首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 62 毫秒
1.
Dental enamel has been widely studied by particle‐induced x‐ray emission (PIXE), but less attention was paid to its demineralization, which leads to caries formation. Using broad‐beam PIXE and µ‐PIXE, we investigated normal enamel and the in vitro formation of pre‐carious lesion in lactic acid solution, aiming also to evaluate intercusp differences within the same tooth. Broad‐beam PIXE was performed using 3.0 MeV protons, and µ‐PIXE maps of Ca, Fe and Zn were collected with 3.1 MeV protons at ~4 µm resolution. In normal enamel a differentiated Ca‐rich surface layer was observed, where Fe and Zn reached their highest levels. In deeper layers, Fe and Zn evidenced quasiperiodic patterns of maxima, possibly due to coupled diffusion‐reaction catalytic processes involved in the enamel growth. Both Fe and Zn appeared to be located in a few distinct types of pools. Near the surface, demineralization induced an increase of Fe, Cu, Zn, Sr and Pb with respect to Ca, attributed to partial hydroxyapatite dissolution and/or to chelate extraction and concentration of trace metals. Ca maps revealed limited changes in the surface layer and a massive loss in the inner enamel; here, Fe was almost depleted and Zn partially removed. The maps of Ca, Fe and Zn demonstrated major intercusp variations in both normal and altered enamel. Thus, broad‐beam PIXE and µ‐PIXE, which do not require (semi)thin sectioning of the tooth as the conventional methods, provide compositional and structural insight of normal dental enamel, of its intercusp variability and of the alterations produced by in vitro demineralization, largely not accessible to the current techniques, and highly relevant for understanding the incipient caries formation. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

2.
A series of Ca3–3x /7Y2x /7(PO4)2:Eu2+ phosphors were synthesized by conventional solid‐state reaction. The photoluminescence spectra elucidate that the evolution of emission hue from violet–blue to blue–greenish could be realized by incorporating Y3+ ions. The aliovalent substitution of Y3+ for Ca2+ forms cationic vacancies (h denotes a vacancy) at Ca(4) site according to the formula 3Ca2+ = 2Y3+ + h, and then facilitates the migration of dopant Eu2+ from Ca(4) site to other sites in host lattice. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)  相似文献   

3.
This paper reports the emission characteristics of leaf and wood biochar (LC500 and WC500) pyrolysis in a drop tube furnace at 1300 °C in argon atmosphere. The char yields at 1300 °C are ~ 65% and ~ 73% respectively for LC500 and WC500. Over 60% Mg, Ca, S, Al, Fe and Si are retained in char after pyrolysis at 1300 °C. The retentions of Na and K in the char from LC500 pyrolysis are lower than those in the char from WC500 pyrolysis due to release via enhanced chlorination as a result of much higher Cl content in LC500. Particulate matter (PM) with aerodynamic diameter of < 10 µm (i.e. PM10) from LC500 and WC500 pyrolysis exhibits a bimodal distribution with a fine mode diameter of 0.011 µm and a coarse mode diameter of 4.087 µm. The PM10 yield for LC500 pyrolysis is ~ 8.2 mg/g, higher than that of WC500 pyrolysis (~2.1 mg/g). Samples in PM1-10 (i.e. PM with aerodynamic diameter 1 µm – 10 µm) are char fragments that have irregular shapes and similar molar ratio of (Na+K + 2Mg+2Ca)/(Cl+2S+3P) as the char collected in the cyclone. In PM1 (i.e. PM with aerodynamic diameter < 1 µm), the main components in sample are inorganic species, and carbon only contributes to ~5% and ~8% the PM1 produced from rapid pyrolysis of LC500 and WC500, respectively. Na, K and Cl are main inorganic species in PM1, contributing ~ 98.8% and ~ 97.5% to all inorganic species. Na, K and Cl from rapid pyrolysis of biochar have a unimodal distribution with a mode diameter of 0.011 µm. In PM1–10, Ca is the main inorganic specie, contributing to ~71.2% and ~65.3% to all inorganic species in PM1–10 from pyrolysis of LC500 and WC500, respectively.  相似文献   

