Rare Earth Silicates Research Articles

Fabrication of luminescent layered lanthanide silicate hybrids by hydrothermal and sol–gel technology

In this paper, some novel lanthanide layered silicate hybrid materials were fabricated with hydrothermal synthesis and sol–gel synthesis technology. The silane crosslingking reagents employed were (3-(methacryloyloxy) propyl-trimethoxysilane) (MTMS, H 2C C(CH 3)COO(CH 2) 3Si(OCH 3) 3), tetraethoxysilane (TEOS, Si(OC 2H 5) 4), (3-chloropropyl) trimethoxysilane (CTMS, Cl (CH 2) 3 Si(OCH 3) 3), and (3-aminopropyl)-triethoxysilane (APES, H 2N(CH 2)3Si(OC 2H 5) 3), respectively. X-ray power diffraction, scanning electron microscopy and N 2 isotherms are used to characterize these hybrids. Especially luminescent spectra are used to investigate the photophysical properties of them, indicating all these hybrids can show the characteristic luminescence of Tb or Eu 3+ ions for the energy transfer process to take place.

Photoluminescent layered Y/Er silicates

The synthesis of new layered rare-earth silicates K3[Y1−aEraSi3O8(OH)2] (AV-22 materials) has been reported. The photoluminescence properties of Y/Er-AV-22 and the material resulting from its thermal degradation, K3[Y1−aEraSi3O9] (Y/Er-AV-23), have been studied and compared. Both materials have a similar chemical makeup and structures sharing analogous building blocks, hence providing a unique opportunity for rationalising the evolution of the photoluminescence properties of lanthanide silicates across dimensionality.

Influence of Impurities on the High-Temperature Water-Vapour Corrosion of Environmental Barrier Rare-Earth Silicates

Corrosion experiments on a number of rare earth di-silicates containing Y, Yb and Lu have been conducted in flowing air with 30 vol.% H2O at 1500°C. Nominally the corrosion rate is slower by a factor 5 to 10 compared to silica. However, alumina impurity incorporation has a profound influence on the process of corrosion as it masks not only the gravimetric results but also changes the surface phase assemblages to contain rare earth garnets and liquid phases. It is presumed that silica loss occurs under those conditions via the liquid phase and does not necessarily follow the same kinetics as direct silicate corrosion. Furthermore, atmospheric silicon hydroxide saturation effects contribute to the corrosion process. Currently all determined corrosion rates of rare earth silicates are viewed as system specific only.

Evolution of Photoluminescence across Dimensionality in Lanthanide Silicates

The dehydratation process of layered lanthanide silicates K3[LnSi3O8(OH)2], Ln = Y, Eu, Tb, and Er, and the structural characterization of the obtained small-pore framework K3LnSi3O9, Ln = Y, Eu, Tb, and Er solids, named AV-23, have been reported. The structure of AV-23 has been solved by single-crystal X-ray diffraction (XRD) methods and further characterized by chemical analysis, thermogravimetry, scanning electron microscopy, and 29Si MAS NMR. The photoluminescence (PL), radiance, and lifetime values of AV-23 have been studied and compared with those of AV-22. Both materials have a similar chemical makeup and structures sharing analogous building blocks, hence providing a unique opportunity for rationalizing the evolution of the PL properties of lanthanide silicates across dimensionality. Although Tb-AV-23 contains a single crystallographic Tb(3+) site, PL spectroscopy indicates the presence of Ln(3+) centers in regular framework positions and in defect regions. PL evidence suggests that Eu-AV-23 contains a third type of Ln(3+) environment, namely, Eu(3+) ions replacing K(+) ions in the micropores. The radiance values of the Tb-AV-22 and Tb-AV-23 samples are of the same order of magnitude as those of standard Tb(3+) green phosphors. For the samples K3(Y1-aEraSi3O9), a = 0.005-1, efficient emission and larger 4I13/2 lifetimes (ca. 7 ms) are detected for low Er(3+) content, indicating that the Er(3+)-Er(3+) interactions become significant as the Er(3)+ content increases.

Atomic Layer Deposition of Lu Silicate Films Using [ ( Me3Si ) 2N ] 3Lu

Rare-earth silicates are promising materials for future microelectronic devices based on high dielectric constant dielectrics. In this work, we report the investigation of Lu silicate films deposited using atomic layer deposition on Si(100). The films were grown from tris[bis(trimethylsilyl)amido]lutetium— (where ), which can supply both Lu and Si. Water or ozone were used as oxygen sources. The films deposited using the latter are shown to be richer in Si. The films are amorphous as grown, have a low physical roughness, an electronic density lower than expected for both crystalline and amorphous stoichiometric Lu silicates, and promote a -rich interfacial layer on Si(100). Crystallization, observed only in films deposited using , has an onset temperature above . Annealing at induces film densification. Signs of crystallization are observed only in the electron diffraction patterns of the annealed film deposited using as oxygen source, but the resulting crystallographic phases cannot be unequivocally identified. The κ values of the dielectric stacks are between 5 and 7. The conduction band offset between the Lu silicate layer and Si(100) measured for the film deposited using is .

