Design and Qualification of Pr–Fe–Cu–B Alloys for the Additive Manufacturing of Permanent Magnets

The direct use of an advanced binder-free additive manufacturing technique, namely laser powder bed fusion (L-PBF), does not easily allow obtaining variously shaped, fully dense Nd–Fe–B magnets with high coercivity. The process inherently leads to the re-melting of the powder and appearance/disappearance of undesired/desired microstructural features responsible for low and large coercivity. In this work, the development of a useful microstructure responsible for high coercivity in Pr₂₁Fe_73.5Cu₂B_3.5 and Nd₂₁Fe_73.5Cu₂B_3.5 alloys and a possible way to produce fully dense permanent magnets via additive manufacturing processes is demonstrated using: (i) suction casting technique, which provides a high cooling rate and thus similar microstructures as in L-PBF but requires only very small amounts of powder; (ii) conventional L-PBF processing using kg of powder, and (iii) a subsequent annealing treatment that is similar to a conventional sintering treatment. The subsequent heat treatment is necessary to develop high coercivity by forming a novel microstructure: hard magnetic (Nd,Pr)₂Fe₁₄B grains embedded in a matrix of intermetallic (Nd,Pr)₆Fe₁₃Cu phase. Furthermore, it is demonstrated that Pr₂₁Fe_73.5Cu₂B_3.5 exhibits a higher coercivity than Nd₂₁Fe_73.5Cu₂B_3.5 because of a finer and more homogeneous grain size distribution of the Pr₂Fe₁₄B phase. The final L-PBF printed Pr₂₁Fe_73.5Cu₂B_3.5 samples provide a coercivity of 0.75 T.     

Photon-assisted capacitance–voltage study of organic metal–insulator–semiconductor capacitors

The results are reported of a detailed investigation into the photoinduced changes that occur in the capacitance–voltage (C–V) response of an organic metal–insulator–semiconductor (MIS) capacitor based on the organic semiconductor poly(3-hexylthiophene), P3HT. During the forward voltage sweep, the device is driven into deep depletion but stabilizes at a voltage-independent minimum capacitance, C_min, whose value depends on photon energy, light intensity and voltage ramp rate. On reversing the voltage sweep, strong hysteresis is observed owing to a positive shift in the flatband voltage, V_FB, of the device. A theoretical quasi-static model is developed in which it is assumed that electrons photogenerated in the semiconductor depletion region escape geminate recombination following the Onsager model. These electrons then drift to the P3HT/insulator interface where they become deeply trapped thus effecting a positive shift in V_FB. By choosing appropriate values for the only disposable parameter in the model, an excellent fit is obtained to the experimental C_min, from which we extract values for the zero-field quantum yield of photoelectrons in P3HT that are of similar magnitude, 10^?5 to 10^?3, to those previously deduced for π-conjugated polymers from photoconduction measurements. From the observed hysteresis we deduce that the interfacial electron trap density probably exceeds 10¹⁶ m^?2. Evidence is presented suggesting that the ratio of free to trapped electrons at the interface depends on the insulator used for fabricating the device.     

Lifetime of excess electrons in Cu–Zn–Sn–Se powders

The method of time-resolved microwave photoconductivity at a frequency of 36 GHz in the range of temperatures of 200–300 K is used to study the kinetics of the annihilation of charge carriers in Cu–Zn–Sn–Se powders obtained by the solid-phase method of synthesis in cells. The lifetime of excess electrons at room temperature is found to be shorter than 5 ns. The activation energy for the process of recombination amounted to E _a ~ 0.054 eV.     

Influence of growth conditions of oxide on electrical properties of AlGaN/GaN metal–insulator–semiconductor transistors

AlGaN/GaN metal–insulator–semiconductor high-electron-mobility transistors(MIS-HEMTs) on a silicon substrate were fabricated with silicon oxide as a gate dielectric by sputtering deposition and electron-beam(EB) evaporation. It was found that the oxide deposition method and conditions have great influences on the electrical properties of HEMTs. The low sputtering temperature or oxygen introduction at higher temperature results in a positive equivalent charge density at the oxide/AlGaN interface(Nequ), which induces a negative shift of threshold voltage and an increase in both sheet electron density(ns) and drain current density(ID). Contrarily, EB deposition makes a negative Nequ, resulting in reduced ns and ID. Besides, the maximum transconductance(gm-max) decreases and the off-state gate current density(I_G-off) increases for oxides at lower sputtering temperature compared with that at higher temperature, possibly due to a more serious sputter-induced damage and much larger Nequ at lower sputtering temperature. At high sputtering temperature, I_G-off decreases by two orders of magnitude compared to that without oxygen, which indicates that oxygen introduction and partial pressure depression of argon decreases the sputter-induced damage significantly. I_G-off for EB-evaporated samples is lower by orders of magnitude than that of sputtered ones, possibly attributed to the lower damage of EB evaporation to the barrier layer surface.     

