Sol-gel synthesis of single-phase Ca5MgSi3O12: Eu, Mn phosphors for white-light emitting diodes

A novel Ca₅MgSi₃O₁₂: Eu²⁺, Mn²⁺ phosphor has been prepared by a sol-gel method. X-ray diffractometer, spectrofluorometer were used to characterize structural and optical properties of the samples. The results indicate that Ca₅MgSi₃O₁₂: Eu²⁺, Mn²⁺ phosphors show two emission bands excited by ultraviolet light. Blue (around 450 nm) and green (around 502 nm) emissions originate from Mn²⁺ and Eu²⁺, respectively. With appropriate tuning the concentration ratios of Eu²⁺ to Mn²⁺, Ca₅MgSi₃O₁₂: Eu²⁺, Mn²⁺ phosphors exhibit different hues and relative color temperatures, which have potential to act as a single-phase phosphor for white-light emitting diode.     

Luminescent properties of long lasting phosphor Ca2MgSi2O7:Eu

Long afterglow phosphors (Ca_1−xEu_x)₂MgSi₂O₇ (0.002 ≤ x ≤ 0.02) were prepared by solid-state reactions under a weak reductive atmosphere. X-ray diffraction pattern, photoluminescence spectra, decay curve, afterglow spectra and thermoluminescence curves were investigated. The phosphors showed two emission peaks when they were excited by 343 nm, due to two types of Eu²⁺ centers existing in the Ca₂MgSi₂O₇ lattice. However, only one emission peak can be found in their afterglow spectra. Energy transfer between Eu²⁺ ions in inequivalent sites was found. A possible mechanism was presented and discussed. The afterglow decay time of Ca_1.998MgSi₂O₇:Eu_0.002 was nearly 12.5 h which means it was a good long lasting phosphor.     

Luminescent properties of Ca2MgSi2O7 phosphor activated by Eu2+, Dy3+ and Nd3+

Long lasting alkaline earth silicates, Ca₂MgSi₂O₇:Eu,Dy,Nd was prepared under a reduction atmosphere through solid state reaction. The obtained phosphor was characterized by means of X-ray diffraction (XRD) and photoluminescence spectrum (PLS). The crystal structure of Ca₂MgSi₂O₇:Eu,Dy,Nd phosphor was refined by Rietveld analysis. The obtained Ca₂MgSi₂O₇:Eu,Dy,Nd phosphor showed a yellow–green emission peaking at 518 nm, which is ascribed to the luminescent emission of the Eu²⁺ that occupied the octa-coordinated Ca²⁺ sites in the Ca₂MgSi₂O₇ host. The electron affinity (ea) value for Eu²⁺ in [EuO₈] was calculated to 1.9 eV. The decay profile and the emission spectrum indicated that when the value of Dy/Eu is increasing, there is a concentration quenching of Eu²⁺.     

Low-temperature sintering of (Zn0.8 Mg0.2)2SiO4-TiO2 ceramics

Effect of Li₂O-B₂O₃-SiO₂ (LBS) glass on the sintering behavior and the microwave dielectric properties of (Zn_0.8 Mg_0.2)₂SiO₄-TiO₂ (ZMST) ceramics were investigated. The Li₂O-B₂O₃-SiO₂ glass lowered the sintering temperature of ZMST ceramics effectively from 1250 to 870 °C. The unknown second phase, which was formed in the ZMST ceramics increased with the addition of LBS glass. With increasing the LBS glass content, the bulk density, dielectric constant (ε_r) and the maximum Q × f value decreased, and the temperature coefficient of resonant frequency (τ_f) shifted to a negative value. (Zn_0.8 Mg_0.2)₂SiO₄-TiO₂ ceramics with 3 wt.% Li₂O-B₂O₃-SiO₂ glass sintered at 870 °C for 2 h shows excellent dielectric properties: ε_r = 8.48, Q × f = 11500 GHz, and τ_f = 0 ppm/°C.     

