectric constant increases with the increase of sintering temperature. The dissipation factor ranged from 0.01 to 0.22 and decreases with the increase of sintering temperature. The Curie constant (C), Curie-Weiss temperature (T₀) and critical exponent of nonlinearity (γ ) were calculated using a Curie-Weiss and modified Curie-Weiss law. The highest value of Curie constant (C=9.06·10⁵ K) was measured in 0.1 at% doped samples. The Curie constant decreased with increasing dopant content. The γ values, ranging from 1.001 to 1.58, point out the sharp phase transition in low doped samples, and the diffuse phase transition in heavily doped BaTiO₃ samples.