High Concentration Of Point Defects Research Articles

Improvement of the Thermoelectric Properties of p-Type Mg3Sb2 by Mg-Site Double Substitution.

A P-type Mg3Sb2-based Zintl phase compound has been considered a promising candidate for thermoelectric applications. Alloying, which introduces a high concentration of point defects, is particularly effective in scattering phonons and reducing lattice thermal conductivity. Herein, alloying in p-type Mg2.995Na0.005Sb2 via the introduction of elements like Yb, Eu, Ca, and Ba was realized, and the room-temperature lattice thermal conductivity has been effectively reduced to ∼1.1 W m-1 K-1. To further intensify the phonon scattering, two groups of elements (Eu and Cd, and Yb and Cd) were chosen for heavy alloying at the Mg site, and the lattice thermal conductivity of Mg1.49Eu0.5Cd1Na0.01Sb2 was further reduced to ∼0.45 W m-1 K-1. Eventually, a peak zT as high as ∼1.0 was achieved at 773 K, and the compound outperforms the previously reported p-type Mg3Sb2 compounds.

Spectrum splitting approach based design simulation for multilayer chalcogenide solar cell architecture

We report design simulation on multilayer solar cell architecture aimed at enhancing the absorption efficiency in antimony sulfide (Sb2S3) absorber based cell via spectrum splitting approach. The poor experimental efficiency chalcogenide absorber (say; ∼8% for Sb2S3) based solar cell has been attributed to inadequate rear contact barrier, a high concentration of point defects (both acceptor and donor) within the bulk, low carrier lifetime, and the presence of defects at the absorber/buffer interface. A hetero-structure design modification vis-à-vis experimental report using bandgap tunability of Sb2S3 via point-focusing spectral splitting technique approach has been implemented to upscale the efficiency. The incident solar spectrum has been split into three well defined regimes for a focussed bandgap matching to enable light absorption and minimize transmission and thermalization losses. The adopted point-focusing spectral splitting technique in this work has resulted in an efficiency of 18.0%, which is an advancement by 2.25 times over the highest reported efficiency of 8.0% in the literature for Sb2S3 absorber based solar cells.

Grain Boundary Migration as a Self-Healing Mechanism of Tungsten at High Temperature

The tungsten components in nuclear fusion reactors need to withstand the radiation cascade damage caused by the neutron bombardment of high temperature and high throughput fusion reaction during service. These damages are mainly present as a high concentration of point defects and clusters, which lead to a series of problems such as irradiation-hardening and decreased thermal conductivity of materials. In this study, molecular dynamics simulations are carried out to study the dynamic interaction between grain boundaries and the void in tungsten at high temperatures (T > 2500 K). Different interatomic potentials of W were tested, and the most appropriate one was selected by the thermodynamic and kinetic properties of W. Simulation results show that the dynamic migration of grain boundary can absorb the void, but the absorption efficiency of grain boundaries is sensitive to their structural characteristics, where the high-angle GBs are more absorptive to the void than the low-angle GBs. It is found that the void absorption cannot be completely attributed to the thermal diffusion mechanism during the GB-void interaction; the dynamic migration of high-angle GBs can significantly accelerate the void absorption. This study reveals a GB migration-induced self-healing mechanism of W at high temperatures.

Dislocations Stabilized by Point Defects Increase Brittleness in PbTe

AbstractDislocations and the residual strain they produce are instrumental for the high thermoelectric figure of merit, zT ≈ 2, in lead chalcogenides. However, these materials tend to be brittle, barring them from practical green energy and deep space applications. Nonetheless, the bulk of thermoelectrics research focuses on increasing zT without considering mechanical performance. Optimized thermoelectric materials always involve high point defect concentrations for doping and solid solution alloying. Brittle materials show limited plasticity (dislocation motion), yet clear links between crystallographic defects and embrittlement are hitherto unestablished in PbTe. This study identifies connections between dislocations, point defects, and the brittleness (correlated with Vickers hardness) in single crystal and polycrystalline PbTe with various n‐ and p‐type dopants. Speed of sound measurements show a lack of electronic bond stiffening in p‐type PbTe, contrary to the previous speculation. Instead, varied routes of point defect–dislocation interaction restrict dislocation motion and drive embrittlement: dopants with low doping efficiency cause high defect concentrations, interstitial n‐type dopants (Ag and Cu) create highly strained obstacles to dislocation motion, and highly mobile dopants can distribute inhomogeneously or segregate to dislocations. These results illustrate the consequences of excessive defect engineering and the necessity to consider both mechanical and thermoelectric performance when researching thermoelectric materials for practical applications.

