Positron Annihilation Measurements Research Articles

Research progress of coincidence Doppler broadening of positron annihilation measurement technology in materials

Positron annihilation technique is an atomic-scale characterization method used to analyze the defects and microstructure of materials, which is extremely sensitive to open volume defects. By examining the annihilation behaviour of positrons and electrons in open volume defects, local electron density and atomic structure information around the annihilation site can be obtained, such as the size and concentration of vacancies, and vacancy clusters. In recent years, positron annihilation spectroscopy has evolved into a superior tool for characterizing features of material compared with conventional methods. The coincident Doppler broadening technique provides unique advantages for examining the local electronic structure and chemical environment (elemental composition) information about defects due to its effectiveness describing high momentum electronic information. The low momentum portion of the quotient spectrum indicates the Doppler shift generated by the annihilation of valence electrons near the vacancy defect. Changes in the peak amplitudes and positions of the characteristic peaks in the high momentum region can reveal elemental information about the positron annihilation point. The physical mechanism of element segregation, the structural features of open volume defects and the interaction between interstitial atoms and vacancy defects are well investigated by using the coincidence Doppler broadening technology. In recent years, based on the development of Doppler broadening technology, the sensitivity of slow positron beam coincidence Doppler broadening technology with adjustable energy has been significantly enhanced at a certain depth. It is notable that slow positron beam techniques can offer surface, defect, and interface microstructural information as a function of material depth. It compensates for the fact that the traditional coincidence Doppler broadening technique can only determine the overall defect information. Positron annihilation technology has been applied to the fields of second phase evolution in irradiated materials, hydrogen/helium effect, and free volume in thin films, as a result of the continuous development of slow positron beam and the improvement of various experimental test methods based on slow positron beam. In this paper, the basic principles of the coincidence Doppler broadening technique are briefly discussed, and the application research progress of the coincidence Doppler broadening technique in various materials is reviewed by combining the reported developments: 1) the evolution behaviour of nanoscale precipitation in alloys; 2) the interaction between lattice vacancies and impurity atoms in semiconductors; 3) the changes of oxygen vacancy and metal cation concentration in oxide material. In addition, coincident Doppler broadening technology has been steadily used to estimate and quantify the sizes, quantities, and distributions of free volume holes in polymers.

Effect of annealing temperature on the structure and optical properties of ZnO thin films

The effect of annealing temperature on the microstructure, defects and optical properties of ZnO thin films are investigated using sol–gel based spin coating method for a range of annealing temperatures from 200 ∘C to 500 ∘C. The correlation among the microstructure, defects, impurity content and the optical band gap of films of thickness about 10–12 nm is elucidated. The particle size increases and the optical band gap reduces with the annealing temperature. At 200 ∘C, amorphous films were formed with particle size less than 10 nm with an optical band gap of about 3.41 eV. As the temperature increases the grain size increases and the defect, impurity content as well as the optical band gap reduces. This could be due to the reduction in the lattice strain. For an average grain size of about 35 nm and above, the band gap asymptotically approaches the theoretical value of ZnO (3.37 eV). The photoluminescence (PL) spectra show a systematic red-shift in the excitonic levels corresponding to the variation in the optical band-gap. The defect emission from Zn-vacancies is observed in the PL spectra and are further supported by the positron annihilation measurements.

Balanced High Thermoelectric Performance in n-Type and p-Type CuAgSe Realized through Vacancy Manipulation.