Thermal Conductivity of Indium–Graphene and Indium-Gallium–Graphene Composites

Samples of graphene composites with a matrix of indium or indium-gallium alloy were prepared in the form of foils using exfoliated graphene dispersions. The thermal conductivity of the composite samples with different thicknesses was determined using the three-omega method. Indium–graphene composite samples with a thickness of 430 μm exhibited a twofold increase in thermal conductivity, whereas indium-gallium–graphene composite samples with a thickness of 330 μm exhibited a threefold improvement in thermal conductivity over that of the matrix at 300 K. The effective medium approximation (EMA) was used to model the thermal conductivity of the composite samples. The graphene platelet size distribution was used to determine the average thermal conductivity of graphene in the composite samples. The interfacial thermal conductance between graphene and indium or indium-gallium alloy determined from EMA was not the limiting factor in the improvement of the thermal conductivity of the composite samples, although the increase in thermal conductivity was found to be slightly lower than predicted theoretically using acoustic and diffuse mismatch models. The smaller size of the graphene platelets obtained by exfoliation prior to dispersion in the matrix appears to be the limiting factor.     

Effect of the chemical composition of Cu–In–Ga–Se layers on the photoconductivity and conversion efficiency of CdS/CIGSe solar cells

The effect of the [Ga]/[In+Ga] ratio of gallium and indium on the microwave photoconductivity of Cu–In–Ga–Se (CIGSe) films and on the efficiency of solar cells fabricated in accordance with the same technology is investigated. According to the observations of a field-emission scanning electron microscopy (FESEM), the grain size decreases with increasing Ga content. With increasing gallium content in the samples, the photogenerated-electron lifetime and the activation energy of the microwave photoconductivity also decrease. The changes in the activation energy of the through conduction in darkness are less than 20%. Analysis of the obtained data shows that the known effect of the gallium gradient on the efficiency should be associated with modification of the internal structure of grains instead of with their boundaries.     

CVD-growth of CNT with the use of catalutic Ct–Me–N–O thin films incorporated in the technology

The process of the formation of carbon nanotube arrays on Ct–Me–N catalytic alloys of low nickel content (10–20 at %) by chemical vapor deposition, where Ct is a catalytic metal from the group of Ni, Co, Fe, and Pd, and Me is a transition metal of group IV–VII of the periodic table, was investigated. It is shown that CNT grow effectively when the alloy contains Ti, V, Cr, Zr, Hf, Nb, and Ta. The addition of nitrogen and oxygen to the alloy’s composition gives rise to a buildup of oxynitrides, expelling of the catalyst, and formation of its clusters on the surface. The replacement of metals in the alloy has an effect on the diameter of the CNT. Moreover, the alloy films 10–500 nm thick can be used for the CNT growth, which is responsible for high degree of homogeneity and the repeatability of the process. CNT growth was not observed when the alloy contained W and Re.     

Numerico–Analytical Model of a Non-Planar Array of Horns

A non-planar array of strip horns is considered. A model of this structure is constructed in the form of a Floquet channel with smoothly varying parameters. The Floquet channel of an infinite array of coupled striplines is chosen as a reference waveguide. The scattering matrix of the Floquet channel in the quasi-periodic mode is found in the approximation of the theory of microwave transmission lines. Expressions are obtained for the elements of the scattering matrix in the mode of excitation of a single channel of a non-planar array. In this mode, the directional pattern of the radiation of the array into a planar waveguide is studied.     

Ab initio studies of the band parameters of III–V and II–VI zinc-blende semiconductors

Electronic band-structure calculations are performed for zinc-blende III–V (AlP, AlAs, AlSb, GaP, GaAs, GaP, InP, InAs, and InSb) and II–VI (ZnS, ZnSe, ZnTe, CdS, CdSe, and CdTe) semiconductors using an ab initio pseudopotential method within a local-density approximation (LDA). Lattice parameters, band gaps, Luttinger parameters, momentum matrix elements and effective masses are studied in detail. It is shown that LDA calculations cannot systematically give accurate band parameters. It is found that LDA band parameters calculated using experimentally determined lattice constants are more accurate than those using LDA lattice constants. We found that inclusion of the d electrons of Group-II atoms in the core gives more accurate band parameters.     