Neutron irradiation induced defects in oxides and their impact on the oxide properties

Understanding the irradiation-induced defects in oxides is of interest for a wide range of applications. ZnO is an interesting oxide with mixed ionic and covalent bonding that contains a variety of point defect structures—making it an excellent model for studying irradiation-induced defects and their impact on properties. Here, we investigate the effects of neutron irradiation on the formation of defects and on the structural, optical, and electrical properties of ZnO single crystals. We observe the formation of vacancies and voids via positron annihilation spectroscopy. Neutron irradiation led to a significant deterioration of the ZnO structure and formed a high concentration of point defects, vacancy clusters, and voids with large disparities in their structure across variable irradiation times. It also led to significant changes in the optical properties and sample color. Irradiation for 444 h induced a high concentration of Cu acceptors as well as a high concentration of Ga donors. Temperature-dependent Hall effect measurements revealed the competing production of donors and acceptors and showed an increase in the slope of the carrier freeze-out curve with increasing irradiation dose. This work demonstrates the combined effects of neutron irradiation in producing a wide range of structural defects, impurities, and dopants in oxides and their enormous impact on modifying the oxide structure and both the optical and electronic properties. It particularly emphasizes the importance of considering the production of new impurities and dopants during the neutron irradiation of oxides.

2.0–2.2 eV AlGaInP solar cells grown by molecular beam epitaxy

We demonstrate 2.0–2.2 eV AlGaInP solar cells grown by molecular beam epitaxy and their performance improvement by rapid thermal annealing (RTA). As grown, these cells exhibit lower performance than their counterparts grown by metal-organic vapor phase epitaxy (MOVPE), indicating a high concentration of point defects. RTA improves all aspects of performance, enabling our 2.0 eV cells to match the efficiency of MOVPE-grown cells. RTA also improves the performance of wider-bandgap AlGaInP solar cells, but the magnitude of improvement decreases with %Al/bandgap energy.

Phase evolution and properties of (VNbTaMoW)C high entropy carbide prepared by reaction synthesis

(VNbTaMoW)C high entropy carbide was fabricated by reaction synthesis using the constituent metal and graphite powders as raw materials. Phase formation and composition uniformity are analyzed using XRD, EDS and SEM. The phase composition for samples sintered at 1600℃ includes TaC, NbC, WC, MoC and VC according to XRD peak fitting and deconvolution. Samples sintered at 1850℃ form a single phase high entropy carbides, which exhibit high microhardness and relatively lower thermal conductivity of 9.2 W/m.K-1 at room temperature compared with binary carbides. Severe lattice distortion and high concentration of point defects are responsible for the low thermal conductivity of the high entropy (VNbTaMoW)C.

Understanding of electronic and optical properties of ZnS with high concentration of point defects induced by hot pressing process: The first-principles calculations

ZnS is one of the important optical materials for thermal imaging, infrared detecting and laser transmission. Herein, the point defects formation mechanism of hot-pressing ZnS and their influence on optical properties were systematically explored by density functional theory (DFT) to fully understand the experimental observation what the transmittance decreases and the black fogs form with the S vacancies increasing for the hot-pressing ZnS ceramics. Zn (VZn) and S (VS) point vacancies with various amounts of defect concentrations in the range from 0 to 50% were systematically studied to corresponding the experimental results. The result indicates that anion VS are energetically more favorable in regards to cation VZn based on their formation energies. This result explains why only VS were observed in the experiment. Electronic structures show that the band gaps vary from 3.858 eV to 1.569 eV, corresponding to the VS concentration from 0 to 25%. Consequently, the VS also lead to the reflectivity increase and the decrease of the optical transmittance. Continuation of increasing the VS concentrations to 50%, ZnS turns into metallic behavior without a distinctive band gap, so the light fails to pass and maybe cause black fogs formation.

Linking Macroscopic and Nanoscopic Ionic Conductivity: A Semiempirical Framework for Characterizing Grain Boundary Conductivity in Polycrystalline Ceramics.