As a liquid-like material, CuAgSe has high carrier mobility and ultralow lattice thermal conductivity. It undergoes an n-p conduction-type transition during β- to α-phase transition with increasing temperature. Moreover, optimization of the thermoelectric performance of CuAgSe is rather difficult, owing to the two-carrier conduction in this material. In this work, we reported the free tuning of the conduction type and thermoelectric performance of CuAgSe by manipulating the cation vacancies. Positron annihilation measurements reveal that the increase in CuAg content can effectively suppress the cation vacancies and reduce the hole carrier concentration, resulting in n-type conduction at high temperatures. Doping with Zn at the Cu sublattice in the CuAg-excessive CuAgSe can further decrease the number of vacancies, leading to a significant decrease in hole carrier concentration. Furthermore, the reduction of vacancies leads to weakening of carrier scattering. As a result, carrier mobility is also enhanced, thus improving the thermoelectric performance of n-type CuAgSe. On the other hand, high-performance p-type CuAgSe can be achieved by decreasing the CuAg content to introduce more cation vacancies. Ultimately, both n-type and p-type CuAgSe with superb thermoelectric performance are obtained, with a zTmax of 0.84 in Cu1.01Ag1.02Zn0.01Se (n-type) and 1.05 in (CuAg)0.96Se (p-type) at 600 K and average zT of 0.77 and 0.94 between 470 and 630 K for n-type and p-type, respectively.

Vacancy Suppression Induced Synergetic Optimization of Thermoelectric Performance in Sb-Doped GeTe Evidenced by Positron Annihilation Spectroscopy.

Synergetic optimization of the electrical and thermal transport performance of GeTe has been achieved through Sb doping in this work, resulting in a high thermoelectric figure of merit ZT of 2.2 at 723 K. Positron annihilation measurements provided clear evidence that Sb doping in GeTe can effectively suppress the Ge vacancies, and the decrease of vacancy concentration coincides well with the change of hole carrier concentration after Sb doping. The decreased scattering by hole carriers and vacancies causes notable increase in carrier mobility. Despite this, the density of states effective mass is not enhanced by Sb doping, a maximum power factor of 4562 μW m-1 K-2 at 723 K is obtained for Ge0.94Sb0.06Te with an optimized carrier concentration of ∼3.65 × 1020 cm-3. Meanwhile, the electronic thermal conductivity κe is reduced because of the decreased electrical conductivity σ with the increase of the Sb doping amount. In addition, the lattice thermal conductivity κL is also suppressed due to multiple phonon scattering mechanism, such as the large mass and strain fluctuations by the substitution of Sb for Ge atoms, and also the unique microstructure including grain boundary, nano-pore, and dislocation in the samples. In conclusion, a maximum ZT of 2.2 is gained at 723 K, which contributes to preferable TE property for GeTe-based materials.

Enhanced Sequestration of Chromium by Mechanochemically Silicified Microscale Zerovalent Iron: Role of the Silicate-Modified Surface

Remediation of chromium-contaminated groundwater remains a significant environmental challenge around the world. Herein, we synthesized silicified microscale zerovalent iron (Si-mZVIbm) using a mechanochemical method and demonstrated that the silicate modification could significantly improve the Cr(VI) removal efficiency (up to 37.5-fold) compared with its un-silicified counterpart. Results of atomic force microscopy, scanning transmission electron microscopy, and positron annihilation measurements revealed that silicate acted as a milling lubricant to boost strain within zerovalent iron particles, inducing more plastic deformation and surface defects. The defect-rich silicified surface accelerates the electron transfer and subsequent in situ generation of Fe(II). More importantly, the surface-modified silicate can act as a ligand to coordinate leached Fe(II) ions, thus strengthening the reduction of Cr(VI) via surface-bound Fe(II) and favoring subsequent co-precipitation of Cr(III) and Fe(III) species on Si-mZVIbm surfaces. During column experiments using real Cr-contaminated groundwater, Si-mZVIbm (4 wt % in sand) was able to reduce the Cr(VI) concentration from 2 to 0.05 mg L–1, the World Health Organization drinking water standard for up to 1720 bed volumes with an empty-bed contact time of 5.1 min. These results demonstrate the potential field applicability of Si-mZVIbm in real contaminated groundwater remediation.

