Lutetium Silicate Research Articles

Mechanical and thermal properties for corrosion products of lutetium silicates against CMAS

AbstractRare earth silicates are promising environmental/thermal barrier coating (E/TBC) materials facing severe CMAS (CaO‐MgO‐Al2O3‐SiO2) corrosion. Previous studies mainly focused on the intrinsic properties of precorrosion coatings, but there were few studies on their CMAS corrosion products that play a crucial role in the performance of coatings in postservice stage. In this work, the mechanical and thermal properties of nine corrosion products between lutetium silicates and CMAS are studied using first‐principles calculations. Their differences of elastic stiffness are attributed to the different crystal structures and bonding strength. The T:O ratio is identified as a factor of the crystal structure for silicate products, and it has a good correlation with their elastic stiffness. Moreover, the divergences of thermal conductivity are dominated by three essential factors, that is, atomic vibration intensity, lattice vibrational anharmonicity, and complexity of crystal structure. Compared with rare earth silicates, six products, that is, the α‐CaSiO3, β‐CaSiO3, Ca2MgSi2O7, Ca2Al2SiO7, CaAl2Si2O8, and Ca2Lu8(SiO4)6O2, showing good damage tolerance and low thermal conductivities, are predicted to be advantageous to E/TBCs. These discoveries reveal the mechanical/thermal properties of corrosion products between lutetium silicates and CMAS and are expected to support the future researches on the performance of E/TBC in the postservice stage.

Elaboration and luminescence of cerium-doped lutetium silicate glass-ceramics via in-situ growth from containerless processed lutetium silicate glass

Amorphous cerium-doped lutetium silicate scintillator was fabricated through aerodynamic levitation method. The lutetium silicate glass scintillator transformed into glass-ceramics after annealing at 950 °C in air. The crystalline phase of the glass-ceramics was composed with lutetium oxyorthosilicate (Lu2SiO5:Ce, LSO:Ce) and lutetium pyrosilicate (Lu2Si2O7:Ce, LPS:Ce). The crystalline parts of the glass-ceramics, i.e., the micro-sized polycrystalline LPS/LSO particles, consist of nano-sized LPS and LSO grains. The in-line transmittance of the LPS/LSO:Ce glass and glass-ceramics are 86% at 600 nm. The luminescence characteristics of the highly transparent LPS/LSO:Ce glass and glass-ceramics were investigated through Photoluminescence (PL), Photoluminescence Excitation (PLE) and Cathodoluminescence (CL). It suggests that combination of containerless processing and in-situ growth of lutetium silicate from glass matrix is a promising route to fabricate highly transparent lutetium silicate scintillators.

Crystal structure of chain silicate Cs3LuSi3O9.

A caesium lutetium(III) silicate, Cs3LuSi3O9, was synthesized by heating a pelletized mixture of Cs2CO3, Lu2O3 and SiO2 at 1273 K. Single crystals of the title compound were grown in a melted area of the pellet. Cs3LuSi3O9 is a single-chain silicate (ortho-rhom-bic space group Pna21) with a chain periodicity of six and is isostructural with Cs3 RE IIIGe3O9 (RE = Pr, Nd and Sm-Yb). The two symmetry-dependent [Si6O18]12- chains in the unit cell lie parallel to the [011] direction. The Lu3+ ions are octa-hedrally coordinated by O atoms of the silicate chains, generating a three-dimensional framework. Cs+ ions are located in the voids in the framework.

Theoretical investigation of formation and diffusion mechanisms for point defects in ytterbium and lutetium silicates

AbstractUnderstanding the mechanism of point defects and the oxygen vacancy diffusion in rare earth (RE) silicates is important to evaluate their performance as environmental barrier coatings. In this work, the defect formation energies are calculated and the similar predominant native point defect type, that is, O Frenkel defect, in RE2SiO5 and RE2Si2O7 (RE = Yb and Lu) is uncovered. Furthermore, the nonstoichiometry related defects and phase composition in RE2SiO5 and RE2Si2O7 is also studied. For RE2SiO5, or is the predominate defect complex accompanied with the RE2Si2O7 separation under SiO2‐rich condition, while or is favorable under RE2O3‐rich condition. In the case of RE2Si2O7, or is the favorable defect complex under SiO2‐rich condition while or dominates the defects, accompanied with the formation of RE2SiO5, under RE2O3‐rich case. Finally, the high oxygen migration barriers are revealed for both RE2SiO5 and RE2Si2O7, indicating the low oxygen permeability in these silicates. These results will not only guide the future performance optimization of RE2SiO5 and RE2Si2O7 through tailoring their composition, but also suggest their potential application as oxidation protective coatings.