Understanding the chemical and charge transport properties of grain boundaries (GBs) with high point defect concentrations (beyond the dilute solution limit) in polycrystalline materials is critical for developing ion-conducting solids for electrochemical energy conversion and storage. Elucidation and optimization of GBs are hindered by large variations in atomic structure, composition, and chemistry within nanometers or Ångstroms of the GB interface, which limits a fundamental understanding of electrical transport across and along GBs. Here we employ a novel correlated approach that is generally applicable to polycrystalline materials whose properties are affected by GB composition or chemistry. We demonstrate the connection between the nanoscopic chemical and transport properties of individual boundaries and the macroscopic ionic conductivity in oxygen-conducting Pr0.04Gd0.11Ce0.85O2-δ. The key finding is that GBs with higher solute concentration have lower activation energy for cross-GB ion conduction through a polycrystalline conductor. The resultant semiempirical framework presented here provides a tool for understanding, designing and optimizing polycrystalline ionic conductors.

Sintering behavior and microwave dielectric properties of Li2Mg3Ti(O1-x/2Fx)6 (0.06≤x≤0.15) ceramics for LTCC application

A novel system Li2Mg3Ti(O1-x/2Fx)6 (0.06 ≤ x ≤ 0.15) ceramics were synthesized through the solid stated reaction method. The calcining temperature, sintering behavior and microwave dielectric properties of samples were investigated systematically. The effects of different calcining temperatures for Li2Mg3Ti(O0.96F0.08)6 ceramics on sintering behavior and microwave dielectric properties were discussed. The optimum calcining temperature was obtained at 700 °C because the high point defect concentrations with associated high diffusivities promoted the solid state synthesis reaction. In addition, the phase compositions, microstructure and microwave dielectric properties of Li2Mg3Ti(O1-x/2Fx)6 (0.06 ≤ x ≤ 0.15) ceramics were studied. The ɛr and Q × f values showed the similar tendency with the relative density as x increased. Moreover, the amount of liquid phase also could be a significant factor affecting dielectric properties, which could be confirmed from the change of morphology of grains. Furthermore, the Li2Mg3Ti(O0.96F0.08)6 ceramics calcined at 700 °C and sintered at 950 °C for 6 h exhibited the excellent microwave dielectric properties of ɛr∼14.57, Q × f∼92,450 GHz and τƒ∼-32.91 ppm/°C.

Theoretical investigations of electronic and thermodynamic properties of Ce doped La2Zr2O7 pyrochlore

With ab-initio calculations, the geometrical structure, electronic band structure, optical and thermodynamic properties of pure and Ce doped La2Zr2O7 are calculated. Doping Ce at Zr site modified the intrinsic band gap of La2Zr2O7. However, increasing the Ce doping concentration has no considerable effect on the band structure. With the increase in Ce doping concentration, the absorption spectra of the doped La2Zr2O7 is shifted to higher wavelengths and the peaks intensity is reduced. Some of the peaks associated with the intrinsic La2Zr2O7 are disappeared and new peaks are introduced in 3.40% Ce doped La2Zr2O7. Perturbations in the phonons density of states are reduced with the increasing Ce doping concentration. The La16Zr13Ce3O56 system with 3.40% of Ce doping provided the most stable phonons curve among the simulated systems. Interesting behavior is observed in the thermal conductivity of the doped models with the increasing temperature. The thermal conductivity of La16Zr15CeO56 model increases compared to the pure La2Zr2O7. However, further increase in the Ce doping concentration drastically reduced the thermal conductivity values in the range 373–1673 K. The La16Zr13Ce3O56 system (3.4% Ce) provided the lowest thermal conductivity attributed to the high concentration of point defects affecting the phonon scattering.

Diamond-based superhard composites: new synthesis approaches and application prospects

We discuss experimental results on high-pressure and high-temperature (HPHT) synthesis of diamond-like cubic boron nitride (cBN) and diamond with a high concentration of point defects. The presence of active fluids allows obtaining cBN with a carbon content up to 10%. The concentration of boron in the diamond lattice is about 2%, and that of vacancies is two times higher. The concentration and positions of substitution atoms were determined by the Rietveld refinement method. Diamond micropowder reactive sintering with fluid phases was used to synthesize new superhard composites. The microstructure and grain boundaries were investigated by transmission and scanning electron microscopy. Tests of the wear resistance of the new diamond–cBN composites demonstrated that these materials offer good prospects for various applications.