In vitro bioactivity of novel tricalcium silicate ceramics

In this study, bone-like apatite-formation ability of tricalcium silicate (Ca₃SiO₅) ceramics in simulated body fluid (SBF) was evaluated and the in vitro degradability was investigated by soaking in Ringer’s solution. The effect of ionic products from Ca₃SiO₅ dissolution on osteobalsts proliferation was investigated. The result indicated that hydroxyapatite (HA) was formed on the surface of the Ca₃SiO₅ ceramics after soaking in SBF for 1 day, and Ca₃SiO₅ ceramics could degraded in Ringer’s solution. The Si ions from Ca₃SiO₅ dissolution at certain concentration range significantly stimulated osteoblasts proliferation. Our results show that Ca₃SiO₅ ceramics possess bone-like apatite-formation ability and degradability, and can release soluble ionic products to stimulate cell proliferation.     

Multiferroic properties of Pb2Fe2O5 ceramics

Pb₂Fe₂O₅ (PFO) powders in monoclinic structure have been synthesized using lead acetate in glycerin and ferric acetylacetonate as the precursor. The powders were pressed into pellets, which were sintered into ceramics at 800 °C for 1 h. The morphology and structure have been determined by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). Polarization was observed in Pb₂Fe₂O₅ ceramics at room temperature, exhibiting a clear ferroelectric hysteresis loop. The remanent polarization of Pb₂Fe₂O₅ ceramic is estimated to be Pr ∼ 0.22 μC/cm². The origin of the polarization may be attributed to the off-centers of shifted Pb²⁺ ions as well as the FeO₆ octahedra in the perovskite-based structure of Pb₂Fe₂O₅. Magnetic hysteresis loop was also observed at room temperature. The Pb₂Fe₂O₅ ceramic shows coexistence of ferroelectricity and ferromagnetism. It provides a new field of research for complex oxides with multiferroic properties.     

Compositional dependence of formation of an apatite layer on glass-ceramics in simulated physiological solution

In simulated physiological solution, an apatite layer is formed on the surface of apatite-containing glass-ceramics having the ability to bond to living bone. In this study, the influence of composition in the system CaO-P₂O₅-SiO₂-MgO, Al₂O₃ on apatite layer formation is investigated. On CaO-P₂O₅-SiO₂ glass-ceramics, an apatite layer was formed rapidly in simulated physiological solution. However, a solution containing an excess of Mg²⁺ prevented apatite layer formation. On glass-ceramics containing MgO, the amount of apatite formed on the surface decreased. An apatite layer was not formed on glass-ceramics containing Al₂O₃. The prevention of apatite layer formation on glass-ceramics containing MgO is attributed to an increase of Mg²⁺ concentration in the solution. It is thought that glass-ceramics containing Al₂O₃ form are Al₂O₃-rich layer, and that this layer prevents the formation of an apatite layer.     

Effect of Bi2O3 and Y2O3 doping methods on electrical properties and PTCR behavior of Ba0.95Ca0.05TiO3 ceramics

Lead-free PTCR ceramics based on Bi₂O₃ and Y₂O₃ doped Ba_0.95Ca_0.05TiO₃ were fabricated by the conventional mixed oxide method, while Bi₂O₃ and Y₂O₃ were doped directly or after pre-calcining, in the molar ratio of Bi₂O₃:Y₂O₃ = 1:1. There were two synthesizing route, i.e. the materials were pre-calcined at 900 °C to obtain BiYO₃ firstly and then doped into the basic materials, and the materials were directly doped into the starting materials, both of which could obtain samples with different electrical properties and PTCR behavior. The samples were characterized by using X-ray diffraction, scanning electron microscope, dielectric constant-temperature and resistivity-temperature measurement instrument. It was revealed that the perovskite lattice, the microstructure and the PTCR behavior of Ba_0.95Ca_0.05TiO₃ varied with different doping contents and methods. A further research was conducted so as to study the electrical properties of ceramics by impedance spectroscopy.     

First-principles calculations of structural, electronic and elastic properties of Ca2MgSi2O7

We studied the structural, electronic and elastic properties of åkermanite, Ca₂MgSi₂O₇, by using the first-principles method. The structure of åkermanite is constructed by interleaved tetrahedral and Ca cation layers, and this characteristic is perfectly presented by three-dimensional (3D) crystal lattice, as well as two-dimensional (2D) contour plots of total electron densities in this paper. The chemical bonding and interaction are investigated by analyzing the bond population and density of states (DOS) of the crystal. Theoretical elastic constants of åkermanite are consistent with the experimental values. Moreover, significant anisotropy for Young’s modulus can be observed.     

