Coincidence Doppler Broadening Spectra Research Articles

Formation of D–VZn complex defects and possible p-type conductivity of ZnO nanoparticle via hydrogen adsorption

The hydrogen adsorption on surfaces and on defect sites of ZnO nanoparticles (NPs) has been studied by using Raman and Fourier transform infrared spectroscopic methods. The presence of hydrogen at defect sites bound to zinc vacancy with different coordinations has been confirmed. To further identify the existence of isolated VZn and H–VZn complexes in the ZnO NPs, coincidence Doppler broadening (CDB) spectroscopic studies have been performed with respect to the CDB spectra of a 99.9999% pure Al single crystal. The broad momentum dip ρL showed between 15–17 × 10−3 m0c suggests the trapping of positrons with the core electrons of 3p Zn. However, positron annihilation takes place between ρL 20–25 × 10−3 m0c and this may occur with an electron belonging to OH bonds (VZn–Hi–O). Here the lattice hydrogen H+ ion acts as a compensating centre, and it can bind with the VZn around the dislocation and stacking faults (SFs) core, which may produce the acceptor-type complex defect for p-type conductivity. Finally, the existence of SFs and dislocation defects, including edges and steps, was confirmed by transmission electron microscopy.

Chemical Information of Chitosan-Based Complex Extracted from Coincidence Doppler Broadening Spectra

Chemical Information of Chitosan-Based Complex Extracted from Coincidence Doppler Broadening Spectra

Chemical Information of Chitosan-Based Complex Extracted from Coincidence Doppler Broadening Spectra

Chemical Information of Chitosan-Based Complex Extracted from Coincidence Doppler Broadening Spectra

Positron annihilation study of ageing precipitation in deformed Fe–Cu–B–N–C

The role of deformation-induced defects and carbon addition on copper precipitation during ageing at 550 °C is investigated in high-purity Fe–Cu–B–N–C alloy samples by positron annihilation spectroscopy. Complementary small-angle neutron scattering (SANS) and hardness tests are utilized to characterize the size distribution of the Cu precipitates formed and their influence on the mechanical properties. Samples with 0 and 8% cold pre-strain are utilized to study the influence of prior tensile deformation on the precipitation kinetics of copper. The time evolution of the coincidence Doppler-broadening spectra indicates that deformation-induced defects enhance the Cu precipitation kinetics, which is confirmed by the SANS results. In the S–W plot, a clear reduction in open volume defects is accompanied by a strong increase of Cu signature during the initial stage of ageing, implying that the open volume defects (mainly dislocations) act as nucleation sites for Cu precipitation. A comparison between the precipitation behaviour of Fe–Cu, Fe–Cu–B–N and Fe–Cu–B–N–C indicates that the addition of carbon does not alter the Cu precipitation mechanism but decelerates the kinetics. Hardness results confirm that carbon counteracts the acceleration of Cu precipitation caused by the addition of B and N.

Identification of Vacancy Formation Sites in LaNi5Cu During Hydrogenation Using in Situ Coincidence Doppler Broadening Technique

To study the formation mechanism of lattice defects introduced during hydrogenation, we developed equipment that can measure in situ positron lifetime and coincidence Doppler broadening (CDB) simultaneously with pressure–composition (P–C) isotherms at room temperature. The equipment, which consisted of a high-pressure sample holder, reduced the intensity of the background signals in the relatively high momentum region of the Doppler broadening spectrum by more than 2 orders of magnitude in comparison to the background shown by conventional Doppler broadening equipment. Further, the quality of data obtained by our instrument was similar to that obtained from ex situ CDB equipment without a high-pressure sample holder. The substitution effect on the positron annihilation behavior was observed from the ratio curves of CDB spectra for LaNi5Cu and LaNi5-xAlx. In addition, using the equipment we developed, we attempted to identify the vacancy formation sites introduced in LaNi5Cu during hydrogenation. Changes i...

Chemical quenching and inhibition of positronium in Cr2O3/Al2O3 catalysts

The pore structure of Cr2O3/Al2O3 catalysts and the surface chemical properties of these pores were characterized by positron lifetime and coincidence Doppler broadening (CDB) measurements. Four lifetime components could be resolved from the positron lifetime spectrum, with two long lifetime components and two short lifetime components. The two long lifetimes τ4 and τ3 are attributed to ortho-positronium (o-Ps) annihilation in large pores and microvoids, respectively. With increasing Cr2O3 content, both τ4 and its intensity I4 show sharp decrease, while τ3 and its intensity I3 keep nearly unchanged. The Doppler broadening S parameters also show sharp decrease with increasing Cr2O3 content. Detailed analysis of the CDB spectrum reveals that the parapositronium (p-Ps) intensity also decreases with increasing Cr2O3 content. This indicates that the change of o-Ps lifetime τ4 is due to the chemical quenching by Cr2O3 but not spin-conversion of positronium. The decrease of o-Ps intensity I4 indicates that Cr2O3 also inhibits positronium formation.

