Search

The tensile, flexural, and fracture toughness properties of seven chemically different amine-cured epoxy resins were studied. Positron annihilation lifetime and pressure–volume–temperature (PVT) experiments were performed on each epoxy resin to characterize the average size and fraction, respectively, of free-volume holes. A negative correlation between hole fraction and hole size was revealed for these chemically different epoxy resins. Better tensile and flexural mechanical properties (higher tensile modulus and lower tensile strain at break; higher flexural modulus, higher flexural strength, and lower flexural strain at break) were clearly observed for epoxy resins with smaller hole size and higher hole fraction. However, no clear relationship between fracture toughness and hole properties was found. The correlations between (tensile and flexural) static mechanical properties and hole properties for chemically different epoxy resins should provide guidance for further improvements in the mechanical properties of carbon-fiber-reinforced polymers. [ABSTRACT FROM AUTHOR]

Doppler broadening of positron annihilation radiation (DBAR) spectroscopy is a critical technique to provide electronic momentum information of materials. For conventional non-beam-based DBAR experiments, due to the continuous distribution of initial kinetic energy (0-545 keV) of positrons from 22Na source, two pieces of identical samples with sufficient thickness (> 1 mm for each sample with a density of 1 g/cm3) are required to stop almost all positrons in the samples. However, in many film studies, it is difficult or even impossible to meet this sample thickness requirement. In this work, judging by the output signals of a specially designed anticoincidence detector, the annihilation of positrons in a film sample and positron source are selectively collected. Furthermore, a new method is proposed to correct high-proportioned source components. Based on this, for the first time, a non-beam-based DBAR spectrometer was developed for a single piece of micron-thickness film. This approach will drastically broaden the application of DBAR spectroscopy in film characterization in future. [ABSTRACT FROM AUTHOR]

Abstract: The pore structure of NiO/γ-Al2O3 catalysts is characterized by positron lifetime and Doppler broadening measurements. A very long lifetime τ 4 of 92ns is resolved from the positron lifetime spectrum measured for pure Al2O3, which could be attributed to the ortho-positronium (o-Ps) lifetime in large pores. It was also found that the fitted lifetime τ 4 and its corresponding intensity I 4 obtained from the lifetime spectra both decrease with narrowing energy window of the stop channel in the fast–fast coincidence lifetime measurement system. This suggests that the ultra long lifetime is primarily due to the self annihilation of o-Ps which emits three gamma-rays. Such 3γ annihilation is further evidenced by measuring the Doppler broadening of annihilation gamma rays in coincidence with the prompt gamma rays (1.28MeV) emitted from the 22Na positron source. In NiO/γ-Al2O3 catalysts both the lifetime τ 4 and its intensity I 4 decreases with increasing NiO content (from 3wt% to 40wt%), which indicates decreasing of the number of 3γ events. The 3γ annihilation parameter analyzed from the coincidence Doppler broadening spectrum shows consistent decrease with increasing NiO content. [Copyright &y& Elsevier]

To precisely characterize the defects in crystalline solids including semiconductors and alloys, it is critical but challenging to improve the time resolution of positron annihilation lifetime (PAL) spectrometers. The effects of geometry and wrapping of scintillators on the time resolution of the PAL spectrometer were studied in detail. Geant4 program simulation was used to clarify the reason why cone-frustum-shaped scintillators have better time resolution than cylinder-shaped scintillators. A figure of merit was proposed to comprehensively evaluate the impacts of the transport and absorption of fluorescent photons on the time resolution of scintillators. Additionally, for the simulations of cone-frustum-shaped scintillators, polished reflective coatings can further improve the time resolution without losing count rate. Eventually, using three cone-frustum-shaped Ba F 2 scintillators and a double-stop setup, we significantly improved the time resolution of PAL spectrometer to a new record of 88.7 ps, the best time resolution up to date. This enables us to distinguish various vacancy types in crystalline solids more precisely in future. • Explained why cone-frustum-shaped scintillators have better time resolution. • Proposed a figure of merit to evaluate the time resolution of PAL spectrometer. • Achieved the best time resolution up to date for the PAL spectrometer. [ABSTRACT FROM AUTHOR]