Dissolution behavior and bioactivity study of glass ceramic scaffolds in the system of CaO–P2O5–Na2O–ZnO prepared by sol–gel technique

Three-dimensional glass ceramic scaffolds from the system CaO–P₂O₅–Na₂O–ZnO have been prepared by coating polyurethane foams with sol–gel derived glass slurry. Main phase catena hexaphosphate (Ca₄P₆O₁₉), minor phases calcium pyrophosphate (β-Ca₂P₂O₇) and calcium metaphosphate (β-Ca(PO₃)₂) were detected in the prepared glass ceramics. In order to assess the potential use in hard tissue engineering, the dissolution and precipitation behavior of the glass ceramics was investigated in vitro after soaking in simulated body fluid (SBF) for different periods of time, and the bioactivity and biocompatibility studies were conducted using mouse MC3T3-E1. Ca₄P₆O₁₉ phase showed a good chemical durability in SBF solution over the period time of soaking. However, there were small quantities of apatite-like deposits formed on the surfaces after soaking 28 days, exhibiting a poor ability of inducing calcification in SBF. In vitro cell culture, a high degree of cell adhesion and spreading was achieved and large number of mineralized deposits composed of Ca, P and Zn were detected in these porous scaffolds. These results confirmed the biocompatibility and bioactivity of the glass ceramics and the positive effects on mouse MC3T3-E1 cell behavior although no continuous apatite layer was formed on scaffold surfaces after soaking in SBF, and also demonstrated that Zn doped this glass ceramics could strongly stimulate the formation of mineralized deposits in vitro culture of MC3T3-E1 cells.     

Influence of doping Nb5+ and Mn2+ on the PTCR effects of Ba0.92Ca0.05(Bi0.5Na0.5)0.03TiO3 ceramics

As a positive temperature coefficient of resistivity (PTCR) material, Ba_0.92Ca_0.05(Bi_0.5Na_0.5)_0.03TiO₃ ceramics with donor doping of Nb⁵⁺ and acceptor doping of Mn²⁺ were prepared by a conventional mixed oxide method. The influence of contents of Nb⁵⁺ and Mn²⁺ on the microstructure and PTCR characteristics of Ba_0.92Ca_0.05(Bi_0.5Na_0.5)_0.03TiO₃ ceramics sintered at 1,360°C for 2 h was investigated. The result showed that the Curie temperature (T _c) was shifted to a lower temperature with increasing of the content of Nb⁵⁺ and the resistance jump (ρ_max/ρ_min) was enhanced with doping of Mn²⁺. The grain size of ceramic sample decreased with increasing of contents of donor Nb⁵⁺ and acceptor Mn²⁺. The Ba_0.92Ca_0.05(Bi_0.5Na_0.5)_0.03TiO₃ ceramic with 0.4 mol%Nb⁵⁺ and 0.04 mol%Mn²⁺ exhibited a low ρ_RT of 5.0 × 102 Ω cm, a typical PTCR effect of ρ_max/ρ_min > 10³, and a T _c of 158°C.     

Photocatalytic properties of TiO2-P2O5 glass ceramics

Binary TiO₂-P₂O₅ glasses with 69 mol% and 76 mol% TiO₂ were prepared and converted into glass ceramics by heat-treatments. XRD measurements show that the main crystalline phases precipitated in the glass ceramics are anatase-type TiO₂ crystals or (TiO)₂P₂O₇ crystals, depending on the concentration of titanium constituent. Photocatalytic activities of the glass ceramics were evaluated by the decomposition of methylene blue (MB) and measuring the water contact angle. It is found that the glass ceramics containing anatase crystals exhibit both photocatalytic oxidation activity and highly photo-induced hydrophilicity under UV irradiation with intensity of 1.0 mW/cm².     