Robustness of semimetallic transport properties of TaAs against off-stoichiometric disorder

TaAs single crystals were grown by a standard chemical vapor transport method. The single-crystallinity and homogeneous distribution of elements were confirmed by transmission electron microscope and x-ray diffraction observations. Positron annihilation measurements revealed that the atomic vacancy concentration was kept below 10−5 at. %. However, inductively coupled plasma analysis showed an As-deficient (7–9 at. %) off-stoichiometry. First-principles calculations implied that the off-stoichiometry could be compensated for with excess Ta antisite defects, thereby inducing metallic states. Nevertheless, excellent semimetallic transport properties of a well-suppressed carrier density (≲1018 cm−3), ultrahigh carrier mobility (≳106 cm2/V/s), and large transverse magnetoresistance (>200000% at 9 T) with the quantum oscillation were obtained at 1.7 K. This indicated the robustness of semimetallic transport properties against the off-stoichiometric disorder and the quenching of metallic conduction associated with excess Ta atoms. The negative longitudinal magnetoresistance, which is considered evidence of a Weyl semimetal (chiral anomaly), was not observed. These data were discussed with theoretical calculations.

Effect of monomer moieties on the aggregate structure and gas transport properties of polyimides: Insights from experiments and simulations

AbstractTFDB‐based PIs (BTDA‐TFDB, PMDA‐TFDB and 6FDA‐TFDB) were prepared by the polymerization of 2,2′‐bis(trifluoromethyl) benzidine (TFDB) with three different dianhydrides, which contained flexible carbonyl bridged unit, rigid‐rod moiety without bridged unit, and bulky CF3 bridged unit in dianhydride moieties, respectively. The influence of dianhydride moieties on the aggregate structure and gas permeation performances of polyimide was systematically investigated in relation to their chain packing and free volumes, using the methods of molecular simulation, positron annihilation measurement and X‐ray diffractogram. Changing the dianhydride from benzophenonetetracarboxylic dianhydride (BTDA) to pyromellitic dianhydride (PMDA) and 4,4′‐(hexafluoroisopropylidene) diphthalic anhydride (6FDA), the gas permeation coefficients of polyimide increased, and the gas permeation selectivity (O2/N2, CO2/N2, H2/N2 and CO2/CH4) decreased. With the introduction of flexible carbonyl bridged unit into dianhydride moiety, the high rotational mobility favored tight chains packing and decreased free volumes, which in turn led to the lowest permeation coefficients of BTDA‐TFDB. The bulky CF3 bridged unit in dianhydride moiety greatly restricted chain packing and increased free volumes, thus resulting in the highest permeation coefficients of 6FDA‐TFDB. From BTDA‐TFDB to PMDA‐TFDB and 6FDA‐TFDB, the permeation selectivity decreased. This was mostly ascribed to the reduction of diffusive selectivity, which was resulted from the increase in free volumes.

Operation-induced degradation mechanisms of 275-nm-band AlGaN-based deep-ultraviolet light-emitting diodes fabricated on a sapphire substrate

Degradation mechanisms of 275-nm-band AlxGa1-xN multiple quantum well deep-ultraviolet light-emitting diodes fabricated on a (0001) sapphire substrate were investigated under hard operation conditions with the current of 350 mA and the junction temperature of 105 °C. The optical output power (Po) initially decreased by about 20% within the operating time less than 102 h and then gradually decreased to about 60% by 484 h. For elucidating the causes for the initial and subsequent degradations, complementary electrical, time-resolved photoluminescence (TRPL), and impurity characterizations were carried out making a connection with the energy band profiles. Because the degradation of the wells was less significant than the Po reduction, the initial degradation is attributed essentially to the decrease in carrier injection efficiency (ηinjection), not in internal quantum efficiency of the wells, most likely due to depassivation of initially H-passivated preexisting nonradiative recombination centers (NRCs) in a Mg-doped p-type Al0.85Ga0.15N electron blocking layer. The principal cause for the subsequent Po reduction until 484 h is attributed to further decrease in ηinjection due to the appearance of certain current bypasses in addition to continuous depassivation of the NRCs in p-type AlxGa1-xN layers. According to our database on the species of vacancy-type defects acting as NRCs in GaN and AlN, which have been identified using the combination of positron annihilation and TRPL measurements, vacancy clusters comprised of a cation vacancy (VIII) and nitrogen vacancies (VN), such as VIIIVN2∼4, are the most suspicious origins of the NRCs in Mg-doped p-type AlxGa1-xN layers.