Mechanism of luminescence and efficient energy storage in Lu2SiO5 : Ce3+single crystals

The development of high energy physics and medicine has raised the necessity of heavy stintillating materials with a large total gamma quantum absorption cross-section, high quantum output and fast response. Cerium doped lutetium silicate Lu2SiO5 : Ce3+ (LSO) has high density, large effective atomic number and high conversion efficiency. In this work we have reported optical absorption spectroscopy and photoluminescence data for LSO single crystals grown using the modified Musatov method. The absorption spectra show the fundamental intrinsic absorption edge of Lu2SiO5 at ~200 nm and four extrinsic absorption bands of Ce3+ activator near 250—375 nm. The band gap is 6.19 to 6.29 eV depending on optical beam direction. We have confirmed that the extrinsic absorption bands correspond to optical transitions in Ce3+ activator ions localized in two crystallographically non-equivalent CeI and CeII positions. We have estimated that oscillator force for the optical transitions in Ce3+ ions. The photoluminescence spectra excited by 3.49 eV photon energy UV laser contain three bands: ~2.96 eV, ~3.13 eV (CeI) and ~2.70 eV (CeII). The energy structure of electron traps in LSO has been studied with thermally stimulated luminescence, the crystals being exposed to UV with different spectral and energy parameters. All the experimental thermally stimulated luminescence curves contain at least two peaks at 345 and 400 K with a 4 : 1 intensity ratio attributable to electron traps at 0.92—0.96 and1.12—1.18 eV. LSO exposure to high pressure mercury lamp radiation having the highest energy has for the first time showed the presence of traps at 0.88 eV. A model of the energy structure of LSO has been developed. The luminescence mechanism in the material is more complex than purely intracenter one. We show that high excitation energies may lead to ionization by the mechanism hva + Ce3+ = Ce4+ + e-. We have assumed that the storage of excitation energy involves not only Ce3+ activator but also the conduction band as well as trap states localized near the conduction band.

Back propagation neural network analysis for the detection of explosives based on tagged neutron

A system for detecting explosives in walls based on the tagged neutron method was established. An associated particle neutron generator ING-27 developed by the All-Russian Research Institute of Automatics (VNIIA) is used as the neutron source, two yttrium lutetium silicate (LYSO) detectors are used for the γ detector, and a silicon detector is used as the α detector. The α-γ coincidence spectra of 300 g ammonium nitrate and TNT samples placed behind 10 cm concrete wall were measured. A neural network algorithm was applied to analyze the data. Through fine selection of time windows of α-γ coincidence spectra, the counts of full-energy peak of N, C and O elements and their proportions are selected as the input eigenvectors. The neural network is trained by the experimental data obtained by this system. Through training, eight groups of test data consisting of 32 γ-ray spectra were identified, and a 98.7% of correct detection rate was achieved.

Energy fluence measurement for pulse hard X-ray

<sec>Pulsed high-energy fluence X-ray source is based on the “Flash II” accelerator. It can be used to carry out effect vulnerability and sensitivity test of unit level system generated electromagnetic pulse (SGEMP). The energy fluence of pulsed hard X-ray is a main parameter of the equipment beam. At present, theoretical calculation method is widely used. Energy fluence can be calculated according to the dose, energy spectrum and energy absorption coefficient of each energy segment.</sec><sec>The principle measuring energy fluence of pulsed hard X-ray by total absorption method is introduced. The photoelectric cell with lutetium silicate (LSO) scintillator is selected as a core component of the detection system, and the measurement system is developed. It is composed of scintillation detector, LSO scintillator, dimmer film, photon collimator, visible light shielding material, power supply and signal collecting system. The conversion coefficient between the incident photon energy and the waveform parameter is calibrated by a standard X-ray source. </sec><sec>The energy fluence measurement experiment is carried out with the high-energy beam source of the “Flash II” accelerator as an experimental platform. In order to meet the requirements of the effect test experiments, the series diode structure is used in the accelerator for forming a high strength and large area uniform X-ray source. In the experiment, the LiF TLD is located in the front of the phototube and used to monitor the dose. According to the measured waveform, the actual energy of the incident photons is calculated. Combined with the receiving area of incident photons, the energy fluence of pulsed hard X-ray is calculated. The average measured value is 35.9 mJ/cm<sup>2</sup> in 5 consecutive experiments. Energy fluence calculated from the measured dose and energy spectrum is 39.8 mJ/cm<sup>2</sup>. The results of the two methods are compared.</sec><sec>It can be found that the experimental result is about 9.8% smaller than the theoretical value. The reasons are as follows. According to the law of exponential decay of rays in matter, in fact, the scintillator cannot absorb all the rays, and some of the rays can penetrate through, the energy of these rays cannot be detected, and thus giving rise to small experimental value. Due to the limited energy point of quasi-monoenergetic source, the sensitivity under the mean photon energy is taken as the sensitivity of the detector, and therefore there is a certain degree of uncertainty. </sec><sec>The successful application of the measurement technology provides a good experimental method for the following similar research, and can also provide a reference for X-ray intensity analysis.</sec>