New Insights into the Effect of Nonstoichiometry on the Electric Response of Strontium Titanate Ceramics

The effect of nonstoichiometry (Sr/Ti ratio from 0.995 to 1.02) on the bulk and grain boundary contributions to the electrical response of strontium titanate (STO) ceramics is investigated. Nonstoichiometric STO exhibits lower electrical resistivity than its stoichiometric counterpart. This decrease is systematic for both Ti- and Sr-excess and with a greater effect on the grain boundary resistivity compared to bulk. Moreover, systematic variations with the degree of nonstoichiometry are observed for bulk and grain boundary conductivity, activation energy, and capacitance. These changes are correlated with a high concentration of point defects and induced by the nonstoichiometry, increasing the charge carrier concentration. As a result, nonstoichiometry can be used to tailor the microstructure and properties of strontium titanate, particularly grain boundary properties.

Time and frequency dependent mechanical properties of LaCoO3-based perovskites: Internal friction and negative creep

The internal friction and creep deformation behavior of La0.8Ca0.2CoO3 and pure LaCoO3 mixed ionic electronic conducting perovskite ceramics have been studied by Dynamic Mechanical Analysis and uniaxial compression under constant applied load, respectively. It was found that both the internal friction and creep strain were almost an order of magnitude higher for Ca2+ doped LaCoO3 as compared to pure undoped LaCoO3. The difference in Ca2+ doped LaCoO3 behavior was attributed to the much higher concentration of point defects (e.g., oxygen vacancies) in the structure and their interaction with other mobile defects, such as ferroelastic domain/twin walls, stacking faults, dislocations, etc. Such interactions of numerous point defects with domain walls produce energetic barriers and slow down the movement of ferroelastic domain walls under applied stress. At the same time, the defects' interactions increase the internal friction resulting in a much higher creep strain of La0.8Ca0.2CoO3 as compared to pure LaCoO3, as the creep strain is determined by the distance between the domain wall and its equilibrium position at the onset of the creep process. Therefore, the high friction will result in the larger distance the wall has to move to reach the equilibrium which in turn results in higher creep strain. The expansion of LaCoO3 under constant applied compressive stress, named here as negative creep, was also discovered to occur during room temperature creep experiments.

Mechanisms on the outstanding high temperature plasticity of AlNp/Al–0.4Cu composites induced by cryogenic treatment

The 8.2AlNp/Al–0.4Cu composites were solution treated by heating to 530 °C for 1 h and then quenched in water and liquid nitrogen, respectively. After quenching in liquid nitrogen, the composites possess nearly the same ultimate tensile strength (UTS) but a significant increase in elongation by 79% (at 350 °C) compared with that of the samples quenched in water. The impact of different quenching process on the microstructure evolution and high temperature tensile properties has been investigated systematically. After quenched in water, there are 69% low–angle grain boundaries (LAGBs), high point defect concentration and a small quantity of dislocations. This microstructure leads to the dynamic recrystallization as well as discordant deformation. The high percentage of high–angle grain boundaries (HAGBs), high dislocation density and low point defect concentration of the composites (quenched in liquid nitrogen) help improve the microstructural stability and contribute to the favorable elongation at 350 °C. The mechanisms revealed in this work may provide a paramount method to achieve a superior combination of strength and elongation for heat resistant Al alloys.

Optimizing the structure and properties of Y2O3 stabilized zirconia: An atmospheric plasma spray (APS) and solution precursor plasma spray (SPPS) based comparative study

The yttria stabilized zirconia (8%YSZ) is widely used to insulate the metallic components of the engine from high temperature and improve the operating temperature of gas turbine engines. With different processing parameters, 8YSZ coatings are prepared by atmospheric plasma spray (APS) and solution precursor plasma spray (SPPS) techniques and the microstructural features and thermodynamics properties are compared. The electron back scattered diffraction (EBSD) analysis indicate that the substitutional point defects (Zr0.86Y0.14O1.93) in the 8YSZ APS coatings are considerably higher than the corresponding SPPS coatings. The replacement of Zr4+ by Y3+ disturbs the charge neutrality of the system which might be compensated by the creation of oxygen vacancy. Both the substitutional point defects and the oxygen vacancies are the sources of phonon scattering, modifying the thermal conductivity of the coating. Pores and cracks are qualitatively and quantitatively analyzed in the microstructure of 8YSZ coatings. Strain tolerant and high thermal cycling life coatings are prepared by SPPS due to the existence of vertical cracks in the microstructure. Comparing the thermal insulation properties of the coatings, the APS coating provided lower thermal conductivities relative to the SPPS coatings which might be due to the high concentration of point defects and low concentration of the mixed oxide phase.