Exploring novel p-n core/shell structure in single α-Fe2O3 nanorods of hierarchical hollow microspheres for ultrasensitive acetone gas sensor

Fabricating metal oxide sensors with super-sensitivity and good distinguishing properties for gases is a challenge due to the difficulties in controlling vacancy-type structural defects of nanomaterials and the data processing. In this paper, we reported the fabrication of hierarchical α-Fe2O3 hollow microsphere (HFHM)-based sensor using a facile and scalable method with uniform-micro spherical carbon templates, as well as the data analysis using artificial intelligence (AI). The shell of the unique hollow microspheres was built by connecting many α-Fe2O3 nanorods, so the superstructures have 0D, 1D, and 3D structural features. In these α-Fe2O3 nanorods, positron annihilation measurements revealed abundant oxygen-vacancy clusters (11 atoms), nanopores (0.53 nm) and p-n core/shell structure. The HFHM-based sensors, hence, exhibited an extremely high sensitivity toward acetone (Response = 320 (200 ppm), limited detection (DL) ∼ 250 ppt) and ethanol (Response = 300 (200 ppm), DL ∼ 500 ppt), as well as a super-fast response time (1–2 s). In particular, by using the Principal Component Analysis (PCA), an applied AI tool, we were able to significantly improve the distinguishing and selective abilities of acetone and ethanol (high-response gas groups) as well as H2, CO and NH3 (low-response gas groups).

Deuterium trapping behavior in tungsten surface due to low-energy ion irradiation

Deuterium (D) irradiation at 421–597 K was performed on polycrystalline tungsten with 100 eV D ions up to a fluence of 4.64 × 1023 m−2. A supersaturated layer in the ion stopping range (< 20 nm) with the concentration of 1.2 at.% under the present surface resolution of ∼ 12 nm, a shallow minimum D concentration area of > 100 nm and a subsequent significant increase in the sub-surface were revealed by combining D (12C, D) 12C elastic recoil detection and D (3He, p) 4He nuclear reaction analysis. The first-principle calculations indicated that the trapping of vacancies for hydrogen (H) atoms can greatly reduce the recombination probability of Frenkel pairs and then act to stabilize the Frenkel pair. The stabilizing effect can be enhanced with increasing number of H atoms. Furthermore, the vacancies from Frenkel pairs can trap more surrounding H atoms, resulting in further H enrichment which could account for the formation of supersaturated layers. The typical location of the blister cavities coincides rather well with the subsequent significant increase in the sub-surface area. D-induced damage in the supersaturated layer and the sub-surface layers was examined by combining scanning electron microscope, transmission electron microscope and positron annihilation measurements.

Vacancy Manipulation Induced Optimal Carrier Concentration, Band Convergence and Low Lattice Thermal Conductivity in Nano‐Crystalline SnTe Yielding Superior Thermoelectric Performance