Positron annihilation lifetime (PAL) spectroscopy has long been considered as a powerful approach to characterize atomic-scale vacancy-type defects and supermicropores, which affect the macroscopic properties of materials. In standard PAL measurements, several million annihilation events are required to achieve accuracy. For a conventional PAL spectrometer which consists of two scintillation detectors and analytical electronics, the two detectors are usually perpendicularly positioned to diminish spectra distortions (caused by backscattering and pileup). The perpendicular placement of detectors leads to a counting rate of 100 – 300 counts/second (cps) while using a 30 μ Ci 22Na source (3.3 – 10 cps/ μ Ci). Consequently, collecting one PAL spectrum with total counts of one million takes several hours. The long time limits its potential to observe changes in the microstructure. To increase the counting rate of PAL measurements, we propose a novel multi-detector PAL spectrometer, which includes three or four detectors, a digital oscilloscope, and an algorithm for any detector to record both start and stop signals. The spectrometer achieves a high counting rate of around 2144 cps (71.5 cps/ μ Ci) with four detectors, which is about ten times that of a conventional one with two detectors perpendicularly positioned. This work enables us to perform in-situ PAL measurements to disclose the minute-scale evolution of microstructure more precisely during fast physical and chemical processes in the future. • A better spectrum unfolding accuracy can be achieved with a larger total count. • The scintillator size and geometry was optimized using Geant4. • A reversible start/stop data acquisition algorithm was developed to double the counting rate. • The novel spectrometer is highly extendable to 3, 4 and more detectors. • The counting rate of ten times higher than the conventional spectrometer was obtained. [ABSTRACT FROM AUTHOR]

Positron annihilation lifetime (PAL) spectroscopy is one of the most powerful methods to quantitatively characterize atomic-scaled lattice imperfections in condensed materials. Generally, one needs to fit a PAL spectrum by solving a local non-linear optimization problem which is more or less affected by initial guesses. Therefore, by using a traditional analysis program for the same PAL spectrum, different results with close goodnesses of fits are yielded by different researchers. Thus, it is difficult to judge the qualities of different results. To overcome this shortage, an efficient Markov Chain Monte-Carlo Bayesian Inference (MCMC-BI) method based on CosmoMC package is applied to analyze both simulated and experimental PAL spectra in the present work. The same level of accuracy of traditional analysis programs is firstly acquired in this work by using MCMC-BI method, which demonstrates that it can be directly used to analyze PAL spectra. Furthermore, the dependence on the initial guesses of PAL analysis is significantly alleviated. Additionally, more precious information is provided by MCMC-BI method, including different lifetime uncertainties in different confidence intervals and the correlations between annihilation parameters. [ABSTRACT FROM AUTHOR]

Silicon photomultipliers (SiPMs) have great potential to replace photomultiplier tubes (PMTs) and exceed their performance in positron annihilation parameter-measurement systems. However, due to the limited active area of a single SiPM, it only covers a small γ -ray emission acceptance range and provides position information over a very small region. Using a SiPM array is a good way to overcome these problems. In this study, a resistor-chain-based SiPM detector was designed and evaluated for positron annihilation lifetime (PAL) measurements. This detector uses a 1 × 4 scintillator array, a 1 × 4 SiPM array, and a resistor-chain readout circuit to obtain position, energy, and timing information about 1275 and 511 keV γ -rays, with the readout pulses serving as both the position, energy, and timing signals. A comprehensive evaluation was conducted to assess the detector in terms of position profile, energy resolution, and time resolution. The position profiles showed that all the crystal elements in the lutetium fine silicate scintillator array were clearly resolved. The detector had an energy resolution of 11. 3 ± 0. 9 % and a time resolution of 198 ± 9 ps. A total of 32 PAL spectra were obtained using two SiPM array detectors. The PAL in yttria-stabilized zirconia (YSZ) sample was accurately verified using the summed PAL spectrum. The developed SiPM detector provides good performance for PAL measurements and could be used to build a high-performance PAL spectrometer. [ABSTRACT FROM AUTHOR]

Positron annihilation lifetime (PAL) spectroscopy is a unique method to study defects and supermicropores in solid materials. Traditionally, the pulsed slow positron beam (accelerator system included) was used to measure the PAL spectrum for a single film with a thickness of several microns, which is more complicated, expensive, and has worse time resolution than a conventional PAL spectrometer. In this paper, we developed a new PAL spectrometer to measure a single piece of micron-thickness film using two lanthanum bromide (LaBr 3) detectors, a specially designed plastic scintillator detector, and a fast digitizer, at a cost equivalent to that of a conventional PAL spectrometer. The reliability and time resolution (around 210 ps, comparable to that of a conventional PAL spectrometer) of this spectrometer were verified by testing samples with thicknesses range from 15 to 60 μ m. Due to the successful construction of the new PAL spectrometer, PAL spectroscopy can now be applied to a single piece of micron-thickness film for the first time without building an accelerator system. [ABSTRACT FROM AUTHOR]