Britholites as natural analogues of actinide matrices: Resistance to radiation damage

Crystallization of borosilicate glasses, used now for solidification of liquid nuclear high-level wastes (HLW), is accompanied by the formation of silicates with the apatite-type structure and high contents of actinides and REE. The stability of these silicates determines the safety of immobilization of radionuclides. As a result of radioactive decay, the crystalline phases of actinides become amorphous with time and their solubility in aqueous solutions increases. One of the ways to assess the radiation stability of compounds is the study of their natural analogues that contain radioactive elements. Minerals of the britholite group are the natural analogues of synthetic rare earth silicates and actinides with the apatite-type structure. Seven britholites of different ages and with different ThO2 + UO2 contents have been studied. The stages of partial damage and complete destruction of sample structures under the effect of radioactive decay were distinguished. The radiation stability of natural britholites surpasses that of their synthetic analogues. The annealing of metamict samples recovers the primary apatite-type structure without formation of any other phases. With an increase in annealing duration from 1 to 5 h, the mineral structure is recovered at a lower (by 200°C) temperature (down to 550–600°C). The temperature conditions in underground storages and the substantially longer occurrence of HLW therein will prevent crystalline matrices with the apatite-type structure from amorphization and thus ensure the retention of their stability.

Quest and Evaluation of Topcoat Materials for Environmental Barrier Coatings of SiC/SiC Composites

SiC fiber reinforced SiC matrix (SiC/SiC) composites are one of the most promising materials for high temperature structural applications such as power generation and propulsion systems. SiC/SiC composites are, however, susceptible to accelerated attacks in water vapor environments through oxidation and volatilization reaction. For protection from such attacks, Environmental Barrier Coatings (EBCs) are indispensable. We have investigated some oxides and rare-earth silicates as topcoat candidate materials for EBCs. Topcoat materials must be stable in the high-water-vapor pressurized environments at high temperatures. Also, it is important that the thermal expansion coefficient of topcoat materials is similar to that of the SiC/SiC composites. In this study, first, zirconium oxides, lutetium silicates and yttrium silicates were selected as topcoat candidate materials. They were exposed in a water-containing atmosphere at a temperature of 1673 K for 100 h under a total pressure 0.96 MPa. Mass changes, structure of crystals and microstructures were investigated after the exposure experiments in order to evaluate the thermal stability of these materials. After their estimation, lutetium silicates were considered to be promising for topcoat materials. Then, lutetium silicates were coated as the topcoat of an EBC system on SiC/SiC composites, and their fracture toughness and microstructures were investigated after exposure to an oxidizing atmosphere. The evaluation results of the topcoat materials are reported in this paper.

Photoluminescent Layered Y(III) and Tb(III) Silicates Doped with Ce(III)

The synthesis and structural characterization of new layered rare-earth silicates K(3)[M(1-a)Ce(a)Si(3)O(8)(OH)(2)], M = Y(3+), Tb(3+), a << 1 (AV-22 materials), have been reported. These materials combine the properties of layered silicates, such as intercalation chemistry, and photoluminescence and may find applications in new types of sensor devices. For mixed Tb/Ce-AV-22, evidence has been found for the energy transfer from the large Ce(3+) 4f( 1) --> 5d(1) broad band to the sharp Tb(3+) 4f (8) lines. This energy transfer allows the fine-tuning of the color emission in the blue-green region of the chromaticity diagram. Upon Ce(3+) excitation (342 nm), the radiance of Tb/Ce-AV-22 is approximately 2 times higher than that measured under direct Tb(3+) excitation, which reinforces the existence of effective room-temperature Ce(3+)-to-Tb(3+) energy transfer.