Relaxation Behavior of Zr-Cu-Al Ternary Bulk Glassy Alloy Studied by Using Positron Annihilation Techniques

Zr-based bulk glassy (BG) alloys show high tensile strength and a high Charpy impact value. In this study, the free volume changes for the hypoeutectic Zr60Cu30Al10 BG alloy during isothermal annealing below glass transition temperature (Tg) have been investigated by positron annihilation lifetime and coincidence Doppler broadening (CDB) measurements. The positron lifetime of hypoeutectic and eutectic BG alloys is almost the same, although the atomic volume ratio, estimated by the density for the eutectic BG alloy is different from that for the hypoeutectic BG alloy. The CDB spectra show a marked difference between eutectic and hypoeutectic BG alloys; that is, the spectrum of the hypoeutectic BG alloy is closes to that of Zr metal than that of the eutectic BG alloy. This result exhibits that the elemental fraction of Zr atoms around free volume in hypoeutectic BG alloy is greater than that in eutectic BG alloy. The CDB ratio profiles for the hypoeutectic BG alloy during annealing shows no appreciable change. The same trend was observed in the eutectic BG alloy.

Positron annihilation study of the hardening behavior in Al-Cu based alloy by electron and heavy ion irradiation

Al-Cu based alloy, which is generally called duralumin (JIS2017), was irradiated with 10 MeV Iodine ions, 200 MeV Xenon ions and 3 MeV electrons at room temperature respectively. The micro Vicker's hardness and positron annihilation coincidence Doppler broadening (CDB) measurements have been performed before and after irradiation. Only in the case of ion irradiation, the Vicker's hardness increases with increasing ion dose. Nevertheless, there was no difference in the profile CDB spectrum for before and after irradiation. On the other hand, we found that the micro hardness of this alloy, which was Xe ion irradiated and subsequently annealed at 423 K, is greater than that of age hardened alloy without irradiation. CDB ratio curve of the age hardened Duralumin is clearly different in the electron momentum range around 0.015-0.025 mc from that of the ion irradiated alloy. The results of three-dimensional atom probe (3DAP) also show that a lot of small clusters were found after ion irradiation but large precipitations have found in annealed Duralumin. These results reveal that a number of small clusters formed in this alloy after ion irradiation, and they should strongly affects the micro hardness.

Effect of thermal annealing on the local structure in ZrCuAl bulk metallic glass

The relaxation and crystallization behaviors of the ternary Zr 50Cu 40Al 10 bulk metallic glass were investigated by the positron annihilation measurements and the extended X-ray absorption fine structure (EXAFS) analysis at the Zr K-edge. The result of positron annihilation coincidence Doppler broadening (CDB) for as-prepared specimen suggests that the fraction of Zr atom around free volume is greater than that of composition for alloy system. According to the results of CDB spectrum and Fourier transformed EXAFS spectrum, the isothermal annealing below the glass transition temperature ( T g ) (structural relaxation) decreases the size of free volume, but it causes little change in the distribution of atoms, either around free volume or around Zr atoms. For the isothermal annealing above crystallization temperature ( T x ), the XRD and EXAFS spectra clearly show the crystallization of specimens. The CDB result also suggests the crystallization. These results demonstrate that the chemical order around free volume is dissociated, and crystallized order is newly formed during the thermal annealing above T x .

Evolution of native point defects in ZnO bulk probed by positron annihilation spectroscopy

This paper studies the evolution of native point defects with temperature in ZnO single crystals by positron lifetime and coincidence Doppler broadening (CDB) spectroscopy, combined with the calculated results of positron lifetime and electron momentum distribution. The calculated and experimental results of the positron lifetime in ZnO bulk ensure the presence of zinc monovacancy, and zinc monovacancy concentration begins to decrease above 600 °C annealing treatment. CDB is an effective method to distinguish the elemental species, here we combine this technique with calculated electron momentum distribution to determine the oxygen vacancies, which do not trap positrons due to their positive charge. The CDB spectra show that oxygen vacancies do not appear until 600 °C annealing treatment, and increase with the increase of annealing temperature. This study supports the idea that green luminescence has a close relation with oxygen vacancies.