4.
Silicon (Si) nanoparticles with average size of 13 nm and orange–red luminescence under UV absorption were synthesized using electrochemical etching of silicon wafers. A film of Si nanoparticles with thickness of 0.75 µm to 2.6 µm was coated on the glass (TiO2 side) of a dye‐sensitized solar cell (DSSC). The cell exhibited nearly 9% enhancement in power conversion efficiency (η) at film thickness of ~2.4 µm under solar irradiation of 100 mW/cm2 (AM 1.5) with improved fill factor and short‐circuit current density. This study revealed for the first time that the Si‐nanoparticle film converting UV into visible light and helping in homogeneous irradiation, can be utilized for improving the efficiency of the DSSCs. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

5.
Simultaneous particle‐induced x‐ray emission (PIXE) and particle‐induced gamma‐ray emission (PIGE) measurements in the 0–270 keV range with a low‐energy hyperpure Ge detector have been performed on the outer surface and on an inner section of a dental composite filling extracted from a patient. The biomaterial has been identified as the fluorine‐releasing Tetric Ceram dental composite. Compared to the native material, the dental filling evidenced changes occurring during the in vivo oral use. These included mainly Cl and K accumulation and loss of Zr, Ba, Yb (by PIXE), and F (in the PIGE region). The changes were similar at the surface and inside of the dental filling for most of the above elements, indicating an unexpectedly high permeability of the material, maybe due to microscopic bulk fracture of the dental composite filling throughout its use. The PIGE spectra detected also Al and traces of Hf and Sm possibly associated with Zr and Yb. Changes in the ratio of the L lines of Yb and in the ratio of the 110‐ and 197‐keV lines of F evidenced alterations in the matrix physical properties of the composite, associated with factors such as surface roughening by wear, grain size decrease by slow dissolution, and compositional variations. Further studies are needed to gain a deeper insight into the complex changes taking place in a dental composite filling. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

6.
Off‐state and vertical breakdown characteristics of AlGaN/AlN/GaN high‐electron‐mobility transistors (HEMTs) on high‐resistivity (HR)‐Si substrate were investigated and analysed. Three‐terminal off‐state breakdown (BVgd) was measured as a function of gate–drain spacing (Lgd). The saturation of BVgd with Lgd is because of increased gate leakage current. HEMTs with Lgd = 6 µm exhibited a specific on‐resistance RDS[ON] of 0.45 mΩ cm2. The figure of merit (FOM = BVgd2/RDS[ON]) is as high as 2.0 × 108 V2 Ω–1 cm–2, the highest among the reported values for GaN HEMTs on Si substrate. A vertical breakdown of ~1000 V was observed on 1.2 µm thick buffer GaN/AlN grown on Si substrate. The occurrence of high breakdown voltage is due to the high quality of GaN/AlN buffer layers on Si substrate with reduced threading dislocations which has been confirmed by transmission electron microscopy (TEM). This indicates that the AlGaN/AlN/GaN HEMT with 1.2 µm thick GaN/AlN buffer on Si substrate is promising candidate for high‐power and high‐speed switching device applications. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

7.
A focusing system based on a polycapillary half‐lens optic has been successfully tested for transmission and fluorescence µ‐X‐ray absorption spectroscopy at a third‐generation bending‐magnet beamline equipped with a non‐fixed‐exit Si(111) monochromator. The vertical positional variations of the X‐ray beam owing to the use of a non‐fixed‐exit monochromator were shown to pose only a limited problem by using the polycapillary optic. The expected height variation for an EXAFS scan around the Fe K‐edge is approximately 200 µm on the lens input side and this was reduced to ~1 µm for the focused beam. Beam sizes (FWHM) of 12–16 µm, transmission efficiencies of 25–45% and intensity gain factors, compared with the non‐focused beam, of about 2000 were obtained in the 7–14 keV energy range for an incoming beam of 0.5 × 2 mm (vertical × horizontal). As a practical application, an As K‐edge µ‐XANES study of cucumber root and hypocotyl was performed to determine the As oxidation state in the different plant parts and to identify a possible metabolic conversion by the plant.  相似文献   