AbstractSynergetic optimization of electrical and thermal transport properties is achieved for SnTe‐based nano‐crystalline materials. Gd doping is able to suppress the Sn vacancy, which is confirmed by positron annihilation measurements and corresponding theoretical calculations. Hence, the optimal hole carrier concentration is obtained, leading to the improvement of electrical transport performance and simultaneous decrease of electronic thermal conductivity. In addition, the incremental density of states effective mass m* in SnTe is realized by the promotion of the band convergence via Gd doping, which is further confirmed by the band structure calculation. Hence, the enhancement of the Seebeck coefficient is also achieved, leading to a high power factor of 2922 µW m−1 K−2 for Sn0.96Gd0.04Te at 900 K. Meanwhile, substantial suppression of the lattice thermal conductivity is observed in Gd‐doped SnTe, which is originated from enhanced phonon scattering by multiple processes including mass and strain fluctuations due to the Gd doping, scattering of grain boundaries, nano‐pores, and secondary phases induced by Gd doping. With the decreased phonon mean free path and reduced average phonon group velocity, a rather low lattice thermal conductivity is achieved. As a result, the synergetic optimization of the electric and thermal transport properties contributes to a rather high ZT value of ≈1.5 at 900 K, leading to the superior thermoelectric performance of SnTe‐based nanoscale polycrystalline materials.

Change in local structure and hardness in the amorphization process of Zr45Cu45Al10 alloy by heavy ion irradiation

Hypo-eutectic alloy of Zr45Cu45Al10 was irradiated with 16 MeV and 200 MeV Au ions at room temperature. Before and after irradiation, effects of the irradiation on the structure and hardness of the alloy were investigated by using X-ray diffraction (XRD), Extended X-ray absorption fine structure (EXAFS), micro-Vickers hardness and positron annihilation measurements. As a result of the irradiation, amorphization was confirmed for both energies of the irradiations, but the behavior of hardness change strongly depends on the irradiation energy. That is, the hardness once decreases and increases with increasing the fluence in the case of 16 MeV irradiation, but in the case of 200 MeV irradiation, no decreasing trend was observed. The EXAFS measurement shows that the changing ratio of Cu-Cu and Cu-Zr bonds around each Cu atom is different between low energy and high energy Au ion irradiation.

A combined experimental and theoretical study of small and large vacancy clusters in tungsten

Tungsten is considered to be used in the future fusion reactors as plasma-facing material. In such extreme environments, defects are induced in materials that modify their macroscopic properties such as the mechanical ones. It is of paramount importance to be able to determine concentration and size of the vacancy defects, from the mono vacancy to the large cavities, to validate the models developed to predict the evolution of the microstructure of irradiated materials. Positrons are very useful non-destructive probes that can characterize vacancy-type defects in materials. We present a combined experimental and theoretical study on detecting and estimating the sizes of vacancy clusters that are invisible with electron microscopy in tungsten, using positron annihilation spectroscopy. We here model the positron annihilation in the tungsten lattice and in vacancy-type defects using state-of-the-art first principles methodology. The Doppler broadening spectra and positron lifetimes in tungsten are calculated with two-component density functional theory with local density approximation and weighted density approximation. Our calculations are in excellent agreement with our experimental results. We show that the sizes of vacancy clusters in tungsten can be well estimated by combining both positron lifetimes and Doppler broadening spectra. We also determine the limit of validity of the canonical calculation method, which here is shown to fail when the vacancy clusters grow beyond their nucleation stage. This work is a first step needed to better interpret the measured positron annihilation characteristics (Doppler and lifetime) in tungsten and then extract quantitative data on small vacancy defects required to improve the understanding of early-stage vacancy defect evolution in tungsten. The method used in this paper could be used to study other metallic materials.

Magneto-structural correlations in Fe-25 at%Al influenced by substitution of Fe by Co and by thermal treatment

The scanning electron microscopy, X-ray diffraction, positron annihilation spectroscopy, Mössbauer spectrometry, and measurements of magnetic characteristics by vibrating sample magnetometer complemented by theoretical simulations are applied in the present investigations of the Fe-Al-Co alloys with 25 at% Al and Co substituting Fe in amount of 15 at% or 25 at%. The alloys prepared by slightly different technologies resulting in different structural morphologies are subjected to thermal treatment followed by slow cooling and water cooling. It is shown that the alloy with lower Co content and the initial A2 structure is more sensitive to the thermal treatments from the viewpoints of changes in morphology, defects concentration and magnetic properties in comparison to the other one with the initial B2 structure. This is reflected almost in all experimentally obtained results.

