Grain boundary oxide layers in NdFeB-based permanent magnets

The microstructure of grain boundaries (GBs) in the commercial NdFeB-based alloy for permanent magnets has been studied. It is generally accepted that the unique hard magnetic properties of such alloys are controlled by the thin layers of a Nd-rich phase in Nd$_{2}$Fe$_{14}$B/Nd$_{2}$Fe$_{14}$B GBs. These GB layers ensure the magnetic isolation of Nd$_{2}$Fe$_{14}$B grains from each other. It is usually supposed that such GB layers contain metallic Nd or Nd-rich intermetallic compounds. However, the commercial NdFeB-based permanent magnets frequently contain a tangible amount of neodymium oxide Nd$_{2}$O$_{3}$ at the triple junctions between Nd$_{2}$Fe$_{14}$B grains. The goal of this work was to check whether the Nd$_{2}$Fe$_{14}$B/Nd$_{2}$Fe$_{14}$B GBs could also contain the thin layers of Nd$_{2}$O$_{3}$ oxide phase. Indeed, the screening with EELS-based elemental analysis permitted to observe that some of these Nd-rich layers in Nd$_{2}$Fe$_{14}$B/Nd$_{2}$Fe$_{14}$B GBs contain not only neodymium, but also oxygen. More detailed analysis of such GBs with high-resolution transmission electron microscopy (HR TEM) showed these GB layers are crystalline and have the lattice of neodymium oxide Nd$_{2}$O$_{3}$. In turn, the Lorentz micro-magnetic contrast in TEM permitted to observe that the Nd-oxide GB layers prevent the migration of domain walls from one Nd$_{2}$Fe$_{14}$B grain to another during remagnetization. This finding proves that the GB oxide layers, similar to those of metallic Nd or Nd-rich intermetallic compounds, can ensure the magnetic isolation between Nd$_{2}$Fe$_{14}$B grains needed for high coercivity. Therefore, the GB oxide layers can be used for further development of NdFeB-based permanent magnets

Silicon nanoparticles with a polymer-derived carbon shell for improved lithium-ion batteries: Investigation into volume expansion, gas evolution, and particle fracture

Silicon (Si) and composites thereof, preferably with carbon (C), show favorable lithium (Li) storage properties at low potential, and thus hold promise for application as anode active materials in the energy storage area. However, the high theoretical specific capacity of Si afforded by the alloying reaction with Li involves many challenges. In this article, we report the preparation of small-size Si particles with a turbostratic carbon shell from a polymer precoated powder material. Galvanostatic charge/discharge experiments conducted on electrodes with practical loadings resulted in much improved capacity retention and kinetics for the Si/C composite particles compared to physical mixtures of pristine Si particles and carbon black, emphasizing the positive effect that the core−shell-type morphology has on the cycling performance. Using in situ differential electrochemical mass spectrometry, pressure, and acoustic emission measurements, we gain insights into the gassing behavior, the bulk volume expansion, and the mechanical degradation of the Si/C composite-containing electrodes. Taken together, our research data demonstrate that some of the problems of high-content Si anodes can be mitigated by carbon coating. Nonetheless, continuous electrolyte decomposition, particle fracture, and electrode restructuring due to the large volume changes during battery operation (here, ∼170% in the voltage range of 600−30 mV vs Li+/Li) remain as serious hurdles toward practical implementation

Influence of carbon on the mechanical behavior and microstructure evolution of CoCrFeMnNi processed by high pressure torsion

In this study, a Cantor type high entropy alloys with the addition of C interstitials (, 0, 0.5 and 2 at.%) were processed via high pressure torsion (HPT) under 6.5 GPa by 0.5, 1 and 3 turns at room temperature. The microstructures and mechanical properties of samples before and following HPT were investigated. In all compositions studied, HPT deformation led to a dramatic grain size refinement down to a nanoscale range and also resulted in a considerable increase in dislocation densit

Fundamentals of interface phenomena in advanced bulk nanoscale materials

The review is devoted to a study of interface phenomena influencing advanced properties of nanoscale materials processed by means of severe plastic deformation, high-energy ball milling and their combinations. Interface phenomena include processes of interface defect structure relaxation from a highly nonequilibrium state to an equilibrium condition, grain boundary phase transformations and enhanced grain boundary and triple junction diffusivity. On the basis of an experimental investigation, a theoretical description of the key interfacial phenomena controlling the functional properties of advanced bulk nanoscale materials has been conducted. An interface defect structure investigation has been performed by TEM, high-resolution x-ray diffraction, atomic simulation and modeling. The problem of a transition from highly non-equilibrium state to an equilibrium one, which seems to be responsible for low thermostability of nanoscale materials, was studied. Also enhanced grain boundary diffusivity is addressed. Structure recovery and dislocation emission from grain boundaries in nanocrystalline materials have been investigated by analytical methods and modeling