Defects in Si-Rich SiO2 Films Prepared by Radio-Frequency Magnetron Co-sputtering Using Variable Energy Positron Annihilation Spectroscopy

Si-rich SiO2 films prepared by rf magnetron co-sputtering method are studied by slow positron beams. The negatively charge point defects (probably Pb centres or peroxy radicals) at the silicon nanocluster (nc-Si)/SiO2 interface are observed by Doppler broadening spectra. Coincidence Doppler-broadening spectra show that positrons have a higher annihilation probability with core electrons nearby oxygen atoms than silicon atoms. The formation of N-related bonds may be the reason for the prevention of the migration reaction of Si and O atoms, hence nc-Si formation is inhibited by annealing in nitrogen compared to in vacuum.

Positron annihilation characteristics of ODS and non-ODS EUROFER isochronally annealed

Yttrium oxide dispersion strengthened (ODS) and non-ODS EUROFER produced by mechanical alloying and hot isostatic pressing have been subjected to isochronal annealing up to 1523K, and the evolution of the open-volume defects and their thermal stability have been investigated using positron lifetime and coincidence Doppler broadening (CDB) techniques. Transmission electron microscopy (TEM) observations have also been performed on the studied samples to verify the characteristics of the surviving defects after annealing at 1523K. The CDB spectra of ODS EUROFER exhibit a characteristic signature that is attributed to positron annihilation in Ar-decorated cavities at the oxide particle/matrix interfaces. The variation of the positron annihilation parameters with the annealing temperature shows three stages: up to 623K, between 823 and 1323K, and above 1323K. Three-dimensional vacancy clusters, or voids, are detected in either materials in as-HIPed condition and after annealing at T⩽623K. In the temperature range 823–1323K, these voids’ growth and nucleation and the growth of other new species of voids take place. Above 1323K, some unstable cavities start to anneal out, and cavities associated to oxide particles and other small precipitates survive to annealing at 1523K. The TEM observations and the positron annihilation results indicate that these cavities should be decorated with Ar atoms absorbed during the mechanical alloying process.

Relaxation of Free Volume in Zr<SUB>50</SUB>Cu<SUB>40</SUB>Al<SUB>10</SUB> Bulk Metallic Glasses Studied by Positron Annihilation Measurements

Relaxation of free volume in Zr 50 Cu 40 Al 10 bulk metallic glass during isothermal annealing below T g (glass transition temperature) has been investigated by using positron annihilation lifetime and coincidence Doppler broadening (CDB) techniques. The mean positron lifetime decreases with increasing annealing time at each annealing temperature. The decrease in lifetime is due to shrinkage and annealing out of the free volume. The relaxation kinetics of free volume obeys the stretched exponential function (KWW: Kohlrausch-Williams-Watts law). An adjustable parameter of the KWW exponent β depending on temperature was determined. The electron momentum distribution around free volume derived from a CDB spectrum during annealing showed no appreciable change at each temperature. These results suggest that long-range atomic diffusion and rearrangement, particularly around the free volume, do not occur essentially during relaxation below Tg.

Positron annihilation study of Mo dispersion in MoO3/Al2O3 catalysts

A series of MoO3/Al2O3 catalysts was prepared by the impregnation method. The positron annihilation lifetime spectra(PALS)and coincidence Doppler broadening (CDB) spectra of the samples were measured to study the pore structure and dispersion of MoO3. The PALS result shows that there are two types of pores with different size in the Al2O3 support. After MoO3 was loaded in the support, Mo species are mainly dispersed inside the large pores of the support, and thus decrease the volume of the large pores. The CDB spectrum shows significant change while MoO3 loading is just 3%. This indicates that the Mo species are dispersed into the pores, and cause the change of the electron momentum distribution seen by positrons. As the MoO3 loading exceeds 18%, the size of the large pores is decreased to the similar size of the small pores, and the positron annihilation parameters show nearly no change with further Mo loading, which indicates saturation of the dispersion of Mo species.

Positron annihilation spectroscopy and specific heat study of Neon ion irradiated MgB2

Abstract Specific heat studies under magnetic field and positron annihilation spectroscopy were carried out on 160 MeV Ne ion irradiated polycrystalline MgB 2 samples. There is an unusual decrease in positron lifetime in the irradiated sample which may be due to neon ion implantation. This was also indicated by change in cell volume. Coincidence Doppler Broadening Spectra of Mg, B, irradiated and unirradiated MgB 2 show that positrons primarily annihilate in boron sublattice in the unirradiated sample whereas there is some similarity of the spectrum of the irradiated sample with that of Mg. There is Mg deficiency in the unirradiated sample whereas predominantly boron vacancies exist in Ne ion irradiated MgB 2 sample. Specific heat measurements show that there is a small increase in electronic part of the specific heat and electron–phonon coupling constant.