8.
Results of measurements made at the SIRIUS beamline of the SOLEIL synchrotron for a new X‐ray beam position monitor based on a super‐thin single crystal of diamond grown by chemical vapor deposition (CVD) are presented. This detector is a quadrant electrode design processed on a 3 µm‐thick membrane obtained by argon–oxygen plasma etching the central area of a CVD‐grown diamond plate of 60 µm thickness. The membrane transmits more than 50% of the incident 1.3 keV energy X‐ray beam. The diamond plate was of moderate purity (~1 p.p.m. nitrogen), but the X‐ray beam induced current (XBIC) measurements nevertheless showed a photo‐charge collection efficiency approaching 100% for an electric field of 2 V µm?1, corresponding to an applied bias voltage of only 6 V. XBIC mapping of the membrane showed an inhomogeneity of more than 10% across the membrane, corresponding to the measured variation in the thickness of the diamond plate before the plasma etching process. The measured XBIC signal‐to‐dark‐current ratio of the device was greater than 105, and the X‐ray beam position resolution of the device was better than a micrometer for a 1 kHz sampling rate.  相似文献   

9.
The local structure of Tm2+ and Yb3+ cubic impurity centers in MeF2: Tm2+ and MeF2: Yb3+ (Me = Ca, Sr, Ba) fluoride crystals, as well as Yb3+ trigonal and tetragonal impurity centers in MeF2: Yb 3+ crystals, is calculated within the shell model in the pair potential approximation.  相似文献   

10.
As previously reported3,4 the optical pumping method was succesfully used to determine the nuclear magnetic moments of135Ba,137Ba,171Yb and173Yb. This method has now been extended to43Ca and87Sr. The nuclear moments of the Ba- and Yb-isotopes have been remeasured by means of an improved experimental technique. Especially the influence of the light shift has been investigated and the nuclear moments have been corrected by eliminating this shift. The magnetic field has been calibrated by means of optical pumping of23Na. The nuclear magnetic moments obtained from the rf-transitions between the Zeeman-levels of the ground state1 S o are: μ(43Ca)=?1.3172 (6)μ N ; μ87Sr)=?1.0924 (7) μ N ; μ(135Ba)=0.83656(2)μ N ; μ(137Ba)=0.93582(2)μ N ; μ(171Yb)=0.491889 (8)μ N ; μ(173Yb)=?0.67744 (3) μ N (diamagnetic correction was applied). The ratios of the nuclear moments of the elements with two odd isotopes are: μ(137Ba)/(135Ba)=1.11865 (3); μ(173Yb)/μ(171Yb)=?1.37723 (7). In comparison with the results of nuclear induction the chemical shifts have been calculated to be: δ(Ca)≦± 5 · 10?4 δ(Sr)=?6(7) ·10?4 δ(Ba)=?7.8(4) ·10?4 δ(Yb)=?23(10) · 10?4.  相似文献   

11.
For microcrystalline silicon based p–i–n solar cells the effect of deposition conditions on the critical oxygen concentration was investigated. All solar cells were prepared by 13.56 MHz plasma‐enhanced chemical vapour deposition. The critical oxygen concentration defines the lowest oxygen concentration in the intrinsic absorber layer causing a deterioration of the solar cell performance. For intentional contamination of ~1.2–1.3 µm thick i‐layers, the oxygen was inserted by a controllable leak at the process gases supply line, i.e. by a gas pipe leak. For µc‐Si:H deposited at a discharge power of 0.53 W/cm2 we find a critical oxygen concentration of 1–2 × 1019 cm–3 in agreement with values commonly reported in literature. However, changing the deposition conditions, we find that the critical oxygen concentration in µc‐Si:H cells is not fixed. At reduced power of 0.20 W/cm2 a much higher value for the critical oxygen concentration of 1 × 1020 cm–3 is observed. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