Synthesis of Scintillating Ce3+-Doped Lu2Si2O7 Nanoparticles Using the Salt-Supported High Temperature (SSHT) Method: Solid State Chemistry at the Nanoscale

The synthesis of lutetium silicate nanoparticles doped with Ce3+ using the new salt-supported high temperature (SSHT) method is reported. This mechanochemical–thermal method enables the synthesis of nanoparticles without aggregation even after long calcination times of 48 h at high temperatures. These nanoparticles are characterized by powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and Fourier transform infrared spectroscopy (FT-IR). Their optical properties, including UV luminescence and X-ray scintillation, are reported. These nanoparticles show strong luminescent and scintillation behavior. This SSHT method is general and can be used for the synthesis of other ternary and quaternary nanoparticle systems.

High Charge Carrier Storage Capacity in Lithium Lutetium Silicate Doped with Cerium and Thulium

It is demonstrate that LiLuSiO4:Ce,Tm has excellent charge carrier storage properties. Comparative studies are performed with the state‐of‐the art X‐ray storage phosphor BaFBr(I):Eu. The thermoluminescence (TL) intensity of the optimized LiLuSiO4:Ce,Tm is four times higher than the TL of BaFBr(I):Eu. After 3 h of storage at room temperature (RT) there is still 94% of initial thermoluminescence and only 48% for BaFBr(I):Eu which demonstrates insignificant thermal fading. The signal can be read out by a blue LED and less efficiently by a red laser. It does require higher stimulation energy than in the case of BaFBr(I):Eu. The self‐irradiation with a dose rate of 13 µGy h−1 due to 176Lu isotopes is not a serious drawback for application.

Evaluation of a SiPM array detector coupled to a LFS-3 pixellated scintillator for PET/MR applications.

SiPM arrays are insensitive to magnetic fields and thus good candidates for hybrid PET/MR imaging systems. Moreover, due to their small size and flexibility can be used in dedicated small field of view small animal imaging detectors and especially in head PET/MR studies in mice. Co-doped LFS-3 scintillator crystals have higher light yield and slightly faster response than that of LSO:Ce mainly due to the co-doped activation of emission centers with varying materials such as Ce, Gd, Sc, Y, La, Tb, or Ca distributed at the molecular scale through the lutetium silicate crystal host. The purpose of this study is to investigate the behavior of the SensL ArraySL-4 (4x4 element array of 3x3 mm2 silicon photomultipliers) optical detector coupled to a 6x6 LFS-3 scintillator array, with 2x2x5 mm3 crystal size elements, for possible applications in small field of view PET/MR imaging detectors. We have designed a symmetric resistive charge division circuit to read out the signal outputs of 4x4 pixel SiPM array reducing the 16 pixel outputs of the photodetector to 4 position signals. The 4 position signals were digitized using free running Analog to Digital Converters. The ADCs sampling rate was 50 MHz. An FPGA (Spartan 6 LX150T) was used for triggering and digital signal processing of the pulses. Experimental evaluation was carried out with 22Na radioactive source and the parameters studied where energy resolution and peak to valley ratio. The first preliminary results of the evaluation shows a clear visualization of the discrete 2x2x5 mm3 LFS-3 scintillator elements. The mean peak to valley ratio of the horizontal profiles on the raw image was measured equal to 11 while the energy resolution was calculated equal to 30% at the central pixels.