A comparison of the effect of rare earth vs Si site doping on the conductivities of apatite-type rare earth silicates

Apatite-type lanthanum silicate (La9.33Si6O26) has been attracting significant recent interest due to its high oxide ion conductivity. In this paper, synthesis and conductivity data for a range of doped samples (Mg, Ca, Sr, Ba, B, Ga and Zn) are reported, in particular, to compare the effect of rare earth vs Si site doping. The results show that Ga, B and Zn favour substitution on the Si site, while Ca, Sr and Ba favour La-site substitution. Mg is shown to be an ambi-site dopant, substituting on either site depending on the starting composition. The samples doped on the Si site show higher conductivities than comparable samples doped on the La site, providing further support for the importance of the silicate network in the conduction process, as initially predicted by atomistic modelling studies. For Ga doping on the Si site, the effect of varying the rare-earth size on the conductivities is also reported.

High-pressure rare earth silicates: Lanthanum silicate with barium phosphate structure, holmium silicate apatite, and lutetium disilicate type X

The phase relations of a wide selection of rare earth disilicates have been investigated up to 10 GPa and 1700 °C using piston cylinder and multi-anvil equipment. Single-crystal X-ray structures have been obtained for the following high-pressure phases: (1) La 2.67(SiO 4) 2: monoclinic, space group C2/ m, Z=2, a=9.419(2), b=5.445(1), c=7.214(1) Å, β=115.71(3)°, R=0.042; disordered Ba 3(PO 4) 2 structure type, with 3× b and 7× b superstructures identified. (2) Ho 8.67(SiO 4) 6(OH) 2: hexagonal, P6 3/ m, Z=1, a=9.3221(4), c=6.7347(2) Å, R=0.026; silicate hydroxyapatite. (3) Lu 2Si 2O 7: tetragonal, P4 12 12, Z=4, a=6.5620(2), c=11.9535(4) Å, R=0.023; type X diorthosilicate structure, and the silicate analogue of tetragonal Er 2Ge 2O 7.

Interfacial reactions and silicate formation in high surface SiO 2 impregnated with La nitrate

Solid state interface reactions in high surface SiO 2 impregnated with lanthanum nitrate were studied at temperature 600–1200 °C with X-ray powder diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), Fourier Transform Infrared spectroscopy (FTIR) and Raman spectroscopy. The results show that at 600–900 °C an amorphous, nanometer thick La O Si layer covering SiO 2 particles exists in the system. At higher temperatures the breakage of the layer into amorphous islands occurs and crystalline silicates with various structures are formed. La 6[Si 4O 13][SiO 4] 2 silicate forms at temperatures from 950 °C as the first, quasi two-dimensional phase, and then, at higher temperatures, A-La 2Si 2O 7 silicate occurs. Mechanism of formation of lanthanide silicates in high surface SiO 2 impregnated with Ln nitrates is proposed.

Development of Silicon Nitride Components for Gas Turbine

Silicon nitride is one of the most practical candidates for ceramic gas turbines. The SN282 is silicon nitride material developed by Kyocera for gas turbines. Several new technologies have been developed to achieve materialization of ceramic gas turbines, such as material, fabrication process, evaluation / analysis technology. Recent technology is focused on recession of silicon-based ceramics under combustion gas. Environmental Barrier Coatings (EBCs) are developed to suppress these recession. We have found rare-earth element silicate and yttrium stabilized zirconium oxide (YSZ) have high corrosion resistance to the combustion gas. These materials were applied to the ceramic gas turbine components. The components with EBCs were evaluated in the actual engine tests. We have confirmed that the EBCs effectively work for the recession resistance.

Electrical properties of (Na 2O) 35.7(RE 2O 3) 7.2(SiO 2) 57.1 (RE = Y, Sm, Gd, Dy, Ho, Er and Yb) glasses and ceramics

Fifteen kinds of sodium rare earth silicate glasses and ceramics with (Na 2O) 35.7(RE 2O 3) 7.2(SiO 2) 57.1 (RE = Y, Sm, Gd, Dy, Ho, Er and Yb) composition were synthesized from a mixture of Na 2CO 3, RE 2O 3 and SiO 2. The densities of the glasses were in fairly good agreement with the theoretical densities and were 0.2–0.41 g cm −3 larger than those of the polycrystalline ceramics. The conductivities of the glasses are 1–2 orders lower than those of the ceramics and the highest electrical conductivity was achieved for the Yb ceramic sample with the smallest ion radius of RE 3+. The electromotive force, EMF, of the potentiometric CO 2 gas sensors using (Na 2O) 35.7(Y 2O 3) 7.2(SiO 2) 57.1 glass and ceramic increased linearly with an increase in the logarithm of CO 2 partial pressure, in accordance with Nernst's law. It was suggested from the slope of Nernst's equation that the two electron-transfer reaction associated with the carbon dioxide molecule takes place at the detection electrode above 450 °C.