Ferromagnetic behaviour of ZnO: The role of grain boundaries

The possibility to attain ferromagnetic properties in transparent semiconductor oxides such as ZnO is very promising for future spintronic applications. We demonstrate in this review that ferromagnetism is not an intrinsic property of the ZnO crystalline lattice but is that of ZnO/ZnO grain boundaries. If a ZnO polycrystal contains enough grain boundaries, it can transform into the ferromagnetic state even without doping with “magnetic atoms” such as Mn, Co, Fe or Ni. However, such doping facilitates the appearance of ferromagnetism in ZnO. It increases the saturation magnetisation and decreases the critical amount of grain boundaries needed for FM. A drastic increase of the total solubility of dopants in ZnO with decreasing grain size has been also observed. It is explained by the multilayer grain boundary segregation

ВЗАИМОДЕЙСТВИЕ МНОГОЗАРЯДНЫХ ПРИМЕСЕЙ С ДИСЛОКАЦИЯМИ В МОНОКРИСТАЛЛАХ ГЕРМАНИЯ

Germanium is a relevant object for research into the influence of dislocations on electronic properties of impurities and conversely the influence of impurities on electronic states of dislocations owing to high structural perfection of germanium single crystals and the abundant data available on properties of impurities and defects. We present the results of studies of radiationless and radiation recombination (by the DLTS and photoluminescence (PL) methods, respectively) of charge carriers in deep levels of plastically deformed germanium single crystals doped with multicharge copper or gold impurities by the diffusion method. The recombination parameters (position of the energy levels in the forbidden gap, the value and activation energy of capture cross−section and ionization entropy) of Cu−2/−3 and Au−1/−2 ions determined by DLTS are independent of dislocation density and in good agreement with those in as−grown samples, which is explained by their position outside the Reed cylinders. The parameters of Cu−2 and Au−1 electron capture account for the dependence of the DLTS signal amplitude on filling pulse frequency. After copper doping the methods of transmission electron microscopy (TEM) revealed no precipitates between the dislocations. The intensity of radiation recombination on dislocations at 4.2 K is significantly reduced by copper doping and restored by heating the samples at temperatures above 500 °С as a result of copper diffusion from the bulk toward the dislocations. The specific features of the luminescence spectra of the heated copper−doped samples within the temperature range 200—400 °C are likely to be due to the reactions of the impurities accumulated near the dislocations on cooling the copper−doped samples. Благодаря высокому структурному совершенству монокристаллов германия и имеющейся обширной информации о свойствах примесей и дефектов в нем, этот полупроводник представляется подходящим объектом для изучения влияния дислокаций на электронные параметры примесей и, наоборот, влияния примесей на электронные состояния дислокаций. Представлены результаты исследования методами DLTS и фотолюминесценции (ФЛ) соответственно безызлучательной и излучательной рекомбинации носителей тока на глубоких уровнях в монокристаллах германия, в которые после пластической деформации введены диффузией многозарядные примеси меди или золота. Методом DLTS определены рекомбинационные параметры (положение энергетических уровней в запрещенной зоне, величина и энергия активации сечения захвата электронов и энтропия ионизации) атомов Cu−2/−3 и Au−1/−2. Установлено, что эти параметры не зависят от плотности дислокаций и хорошо согласуются с таковыми в недеформированных образцах, что объясняется их расположением вне цилиндров Рида. Показано, что параметры процесса захвата электронов на атомы Cu−2 и Au−1 объясняют зависимость амплитуды DLTS сигнала от частоты заполняющего импульса. После легирования медью образцы исследовали методом просвечивающей электронной микроскопии. Преципитаты между дислокациями не выявлены. Обнаружено, что интенсивность излучательной рекомбинации на дислокациях при 4,2 К, сильно пониженная после введения меди, восстанавливается после нагрева образцов при температурах выше 500 °C вследствие диффузии меди из объема к дислокациям. Особенности спектров ФЛ после нагрева образцов с медью в интервале 200—400 °C обусловлены, вероятно, реакциями примесей, собранными вблизи дислокаций при охлаждении образцов после введения меди.