Removing the Oxygen-Induced Donor-like Effect for High Thermoelectric Performance in n-Type Bi2Te3-Based Compounds.

Bismuth telluride-based alloys are the best performing thermoelectric materials near room temperature. Grain size refinement and nanostructuring are the core stratagems for improving thermoelectric and mechanical properties. However, the donor-like effect induced by grain size refinement strongly restricts the thermoelectric properties especially in the vicinity of room temperature. In this study, the formation mechanism for the donor-like effect in Bi2Te3-based compounds was revealed by synthesizing five batches of polycrystalline samples. We demonstrate that the donor-like effect in Bi2Te3-based compounds is strongly related to the vacancy defects (VBi‴ and VTe···) induced by the fracturing process and oxygen in air for the first time. The oxygen-induced donor-like effect dramatically increases the carrier concentration from 2.5 × 1019 cm-3 for the zone melting ingot and bulks sintered with powders ground under an inert atmosphere to 7.5 × 1019 cm-3, which is largely beyond the optimum carrier concentration and seriously deteriorates the thermoelectric performance. Moreover, it is found that both avoiding exposure to air and eliminating the thermal vacancy defects (VBi‴ and VTe···) via heat treatment before exposure to air can effectively remove the donor-like effect, producing almost the same carrier concentration and Seebeck coefficient as those of the zone melting ingot for these samples. Therefore, a defect equation of oxygen-induced donor-like effect was proposed and was further explicitly corroborated by positron annihilation measurement. With the removal of donor-like effect and improved texturing via multiple hot deformation (HD) processes, a maximum power factor of 3.5 mW m-1 K-2 and a reproducible maximum ZT value of 1.01 near room temperature are achieved. This newly proposed defect equation of the oxygen-induced donor-like effect will provide a guideline for developing higher-performance V2VI3 polycrystalline materials for near-room-temperature applications.

Relationship between the elastic properties and structure of BaO-TiO2-GeO2-SiO2 glasses

The macroscopic elastic modulus of a solid-state matter is generally affected by the packing density and average bond strength of the constituent atoms. However, because of the diversity of network connections in oxide glasses, an understanding based on the network structure is also required for precise structural categorisation. Here, we examine the relationship between the elastic modulus and structural parameters, viz., the correlation length of the Boson peak and cavity size calculated from the positron annihilation measurement, of BaO-TiO2-GeO2-SiO2 glasses. The Poisson's ratio correlates with the ratio of the longitudinal and transversal sound velocities, and thermal stability parameter ΔT. The Boson peak frequencies of these glasses at the GeO2-rich region exhibit an apparent plateau behavior, indicating that the rate of change in the sound velocity is similar to that in the correlation length. Although the numerical calculation of the cavity in this glass by positron annihilation spectroscopy is unavailable because of the d-electron of Ti cations, the τ1 value shows a similar compositional dependence as the correlation length of the Boson peak. It is expected that a combination of elastic modulus and inelastic light scattering of glasses will be helpful in understanding glass-forming melts.

Zn-Induced Defect Complexity for the High Thermoelectric Performance of n-Type PbTe Compounds.