Characterization of rare earth aluminosilicate glasses

Rare earth glasses are commonly used in lasers, sensors and radiation shield glasses. Rare earth aluminosilicate-based glasses have been successfully used as in vivo radiation delivery vehicles, in treatment of primary hepatocellular carcinoma, irradiation of diseased synovial membrane for the treatment of rheumatoid arthritis, and treatment of prostate tumors. These glasses have also been found useful to promote liquid-phase sintering of covalent ceramics and, specifically, silicon carbide-based ceramics. Usually, the SiO 2–Al 2O 3–RE 2O 3 system is used as sintering additives. In this work, several rare earth silicate glasses (RE = Y, La, Nd, Dy and Yb) were prepared based on 60%SiO 2–20%Al 2O 3–20%RE 2O 3 (mol%) compositions. These glasses were characterized by infrared spectroscopy and nuclear magnetic resonance. Moreover, the liquid phase sintering of silicon carbide ceramics was studied using dilatometric experiments with 10 vol.% additives of the 60%SiO 2–20%Al 2O 3–20%RE 2O 3 system.

Final state effects in VUV and soft X-ray absorption spectra of transition metal oxides and silicate alloys: comparisons between experiment and ab initio calculations

This paper uses X-ray absorption spectroscopy and vacuum ultra-violet spectroscopic ellipsometry to study the electronic structure of high- k transition metal (TM) oxide gate dielectrics. The results are applicable to TM and rare earth (RE) silicate and aluminate alloys, as well as complex oxides comprised of mixed TM/TM and TM/RE oxides. These studies identify the nature of the lowest conduction band d * states, which define the optical band gap, including their relationship to the band gap, E g, of the oxide.

Recent developments in the MOCVD and ALD of rare earth oxides and silicates

Lanthanide, or rare-earth oxides are currently being investigated as alternatives to SiO 2 as the dielectric insulating layer in sub-0.1 μm CMOS technology. Metalorganic chemical vapour deposition (MOCVD) and atomic layer deposition (ALD) are promising techniques for the deposition of these high- κ dielectric oxides and in this paper some of our recent research into the MOCVD and ALD of PrO x , La 2O 3, Gd 2O 3, Nd 2O 3 and their related silicates are reviewed.

Conduction band states of transition metal (TM) high- k gate dielectrics as determined from X-ray absorption spectra

This paper uses X-ray absorption spectroscopy to study the electronic structure of the high- k gate dielectrics including TM and RE oxides. The results are applicable to TM and rare earth (RE) silicate and aluminate alloys, as well as complex oxides comprised of mixed TM/TM and TM/RE oxides. These studies identify the nature of the lowest conduction band d ∗ states, which define the optical band gap, E g, and the conduction band offset energy with respect to crystalline Si, E B. E g and E B scale with the atomic properties of the TM and RE atoms providing important insights for identification high- k dielectrics that meet performance targets for advanced CMOS devices.

Rare earth silicate environmental barrier coatings for SiC/SiC composites and Si 3N 4 ceramics

Rare earth silicates have been investigated to evaluate their potential as an advanced environmental barrier coating (EBC) having higher temperature capability than current EBCs. Volatility data in high steam environments indicate that rare earth monosilicates (RE 2SiO 5; RE: rare earth element) have lower volatility than the current barium strontium aluminum silicate (BSAS) EBC top coat in combustion environments. Rare earth silicates also exhibit superior chemical compatibility compared to BSAS. The superior chemical compatibility and low volatility are key attributes to achieve higher temperature capability. The chemical compatibility is especially critical for EBCs on Si 3N 4 because of the high chemical reactivity of some of the oxide additives in Si 3N 4. In simulated combustion environments, EBCs with a rare earth monosilicate top coat exhibit superior temperature capability and durability compared to the current state-of-the art EBCs with a BSAS top coat.

Single crystal growth and oxide ion conductivity of oxyapatite type Sr-bearing neodymium silicate

Single crystals of Sr-bearing neodymium silicates with the oxyapatite structure were grown by the floating zone method. The grown crystals were free from any macroscopic defects and their quality was sufficient for electrical measurements. Electrical conductivity was enhanced by about three orders of magnitude by the introduction of cation vacancies to Sr 2Nd 8(SiO 4) 6O 2, a compound without cation or oxygen ion vacancies. The introduction of oxygen ion vacancies to Sr 2Nd 8(SiO 4) 6O 2 and Nd 9.33(SiO 4) 6O 2 did not significantly affect their electrical conductivity. Cation vacancies may play an important role for oxygen ion conduction in oxyapatite type rare-earth silicates.

Photoluminescent Layered Lanthanide Silicates.

AbstractFor Abstract see ChemInform Abstract in Full Text.