Annihilation of positronium (Ps) in static magnetic fields is investigated by positron annihilation lifetime (PAL) and Doppler broadening (DB) measurements of γ-Al 2 O 3 nanopowder. The properties of four eigenstates of Ps atoms (| 1,1 ⟩ , | 1 , − 1 ⟩ , | + ⟩ (long-lived eigenstate mixed from | 1,0 ⟩ and | 0,0 ⟩ due to Zeeman effect), and | − ⟩ (short-lived eigenstate mixed from | 1,0 ⟩ and | 0,0 ⟩ due to Zeeman effect)) were revealed from PAL and DB results. Thereafter, with magnetic field increasing from 0.105 to 0.760 T, clear and regular variation of Ps annihilation parameters are observed: (1) The mean lifetime of the three long-lived Ps states (| 1,1 ⟩ , | 1 , − 1 ⟩ , and | + ⟩) shows clear quenching effect; (2) The formation probability of each Ps eigenstate (| 1,1 ⟩ , | 1 , − 1 ⟩ , | + ⟩ , and | − ⟩) keeps unchanged; (3) The intensity of the narrow component (derived from multi-Gaussian fitting) of DB spectrum exhibits a gradual increase which is in good agreement with the theoretical calculation from PAL results (2γ decay of | + ⟩ and | − ⟩ states, and pick-off quenching of long-lived Ps states); (4) The intensity of the low energy region of DB spectrum is diminished by the decrease of 3γ decay of | + ⟩ state. The quantitative correlations between magnetic field and o-Ps annihilation parameters will be helpful for the further understanding of complex Ps quenching effects, such as the effect mixed from orhto-para spin conversion and magnetic quenching. • Magnetic quenching of positronium was investigated in nanometer-scaled porous γ-Al 2 O 3 which is very suitable for theformation of positronium. • Magnetic quenching of positronium was investigated by using both PAL and DB experimental techniques for the first time. • The multi-Gaussian fitting method was utilized to deduce the p-Ps intensity of each sample (the intensity of the narrowest component of each Doppler broadening spectrum). [ABSTRACT FROM AUTHOR]

Positron annihilation lifetime (PAL) spectroscopy is a unique technique to characterize atomic-scaled microstructure of materials, especially the vacancy-type defects. Compared to photomultiplier tube, silicon photomultiplier (SiPM) has a higher quantum efficiency, a more compact size, and a much lower price. Considering the evident advantages of SiPM, we developed a SiPM-based PAL (SiPM-PAL) spectrometer by using LFS-3 and LYSO scintillators in the sizes of 3 × 3 × l mm3. The performance of the SiPM-PAL spectrometer was studied with four different scintillator lengths (l) of 5.0, 9.5, 13.8, and 20.0 mm. Under the same experimental conditions, with LFS-3 scintillator length decreasing from 20.0 to 5.0 mm, the time resolution (in FWHM) of SiPM-PAL spectrometer was significantly improved from 174.9 to 135.9 ps. While for the SiPM-PAL spectrometer with LYSO scintillators, the time resolution improved from 181.9 to 137.0 ps with scintillator length decreasing from 20.0 to 5.0 mm. Compared to conventional PAL spectrometer using photomultiplier tubes, a novel, small size, and low cost PAL spectrometer is successfully constructed by using SiPMs. Easy customization and superiority in time resolution and small size of SiPM-based scintillation detectors guarantee high flexibility and adaptability to the SiPM-PAL spectrometers, positron micro-beam facilities, and angular correlation of positron annihilation radiation measurements. [ABSTRACT FROM AUTHOR]

Ag/SiO 2 /Si multilayer samples with different SiO 2 layer thicknesses were prepared through deposition of Ag layer onto SiO 2 /Si substrates at different substrate temperatures (T sub) by RF magnetron sputtering method. Microstructure of the multilayer was characterized by XRD θ /2 θ scan and φ -scan analysis, depth-resolved XPS analysis, and Doppler broadening of annihilation radiation (DBAR) spectra of positron measurements. The experimental results indicate that, at T sub = 25 °C, the Ag deposition forms Ag/SiO 2 /Si multilayer structure. With T sub increasing to 200 °C, Ag atoms are found to diffuse into the SiO 2 layer. This thermal diffusion process plays a critical role in removing SiO 2 layer on the Si substrate. At T sub of 600 °C, the 31-nm-thick SiO 2 layer is partially desorbed from the Si surface as a result of Ag diffusion, and eventually results in a deposition of Ag layer directly on the Si substrate. This is also confirmed by the XRD φ -scan measurements, which indicates epitaxial growth of Ag atoms directly on the surface of Si substrate at T sub of 600 °C. • The deposition of Ag on SiO 2 /Si substrate at 25 °C is easily forming Ag/SiO 2 /Si multilayer structure. • Compared with Ag single crystal, Ag films have the smaller positron effective diffusion lengths due to its higher density of structural defects. • Depth-resolved XPS results show the high diffusion nature of surface O when the substrate temperature up to 600 °C. [ABSTRACT FROM AUTHOR]