Formulation and characterization of glass–ceramics based on Na2Ca2Si3O9–Ca5(PO4)3F–Mg2SiO4-system in relation to their biological activity

Glasses having a chemical composition based on combeite [Na₂Ca₂Si₃O₉]–fluoroapatite [Ca₅(PO₄)₃F] and forsterite [Mg₂SiO₄] system were crystallized through controlled heat-treatment. Two forms of sodium calcium silicate e.g. combeite Na₂Ca₂Si₃O₉ and pectolite Na₂CaSi₃O₈, were formed together with diopside (CaMgSi₂O₆) and monticellite (CaMgSiO₄) in addition to fluoroapatite (Ca₅(PO₄)₃F) phases by thermal treatment of the glasses. Selected glass–ceramics were exposed to a simulated body fluid solution (SBF) which is close to human plasma for 3 weeks. Energy dispersive X-ray analysis (EDX) and inductive coupled plasma (ICP) analysis confirmed the formation of an apatite layer which indicate bioactivity in the all crystallized sample. A decreasing of surface bioactivity with increasing Mg₂SiO₄/Na₂Ca₂Si₃O₉ replacement was observed as indicated by the decrease in the amount of apatite layer on the surface of the crystallized specimens. The Vicker’s microhardness of the studied glass–ceramic materials are between 5,047 and 6,781 MPa.     

Fabrication and optical properties of fibers with an Al2O3-P2O5-SiO2 glass core

A process has been developed for the fabrication of preforms and fibers with Al₂O₃-P₂O₅-SiO₂ glass cores, and their optical properties have been investigated. The results demonstrate that the refractive index and optical losses of the glasses studied are nonadditive functions of Al₂O₃ and P₂O₅ contents in the range 0–20 mol %. Original Russian Text ? M.M. Bubnov, V.N. Vechkanov, A.N. Gur’yanov, K.V. Zotov, D.S. Lipatov, M.E. Likhachev, M.V. Yashkov, 2009, published in Neorganicheskie Materialy, 2009, vol. 45, No. 4, pp. 496–501.     

Recent advances in silicate-based crystalline bioceramics for orthopedic applications: a review

The present article critically reviewed the potentiality of Mg–Ca silicate-based crystalline bioceramics such as MgSiO₃, Mg₂SiO₄, CaSiO₃, Ca₂SiO₄, Ca₃SiO₅, CaMgSi₂O_6, Ca₂MgSi₂O₇, Ca₇MgSi₄O₁₆, CaMgSiO₄ and Ca₃MgSi₂O₈ as new generation orthopedic prosthetic implants. Mg²⁺, Ca²⁺ and Si⁴⁺ ions are abundant in bone and play a crucial role in various bone metabolic activities such as enhancing osteogenesis and inhibiting osteoporosis. The release rate of Mg²⁺, Ca²⁺ and Si⁴⁺ ions from these bioceramics depends on the crystal structure which consequently, influences their bioactivity and biocompatibility. In addition, the release rate of these ions can be tuned by tailoring the processing parameters/routes and compositional modifications and subsequently, bioactivity, cellular response as well as bone regeneration ability can be improved. Toward this end, the present article thoroughly reviewed and analyzed the influence of crystal structure, processing parameters/routes and compositional alteration on in vitro/in vivo biocompatibility and degradation behavior of the above ceramics. Further, a correlation between structure, processing and properties has been established.

     

Piezoelectric properties enhanced of Sr0.6(BiNa)0.2Bi2Nb2O9 ceramic by (LiCe) modification with charge neutrality

Aurivillius-type ceramics, Sr_0.6−x(LiCe)_x/2.5(BiNa)_0.2Bi₂Nb₂O₉(SLCBNBNO) with the charge neutrality, were synthesized by using conventional solid-state processing. Phase analysis was performed by X-ray diffraction analyses (XRD) and Raman spectroscopy. Microstructural morphology was assessed by the scanning electron microscopy (SEM). Structural, dielectric, piezoelectric, ferroelectric, and electromechanical properties of the SLCBNBNO ceramics were investigated. Piezoelectric properties were significantly enhanced compared to Sr_0.6(BiNa)_0.2Bi₂Nb₂O₉ (SBNBN) ceramic and the maximum of piezoelectric coefficient d₃₃ of the SBNBN-LC6 ceramic was 32 pC/N with higher Curie temperature (T_c ∼590 °C). In addition, mechanisms for the piezoelectric properties enhanced of the SBNBN-based ceramics were discussed.     