Plastic flow of metals under cyclic change of deformation path conditions

Reverse dislocation slip in metal, forced by cyclic change of deformation path (loading scheme), causes localized plastic flow with formation of dislocation dipoles which subsequently collapse leading to high concentration of point defects. Very low migration energy of point defects, especially self-interstitial atoms, indicating low bonding in a crystal lattice, favors low plastic flow viscosity which in turn leads to low flow stress and high plasticity (superplasticity) of metal. This paper presents the experimental results of KOBO extrusion and KOBO complex rolling, at low (room) temperature. The first method was used to produce thin wires made of MgLi4 magnesium alloy (extrusion ratio λ∼10000), and fully compacted wires made of AZ91 magnesium alloy in the form of machining chips. Basing on the second method, it has been proved possible to change the mechanical properties of strips made of 7075 aluminum alloy without any reduction in their thickness.

Accumulation of dislocation loops in the α phase of Zr Excel alloy under heavy ion irradiation

In-situ heavy ion irradiations were performed on the high Sn content Zr alloy ‘Excel’, measuring <a> type dislocation loop accumulation up to irradiation damage doses of 10 dpa at a range of temperatures. The high content of Sn, which diffuses slowly, and the thin foil geometry of the sample provide a unique opportunity to study an extreme case where displacement cascades dominate the loop formation and evolution. The dynamic observation of dislocation loop evolution under irradiation at 200 °C reveals that <a> type dislocation loops can form at very low dose (0.0025 dpa). The size of the dislocation loops increases slightly with irradiation damage dose. The mechanism controlling loop growth in this study is different from that in neutron irradiation; in this study, larger dislocation loops can condense directly from the interaction of displacement cascades and the high concentration of point defects in the matrix. The size of the dislocation loop is dependent on the point defect concentration in the matrix. A negative correlation between the irradiation temperature and the dislocation loop size was observed. A comparison between cascade dominated loop evolution (this study), diffusion dominated loop evolution (electron irradiation) and neutron irradiation suggests that heavy ion irradiation alone may not be enough to accurately reproduce neutron irradiation induced loop structures. An alternative method is proposed in this paper. The effects of Sn on the displacement cascades, defect yield, and the diffusion behavior of point defects are established.

Impact of intrinsic point defect concentration on thermal transport in titanium dioxide

The thermal conductivity of functional oxide materials can be significantly impacted by variations in point defect concentration, especially at high concentrations where defect interactions can result in extended defects and secondary phase formation. In this work, we systematically study the impact of high point defect concentrations on thermal transport in rutile TiO2. Using atmospherically controlled annealing, we vary equilibrium point defect concentrations and measure the resulting thermal conductivity using time domain thermoreflectance. We verify our results with analytical modeling and find that it is not until very high defect concentrations (> 0.5 mol.%) that the phonon thermal conductivity is impacted. We vary the partial pressure of oxygen to low enough levels that sub-stoichiometric Magnéli phases form and find that these highly defective phases severely reduce the thermal conductivity and anisotropy from intrinsic levels.

Radiation damage induced in Zircaloy-4 by 2.6 MeV proton irradiation

Radiation damage induced in Zircaloy-4 by 2.6 MeV proton irradiation at low doses was studied. Our aim is to emulate the effect of neutron irradiation during early stages of irradiation. X-ray diffraction analysis reveals that the domain size remains constant, while the microstrain increases after irradiation to a dose of 0.017 dpa. The micro-hardness, nano-hardness and Young’s modulus are found to decrease after irradiation to a dose of 0.017 dpa. These indicate, respectively, the formation of a high concentration of point defects, probably vacancy type, and a decrease in the second phase precipitates size.