Three- and four-photon excited upconversion luminescence in terbium doped lutetium silicate powders by femtosecond laser irradiation

Frequency upconversion (UC) luminescence was investigated in terbium (Tb3+) doped lutetium silicate powders when the samples were irradiated with femtosecond lasers operating either at 800 nm or 1500 nm. The samples with three different Tb3+ concentrations were prepared by the combustion synthesis method. Rietveld analysis of the X-ray powder diffraction data showed the predominance of monoclinic Lu2SiO5 phase. UC luminescence signals induced by three- and four-photon absorption were identified. The mechanisms that originate the anti-Stokes luminescence were discussed.

Phase transition and elastic and optical properties of Lu2SiO5

Cerium-doped lutetium silicate (LSO:Ce) crystal is a well-known scintillator. However, there are two different structures reported for LSO crystal at ambient conditions in experiments. The structural, elastic, electronic and optical properties of the C2/c and P21/c phases of LSO crystal have been investigated using first-principles method. The C2/c phase is found to be stable at ambient conditions. The phase transition from C2/c phase to P21/c phase is predicted to occur at 6.3GPa, together with a volume contraction of 3.7%. The bulk modulus of the C2/c phase is larger than the P21/c phase. The hybridization of O-2p and Si-3p is found in the upper valence band of LSO crystal. The C2/c and P21/c phases are predicted to be insulators with calculated gaps of 4.67 and 4.80eV, respectively. The refractive index of LSO crystal is found to be about 1.9 for both phases.

Microstructure and luminescent properties of Ce:Lu2SiO5 ceramic scintillator by spark plasma sintering

Spark plasma sintering (SPS) processing was employed to fabricate cerium (Ce3+) ions doped lutetium silicate (Lu2SiO5) translucent scintillation ceramics. Polycrystalline Ce:Lu2SiO5 ceramic bulks were densified from as-prepared powder by spark plasma sintering at 1350°C for 5min under pressure of 50MPa. Translucent monolithic Ce:Lu2SiO5 ceramic specimen was obtained with excellent luminescent characteristics after being annealed in air at 1000°C for 15h. Under 360nm UV excitation, a broad emission peak centered at about 425nm was detected for Ce:Lu2SiO5 ceramic. The luminescence intensity of annealed ceramic sample (doped by 0.5mol% Ce3+) was 3 times greater than that of BGO single crystal under X-ray excitation. The decay constant under γ-ray excitation of Cs-137 radioactive source was measured to 44.36ns. The good luminescent characteristics made Ce:Lu2SiO5 polycrystalline ceramics a promising scintillator candidate with high performance for radiation detection in future.

Doped Lutetium Silicates Scintillators Prepared by Sol-Gel Method. The Effect of Stoichiometry on Phase Relations and Luminescent Properties

Sol-gel method was used for preparation of the Lu2Si2O7 compounds doped with Ce3+. XRD and Electron microscopy (SEM, HR TEM) were used for characterization of the structure and morphology of the samples, respectively. Different samples were prepared in order to study the effects of overstoichiometric ratio Si/Lu >1. Higher concentration of SiO2 leads to phase separation of silica and Lu2Si2O7. Luminescence spectra were measured at room temperature to characterize emission centres created by the doped trivalent rare earth ion.

Prepration and photoluminescence properties of Ce doped lutetium silicate nanopowders by sol–gel method

Cerium doped lutetium silicate powders were prepared by sol–gel process with different cerium concentrations (0, 0.01 and 0.03 mol%). The synthesized lutetium silicate powders were investigated by X-ray diffraction (XRD), TG-DTA, SEM, and Photoluminescence excitation and emissions. The crystallization of lutetium silicate is observed at 1.073 °C in S2 (and S3: 1.058 °C) sample. The formation of lutetium silicate phases was confirmed by XRD analysis for the powders heated at 1.200 °C, of which mixed phases containing Lu 2SiO 5 and Lu 2Si 2O 7 phase were observed. Lutetium silicate powders were homogeneous and constituted of small spherical particles of about 400 nm. Lutetium silicate powder itself was not scintillate, but scintillation yield was higher in Lu 2SiO 5:Ce 3+ powder. Very broad spectra were observed in the range of 350–530 nm, and emission intensity increases at a Ce doping of 0.01%, but decreases at 0.03%.