12.
The identification of iron sulfates on Mars by the Mars Exploration Rovers (MERs) and the Mars Reconnaissance Orbiter emphasized the importance of studying iron sulfates in laboratory simulation experiments. The copiapite group of minerals was suggested as one of the potential iron sulfates occurring on the surface and subsurface on Mars, so it is meaningful to study their spectroscopic features, especially the spectral changes caused by cation substitutions. Four copiapite samples with cation substitutions (Fe3+, Al3+, Fe2+, Mg2+) were synthesized in our laboratory. Their identities were confirmed by powder X‐ray diffraction (XRD). Spectroscopic characterizations by Raman, mid‐IR, vis‐NIR and laser‐induced‐breakdown spectroscopy (LIBS) were conducted on those synthetic copiapite samples, as these technologies are being (and will be) used in current (and future) missions to Mars. We have found a systematic ν1peak shift in the Raman spectra of the copiapite samples with cation substitutions, a consistent atomic ratio detection by LIBS, a set of systematic XRD line shifts representing structural change caused by the cation substitutions and a weakening of selection rules in mid‐IR spectra caused by the low site symmetry of (SO4)2− in the copiapite structures. The near‐infrared (NIR) spectra of the trivalent copiapite species show two strong diagnostic water features near 1.4 and 1.9 µm, with two additional bands near 2.0 µm. In the vis‐NIR spectra, the position of an electronic band shifts from 0.85 µm for ferricopiapite to 0.866 µm for copiapite, and this shift suggests the appearance of a Fe2+ electronic transition band near 0.9 µm. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

13.
环带钾长石、榍石和锆石的显微结构与微区组成特征分析   总被引:1,自引:0,他引:1  
南秦岭沙河湾石英二长岩中的钾长石、榍石和锆石具有明显的成分环带。利用电子探针(EPMA)波谱仪(WDS)、扫描电子显微镜(SEM)及其附带的能谱仪(EDS)和激光剥蚀电感耦合等离子体质谱(LA-ICP-MS)对造岩矿物环带钾长石、榍石和锆石的显微结构与微区组成进行表征。结果表明:钾长石中K+被Na+,Ca2+,Ba2+,Fe2+和Ce3+不同程度替代。榍石中Ca2+被V3+,Ce3+和Ba2+等替代,Ti4+被Fe2+和Al3+等替代。锆石含Fe,Th,U,Nb,Ta,Y,Hf,Yb和Pb等稀土和微量元素。钾长石中元素浓度由高到低为Si,Al,K,Ca,Na,Mg和Ba,其中K和Na互补,较亮处Ba含量高,越靠近边缘,Si升高、K升高与Na降低趋势越明显。榍石中元素浓度由高到低为Ca,Si,Ti,Ba,V,Ce,Al和Fe,较亮处Fe含量较高。锆石中元素浓度由高到低为Zr,Si,Nd,Ce,Hf,U,Pb和Th,Hf与Zr呈明显的互补关系,核部Zr含量较边部高,核部Hf、U和Th含量较边部低。  相似文献   

14.
路芳  张兴华  卢遵铭  徐学文  唐成春 《物理学报》2012,61(14):144209-144209
利用固相反应法制备了Sr和Ba替代的Ca2.955-xMxSi2O7: 0.045Eu2+ (M= Sr, Ba, x= 0.1-0.5)系列荧光粉, 利用较大离子半径的Sr和Ba元素替代Eu掺杂Ca2.955-xMxSi2O7 中的Ca元素,研究Sr和Ba替代对样品结构和发光特性的影响. X射线衍射测试结果表明,少量Sr和Ba替代不会改变基质的晶体结构, 样品仍然为单斜晶系.未替代前, Ca2.955Si2O7: 0.045Eu2+ 样品的发射峰在574 nm左右,随着Sr含量的增加,样品的发射峰发生蓝移; 而Ba含量在x= 0.1-0.4时不会引起发射峰位置的移动, 但x= 0.5样品的发射峰发生蓝移.同等含量的Sr和Ba部分替代样品中的Ca元素, Ba替代样品的光谱强度较强.  相似文献   