Improvement of alkali corrosion resistance of mullite ceramics at high temperature by depositing Ca0.3Mg0.2Zr2(PO4)3 coating

Ca_0.3Mg_0.2Zr₂(PO₄)₃ coating was deposited on the mullite ceramic to improve its alkali corrosion resistance at high temperatures, using sol–gel method and dip-coating technique. The phase composition and microstructure of the coating were characterized by X-ray diffraction and scanning electron microscopy (SEM). Results show that homogeneous, dense and single-phase Ca_0.3Mg_0.2Zr₂(PO₄)₃ coating was successfully deposited on mullite ceramics. SEM microstructural examination revealed the excellent bonding between Ca_0.3Mg_0.2Zr₂(PO₄)₃ coating and mullite ceramics. The effectiveness of the prepared coating to improve the alkali corrosion resistance of mullite ceramics was assessed through the measurements of mass loss and flexural strength degradation after 96 h and longer exposure time at alkali corrosion condition at 1000 °C. A significant enhancement of the alkali corrosion resistance for Ca_0.3Mg_0.2Zr₂(PO₄)₃-coated mullite samples was observed. Therefore, the effectiveness of the Ca_0.3Mg_0.2Zr₂(PO₄)₃ material as protection coating for mullite ceramic is confirmed.     

Microwave dielectric properties and microstructures of the 11Li2O-3Nb2O5-12TiO2 ceramics with B2O3 addition

The effects of B₂O₃ addition, as a sintering agent, on the sintering behavior, microstructure and microwave dielectric properties of the 11Li₂O-3Nb₂O₅-12TiO₂ (LNT) ceramics have been investigated. With the low-level doping of B₂O₃ (≤2 wt.%), the sintering temperature of the LNT ceramic could be effectively reduced to 900 °C. The B₂O₃-doped LNT ceramics are also composed of Li₂TiO₃ss and “M-phase” phases. No other phase could be observed in the 0.5-2 wt.% B₂O₃-doped ceramics sintered at 840-920 °C. The addition of B₂O₃ induced no obvious degradation in the microwave dielectric properties but increased the τ_f values. Typically, the 0.5 wt.% B₂O₃-doped ceramics sintered at 900 °C have better microwave dielectric properties of ?_r = 49.2, Q × f = 8839 GHz, τ_f = 57.6 ppm/°C, which suggest that the ceramics could be applied in multilayer microwave devices requiring low sintering temperatures.     

Development of mechanoluminescent micro-particles Ca2MgSi2O7:Eu,Dy and their application in sensors

We have revealed that Ca₂MgSi₂O₇:Eu micro-particles emits green light under the application of a mechanical stress, called as mechanoluminescence (ML). The ML showed a similar spectrum as photoluminescence (PL), which indicated that ML is emitted from the same center of Eu²⁺ ions as PL. Such a green light of ML emission can be seen by the naked eye when pressing the sample. Furthermore, using lab-made AFM-ML system, the ML of single micro-particle under the application of micro force (10 μN) has been investigated.     

In vitro biocompatibility study of calcium phosphate glass ceramic scaffolds with different trace element doping

Porous calcium phosphate based glass ceramics (CaO-P₂O₅-Na₂O) containing different trace elements (2.0 mol% Mg, Sr and Zn respectively) were prepared by coating polyurethane foams with sol-gel derived glass slurry. After heat treatment at suitable temperatures, main phase catena hexaphosphate (Ca₄P₆O₁₉) and minor phase calcium pyrophosphate (β-Ca₂P₂O₇) crystallized from the glass matrix. These scaffolds were soaked in simulated body fluid (SBF) to determine the solubility and apatite formation, and mouse MC3T3-E1 cells were used to investigate the bioactivity and biocompatibility. The Sr doped scaffold showed a higher degradability than those samples containing Zn or Mg, inducing the formation of an apatite layer with a high (Sr + Ca)/P molar ratio of 1.64, whereas only some discontinuous CaP layers and spare apatite agglomerates were found on the scaffolds doped with Mg ((Mg + Ca)/P = 1.12) and Zn ((Zn + Ca)/P = 1.55) respectively. In vitro cell culture, a high degree of cell adhesion and spreading was achieved on the samples containing Sr or Zn, while only a few cells adhered to the Mg doped sample. These results implied that the bioactivity and biocompatibility of the scaffolds were not only strongly associated with the apatite forming ability, but also related with the Ca/P molar ratios of the deposits.