Corrosion Behavior of Lu-Si-O System in Water Vapor

采用溶胶-凝胶法制得三种镥硅酸盐体系粉体材料.以氧化物的摩尔比来表示此三种粉体,分别为：Lu2O3.SiO2、Lu2O3.2SiO2和Lu2O3.2.26SiO2.在1400℃、50%H2O-50%O2静态常压气氛下,研究了它们的耐水蒸气腐蚀性能.以单位面积重量变化率表征材料的耐水蒸气腐蚀性能,结合X射线衍射（XRD）、傅里叶红外光谱（FTIR）和扫描电镜能谱分析（SEM-EDS）等分析手段,揭示了镥硅酸盐体系在高温水蒸气环境中的腐蚀机制和反应机理.结果表明：三种原始粉体主要物相依次为：Lu2SiO5＋Lu2Si2O7、Lu2Si2O7＋SiO2和Lu2Si2O7＋SiO2.在水蒸气作用下,Lu2SiO5相与Al2O3反应生成新相Lu3Al5O12,而Lu2Si2O7相并未受到水蒸气的作用而发生任何反应,表现出优异的化学稳定性.

A Multi-Slice Dual Layer MR-Compatible Animal PET System

A multi-slice depth-of-interaction (DOI) MR-compatible PET system for imaging small animals such as mice or rats was developed and tested It employs DOI block detectors made of mixed lutetium silicates (MLSs), optical fibers, and multi-channel position sensitive photo-multiplier tubes (PSPMTs). The size of a single MLS was 2.5 mm times 3.5 mm times 3.5 mm and eight MLSs were combined to form a DOI block detector using the Anger principle and connected to four optical fibers with light guides. Thirty-two sets of block detectors were arranged in a 51 mm diameter circular ring. The scintillation light signals from the DOI detectors are read from the side of the detector ring. The configuration makes the diameter of the detector ring small and easy to use in the small imaging area of the MRI. Position histograms showed good separation for all block detectors. The timing resolution was 6 ns FWHM. The measured transaxial spatial resolution was 2.8 mm FWHM at the center of the image and the axial resolution was 3 mm FWHM. Sensitivity was around 0.23% at the center of the axial field of view (FOV). Simultaneous imaging of PET and MRI was also tested using a 0.15 T permanent magnet system with a 30-mm FOV. The detector part of the PET system was set inside the gradient coil of the MRI system and the RF coil was positioned inside the FOV of the PET system. No difference was found in the PET images among outside MRI, inside MRI without gradient and RF, and inside MRI with the gradient and the RF sequences. Also there is no noticeable change of the MRI images with and without the detector ring of the PET system. We also successfully obtained simultaneously measured rat brain with the PET and MRI. These results indicate that the developed MR-compatible PET system can be used for simultaneous measurements with an MRI system.

Development of New Technologies for the Manufacturing of Nanocrystalline Scintillation Materials

Single crystal scintillators are widely applied in many areas of science and technology. Production of single crystal scintillators is a rather complex, long-standing and power-intensive process. As an alternative to single crystals scintillation ceramic materials made of nanocrystalline powders were being considered. Sol-gel and co-precipitation methods have been used for establishing possibilities of fabricating rare earth metal ion-doped scintillators. By these methods, if modified, it is possible to regulate sizes of powder particles. Finely dispersed powders of single-phase lutetium silicates, titanates, aluminates, tungstates, molybdates and other materials were synthesized. Main goal of the present work was to study the influence of different factors on purity of LSO and LPS phases. Pyrolysis procedure of amorphous gels promoted formation of phase mixtures (LSO, LPS, Lu2O3 and SiO2) due to disturbance of the Lu:Si ratio under the influence of fluorine- and chlorine-containing compounds. Optimum modes for preparing nanophase powders of Ce3+-doped lutetium silicates (LSO, LPS) were defined. The synthesized samples were studied by using X-ray diffraction, electron microscopic and thermal analysis methods. LPS luminescence characteristics were established.

High Efficiency of Lutetium Silicate Scintillators, Ce-Doped LPS and LYSO Crystals for Medical Applications

The paper presents two cerium doped lutetium silicate crystals: pyrosilicate Ce:Lu2Si2O7 (LPS) and Ce: Lu2(1-x)Y2xSiO5 (LYSO). These two crystals exhibit the expected requirements for gamma detection: high density and high atomic number, high scintillation light yield, good energy resolution and fast response. LPS and LYSO crystals doped with cerium were grown by the Czochralski process. The crystal growth parameters were studied and optimized. Development of scintillators requires good understanding of the scintillation process. The location within the forbidden band gap of the localized lanthanide energy levels is analyzed by time resolved spectroscopy and thermoluminescence studies.

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.