15.
A new ultrahigh‐energy‐resolution and wide‐energy‐range soft X‐ray beamline has been designed and is under construction at the Shanghai Synchrotron Radiation Facility. The beamline has two branches: one dedicated to angle‐resolved photoemission spectroscopy (ARPES) and the other to photoelectron emission microscopy (PEEM). The two branches share the same plane‐grating monochromator, which is equipped with four variable‐line‐spacing gratings and covers the 20–2000 eV energy range. Two elliptically polarized undulators are employed to provide photons with variable polarization, linear in every inclination and circular. The expected energy resolution is approximately 10 meV at 1000 eV with a flux of more than 3 × 1010 photons s?1 at the ARPES sample positions. The refocusing of both branches is based on Kirkpatrick–Baez pairs. The expected spot sizes when using a 10 µm exit slit are 15 µm × 5 µm (horizontal × vertical FWHM) at the ARPES station and 10 µm × 5 µm (horizontal × vertical FWHM) at the PEEM station. The use of plane optical elements upstream of the exit slit, a variable‐line‐spacing grating and a pre‐mirror in the monochromator that allows the influence of the thermal deformation to be eliminated are essential for achieving the ultrahigh‐energy resolution.  相似文献   

16.
Fabrication and testing of a prototype transmission‐mode pixelated diamond X‐ray detector (pitch size 60–100 µm), designed to simultaneously measure the flux, position and morphology of an X‐ray beam in real time, are described. The pixel density is achieved by lithographically patterning vertical stripes on the front and horizontal stripes on the back of an electronic‐grade chemical vapor deposition single‐crystal diamond. The bias is rotated through the back horizontal stripes and the current is read out on the front vertical stripes at a rate of ~1 kHz, which leads to an image sampling rate of ~30 Hz. This novel signal readout scheme was tested at beamline X28C at the National Synchrotron Light Source (white beam, 5–15 keV) and at beamline G3 at the Cornell High Energy Synchrotron Source (monochromatic beam, 11.3 keV) with incident beam flux ranges from 1.8 × 10?2 to 90 W mm?2. Test results show that the novel detector provides precise beam position (positional noise within 1%) and morphology information (error within 2%), with an additional software‐controlled single channel mode providing accurate flux measurement (fluctuation within 1%).  相似文献   

17.
Metal oxides with the nominal chemical compositions Li5La3M2O12 (M=Nb, Ta), possessing a garnet-like structure, exhibit ionic bulk conductivities of the order of magnitude of ∼10−6 S/cm at 25 °C. Partial substitution of La by alkaline earth elements (Ca, Sr, Ba) in Li5La3M2O12 yields new members of compounds with garnet-like structure with the composition Li6ALa2M2O12 (A=Ca, Sr, Ba). Among the investigated compounds, so far, the Ba-compound Li6BaLa2Ta2O12 exhibits the highest bulk conductivity of 4.0×10−5 S/cm at 22 °C with an activation energy of 0.40 eV. All Ta-members were found to be stable against chemical reaction with molten elemental lithium. Li6ALa2Ta2O12 (A=Sr, Ba) exhibit also high electrochemical stability of ∼6 V vs. lithium and chemical stability against reaction with LiCoO2 cathode material. A novel high voltage thin-film battery was constructed using spinel-type Li2MMn3O8 (M=Co, Fe) as positive electrode, LiPON as electrolyte and Al as negative electrode material. Li2MMn3O8 (M=Fe, Co) electrodes show two reversible plateaus during the charging and discharging cycle at ∼4 and ∼5 V vs. Li. The former plateau is due to the valence change of Mn3+ to Mn4+ and the latter one is due to the oxidation of M3+ to M4+. The chemical diffusion coefficient ( ) was found to be in the range 10−13–10−12 cm2/sec for any composition x of Li2−xMMn3O8 (M=Fe, Co) in the range from 0.1 to 1.6 by employing the galvanostatic intermittent titration technique (GITT). AC impedance studies revealed an electrolyte-electrode charge transfer resistance of 260–290 Θ and an electrode double layer capacity of ∼45–70 μF for an electrode area of 6.7 cm2 at room temperature. The chemical diffusion coefficient of the Al,LiAl negative electrode is about three orders of magnitude higher than that of the positive electrode materials. Accordingly, we believe that the diffusion of Li into and out of the cathode material is the rate determining process. Paper presented at the Patras Conference on Solid State Ionics - Transport Properties, Patras, Greece, Sept. 14 – 18, 2004.  相似文献   