Although defect engineering is the core strategy to improve thermoelectric properties, there are limited methods to effectively modulate the designed defects. Herein, we demonstrate that a high ZT value of 1.36 at 775 K and a high average ZT value of 0.99 in the temperature range from 300 to 825 K are realized in Zn-containing PbTe by designing complex defects. By combining first-principles calculations and experiments, we show that Zn atoms occupy both Pb sites and interstitial sites in PbTe and couple with each other. The contraction stress induced via substitutional Zn on Pb sites alleviates the swelling stress by Zn atoms occupying the interstitial sites and promotes the solubility of interstitial Zn atoms in the structure of PbTe. The stabilization of Zn impurity as a complex defect extends the region of PbTe phase stability toward Pb0.995Zn0.02Te, while the solid solution region in the other direction of the ternary phase diagram is much smaller. The evolution of defects in PbTe was further explicitly corroborated by aberration-corrected scanning transmission electron microscopy (Cs-corrected STEM) and positron annihilation measurement. The Zn atoms compensate the Pb vacancies (VPb) and Zn interstitials (Zni) significantly improve the electron concentration, producing a high carrier mobility of 1467.7 cm2 V-1 s-1 for the Pb0.995Zn0.02Te sample. A high power factor of 4.11 mW m-1 K-2 is achieved for the Pb0.995Zn0.02Te sample at 306 K. This work provides new insights into understanding the nature and evolution of the defects in n-type PbTe as well as improving the electronic and thermal transport properties toward higher thermoelectric performance.

Nonlinear optical birefringence in Li2SO4-MgO-P2O5 amorphous system -influence of Cu ions

Li2SO4-MgO-P2O5 glasses doped with varied contents of CuO (from 0- 1.0 mol%) were prepared. The optical birefringence induced by a High Q-2 femtosecond pulsed laser is explored as a function of copper oxide concentration in these glasses. The magnitude of the light-induced birefringence exhibited a gradual decrease with the increase of copper oxide content. The observed behavior is discussed in terms of polymerization of the glass network, the degree of which rises with concentration of Cu+ ions occupying Td sites in the glass matrix. However, the samples containing low content of CuO are dominated by Cu2+ ions that act as modifiers and induce structural imperfections by breaking of P-O-S bonds. Accordingly, the samples mixed with lower content of CuO are more favorable for inducing higher magnitudes of the optical birefringence. This conclusion is corroborated by results of electrical conductivity, positron annihilation and ultrasonic measurements.

Irradiation of Ni-Cu ferrite nanoparticles by 241Am-9Be source and investigation of their structural and magnetic properties using positron annihilation spectroscopy

Single-phase Ni-Cu ferrite nanoparticles have been prepared by sol–gel method and exposed using an Am-Be source for time periods of one and two months. The effects of irradiation on structure and magnetic properties of prepared nanoparticles are investigated by X-ray diffraction, positron annihilation spectroscopy and vibrating sample magnetometer. The X-ray measurements showed that unirradiated and irradiated samples have cubic spinel structure and variation of structural parameters with irradiation is nonuniform. The positron annihilation measurements showed that the main effect of irradiation on samples is variation in momentum distribution of core electrons in metallic cations. The magnetic measurements represent that irradiation cause an increase in saturation magnetization and a decrease in coercivity. The increase in saturation magnetization is attributed to cation redistribution because of atomic displacement. The decrease in coercivity is attributed to the variation of magnetocrystalline anisotropy constant due to the effect of irradiation on the core electrons of cations.

Investigation of the formation of defects under fast neutrons and gamma irradiation in 3C–SiC nano powder

In this work cubic phase, silicon carbide nano-powders were irradiated at the high-flux pulsed reactor IBR–2 (Dubna, Russia). The 3C–SiC powder was irradiated with neutron doses up to 1015 n/cm2. The irradiated samples were then analyzed using X-ray diffraction, Raman spectroscopy, Positron annihilation spectroscopy, and Fourier Transform Infrared Spectroscopy. The XRD analysis showed a slight decrease in the lattice parameters with the increase in neutron fluences. The results obtained from positron annihilation measurements were compared to the theoretical calculations, to recognizing the type of structural defect in the samples. A positron lifetime component 355 ps associated with the calculated values for clusters containing of 13–21 vacancies was identified. The concentration of these defects was estimated to be in the region of 5 ppm, and was very similar to the one identified on the unirradiated sample. The results also indicate high irradiation resistivity of the 3C–SiC after irradiation.