Quantitative chemical analysis of vacancy‐solute complexes in metallic solid solutions by coincidence Doppler broadening spectroscopy

AbstractLow background measurements of 1D momentum spectra of the positron annihilation radiation by the coincidence Doppler broadening (CDB) method are currently used to obtain the quantitative chemical analysis of the environment of vacancy‐like defects in supersaturated solid solution of metals. Fractional concentrations are obtained by fitting the CDB spectra with linear combination of reference spectra. Examples regarding Al‐based alloys are presented to show the quality of the fitting and the consistency of the CDB analysis with positron lifetime measurements. The appropriate choice of the references is considered a key factor for reducing systematic errors. Nevertheless the method still awaits a theoretical validation. Preliminary first‐principles calculations regarding Cu‐decorated vacancies in aluminium are presented. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Free Volume Relaxation Process in Zr50Cu40Al10 Bulk Metallic Glass Studied by Positron Annihilation Techniques

AbstractStructural relaxation around free volume in Zr50Cu40Al10 bulk metallic glass (BMG) during isothermal annealing at 473, 573 and 673 K which are below glass transition temperature Tg =675 K have been investigated by positron annihilation lifetime (PAL) and coincidence Doppler broadening (CDB) measurements. The trends of change in positron lifetime, which correspond to the size of free volume at each annealing temperature, have a good correlation with their density change. These annealing processes obey a stretched exponential relaxation function (KWW: Kohlrausch-Williams-Watts law). Fitting parameters of KWW function, with relaxation time t0 and β, in each temperature were determined. These relaxation parameters depend on the annealing temperature, suggesting the distribution of activation energy for structural relaxation. Moreover, the profile of electron momentum distribution around free volume derived by CDB spectrum during annealing showed no appreciable change at each temperature. These facts suggest that long range chemical ordering, particularly around the free volume, dose not take place essentially.

Free volume change in crystallization process of Zr–Cu–Al metallic glass studied by positron annihilation techniques

Positron annihilation lifetime (PAL) and coincidence Doppler broadening (CDB) experiments have been performed to study free volume changes and atomic relaxation behavior in Zr–Cu–Al bulk metallic glass. Zr 50Cu 40Al 10 metallic glass was annealed isothermally at 673 K, which is below the glass transition temperature ( T g). During this annealing, PAL was observed to decrease, indicating a decrease in free volume; whereas, no significant change in CDB spectra was detected. This observation implies that the relaxation below T g takes place without a change in atomic configuration. As for specimens annealed at 723 K (> T g) for 100 s and subsequently annealed isothermally at 673 K for 18,000 s, PAL decreased similarly, suggesting a decrease in free volume. In addition, CDB spectra showed a systematic fluctuation of electron momentum distribution, especially in a high momentum region. This indicates that a chemical reordering took place during crystallization process.

Electron-positron momentum distributions associated with isolated silicon vacancies in3C−SiC

Two-dimensional angular correlation of annihilation radiation (2D-ACAR) and coincidence Doppler broadening (CDB) of annihilation radiation measurements have been performed on electron-irradiated $n$-type $3C\text{\ensuremath{-}}\mathrm{Si}\mathrm{C}$ in which isolated silicon vacancies are responsible for positron trapping. After irradiation, the intensity of the CDB spectrum increased and decreased in low- and high-momentum regions, respectively. These features were explained by a theoretical calculation considering silicon vacancies. The central region of the 2D-ACAR spectra became isotropic after iradiation, while the overall anisotropies extending within the Jones zone were conserved suggesting that isolated silicon vacancies have tetrahedral symmetry, as expected from a previous electron spin resonance study.

Relaxation and Crystallization Behavior of the Zr<SUB>50</SUB>Cu<SUB>40</SUB>Al<SUB>10</SUB> Metallic Glass

We examined relaxation and crystallization behavior of the ternary Zr50Cu40Al10 metallic glass by using positron annihilation and transmission electron microscopy (TEM). Observed changes in positron annihilation lifetime of the alloy annealed isothermally at 673 K, which is below the glass transition temperature, correlate well with observed density changes; while coincidence Doppler broadening (CDB) spectra exhibit no significant change. These observations demonstrate that free volume decreases without a rearrangement of atoms during structural relaxation. On the other hand, CDB spectrum has exhibited considerable changes when the same alloy was annealed at 773 K. TEM observations suggested that crystallization occurs via growth of spherulites of several hundred nm in diameter, which themselves are composed of radially grown grains. Chemical analysis revealed that Cu atoms are partitioned out during the growth of the spherulites. Diffraction study indicated that the unit cell of the crystalline phase belong to an orthorhombic system with a ¼ 0:892, b ¼ 0:550, and c ¼ 1:060 nm; while remaining interspherulite regions are found to crystallize into an fcc phase with a ¼ 1:28 nm, which is probably isostructural to the � 3 phase.