18.
The fundamental performance of microangiography has been evaluated using the S‐band linac‐based inverse‐Compton scattering X‐ray (iCSX) method to determine how many photons would be required to apply iCSX to human microangiography. ICSX is characterized by its quasi‐monochromatic nature and small focus size which are fundamental requirements for microangiography. However, the current iCSX source does not have sufficient flux for microangiography in clinical settings. It was determined whether S‐band compact linac‐based iCSX can visualize small vessels of excised animal organs, and the amount of X‐ray photons required for real time microangiography in clinical settings was estimated. The iCSX coupled with a high‐gain avalanche rushing amorphous photoconductor camera could visualize a resolution chart with only a single iCSX pulse of ~3 ps duration; the resolution was estimated to be ~500 µm. The iCSX coupled with an X‐ray cooled charge‐coupled device image sensor camera visualized seventh‐order vascular branches (80 µm in diameter) of a rabbit ear by accumulating the images for 5 and 30 min, corresponding to irradiation of 3000 and 18000 iCSX pulses, respectively. The S‐band linac‐based iCSX visualized microvessels by accumulating the images. An iCSX source with a photon number of 3.6 × 103–5.4 × 104 times greater than that used in this study may enable visualizing microvessels of human fingertips even in clinical settings.  相似文献   

19.
MMoO4 (M=Ca, Ba) particles were synthesized by a metathetic reaction in ethylene glycol assisted by cyclic microwave irradiation followed by further heat-treatment. The MMoO4 (M=Ca, Ba) particles were well crystallized after heat-treatment at 400–600 °C for 3 h. The microstructures exhibited fine morphologies with sizes of 0.5–1 μm and 1.5–2 μm for the CaMoO4 and BaMoO4 particles, respectively. The synthesized MMoO4 (M=Ca, Ba) particles were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy and transmission electron microscopy. The optical properties were examined by photoluminescence emission and Raman spectroscopy.  相似文献   

20.
At the National Synchrotron Radiation Research Center (NSRRC), which operates a 1.5 GeV storage ring, a dedicated small‐angle X‐ray scattering (SAXS) beamline has been installed with an in‐achromat superconducting wiggler insertion device of peak magnetic field 3.1 T. The vertical beam divergence from the X‐ray source is reduced significantly by a collimating mirror. Subsequently the beam is selectively monochromated by a double Si(111) crystal monochromator with high energy resolution (ΔE/E? 2 × 10?4) in the energy range 5–23 keV, or by a double Mo/B4C multilayer monochromator for 10–30 times higher flux (~1011 photons s?1) in the 6–15 keV range. These two monochromators are incorporated into one rotating cradle for fast exchange. The monochromated beam is focused by a toroidal mirror with 1:1 focusing for a small beam divergence and a beam size of ~0.9 mm × 0.3 mm (horizontal × vertical) at the focus point located 26.5 m from the radiation source. A plane mirror installed after the toroidal mirror is selectively used to deflect the beam downwards for grazing‐incidence SAXS (GISAXS) from liquid surfaces. Two online beam‐position monitors separated by 8 m provide an efficient feedback control for an overall beam‐position stability in the 10 µm range. The beam features measured, including the flux density, energy resolution, size and divergence, are consistent with those calculated using the ray‐tracing program SHADOW. With the deflectable beam of relatively high energy resolution and high flux, the new beamline meets the requirements for a wide range of SAXS applications, including anomalous SAXS for multiphase nanoparticles (e.g. semiconductor core‐shell quantum dots) and GISAXS from liquid surfaces.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号