Use of benzothiadiazole–triphenylamine amorphous polymer for reproducible performance of polymer–fullerene bulk-heterojunction solar cells

An amorphous polymer, poly(BTD-TPA), which consists of benzothiadiazole and triarylamine units, can be successfully utilized to fabricate bulk heterojunction (BHJ) organic photovoltaics (OPVs), and the OPV performance can be demonstrated to be independent of the casting solvent or thermal annealing temperature. The OPV based on poly(BTD-TPA):PC₇₀BM (1:4) that was fabricated using chloroform (boiling point of 61 °C) and annealed at 60 °C for 10 min exhibited a power conversion efficiency (PCE) of 2.81% under simulated solar irradiation through an air mass of 1.5 at 100 mW cm⁻². On the other hand, the OPV fabricated using o-dichlorobenzene (boiling point of 181 °C) and annealed at 110 °C for 10 min exhibited a PCE of 2.65%. Almost the same PCEs and incident photon to current conversion efficiencies (IPCEs) were obtained in both OPVs. The use of an amorphous film of poly(BTD-TPA) in the fabrication of OPVs offers great advantages over the use of a polycrystalline film of regioregular poly(3-hexylthiophene) (P3HT) in terms of high reproducibility of the OPV performance.     

Modeling of copper–carbon solid solutions

The atomistic simulations in the framework of the Generalized Simulated Annealing approach (GSA) and classical force fields lead to very reasonable relaxed geometries around the carbon interstitial in O-, T-, and TS-sites. We have thus shown that a highly efficient energy-sampling and relaxation scheme, implemented with tight constraints on a limited volume, provides a powerful steering mechanism for selection of geometries suitable for detailed investigation by first-principles methods. The results, based upon harmonic interactions between Cu atoms and a van der Waals interaction between Cu and C, predict the relaxed O-site to be more stable than the T-site by ∼1.2 eV, in accordance with general expectations. The TS barrier to OO diffusion is found to be ∼0.8 eV, at a temperature of 0 K; the TS exhibits a strong local axial distortion of the pseudo-octahedral environment. The Density Functional results indicate a charge transfer of ∼1 e to carbon, mostly from the first neighbor shell, in all relaxed environments studied. Bond-order data show the Cu–C interaction to be bonding in nature, despite the net ‘repulsive interaction’ leading to a surface state of lower net energy.     

Effect of Nonlinearity of the Phonon Spectrum on the Thermal Conductivity of Nanostructured Materials Based on Bi–Sb–Te

A theoretical investigation of the lattice thermal conductivity of nanostructured materials based on Bi–Sb–Te is presented. The calculations were based on relaxation time approximation and took into account both the real phonon spectra, obtained from first-principles by use of density functional theory, and the anisotropy of phonon relaxation time. Phonon relaxation time data were determined from experimental values of the lattice thermal conductivity. The decrease of the thermal conductivity caused by the nanostructure was compared with results from calculations based on the linear Debye approach. Estimation showed that phonon boundary scattering can lead to a 55% decrease of thermal conductivity for a grain size of ~20 nm in the Debye approximation. Taking the nonlinearity of the acoustic phonon spectrum into account leads to a 20% larger decrease of the thermal conductivity because of boundary scattering. The reason is that consideration of the real phonon spectrum increases the relative contribution to thermal conductivity of acoustic phonons with low frequencies that are scattered more strongly at nanograin boundaries. Similarly, estimation of lattice thermal conductivity reduction as a result of phonon scattering by nanoinclusions gave an 8% larger decrease when the real phonon spectrum was used rather than the linear Debye approximation. For such a substantial decrease of lattice thermal conductivity, the effect of the optical phonons was estimated; it was shown that optical phonons can reduce the change of thermal conductivity as a result of grain boundary scattering by no more than 10%. Finally, the minimum lattice thermal conductivity was estimated to be 0.07 W/m K because of acoustic modes (0.09 W/m K in the Debye approach) and 0.14 W/m K when the contribution of optical modes was also taken into consideration.     

Altering regularities on resistances of doped Au–alkanedithiol–Au junctions

The electronic transport properties of alkanedithiol molecular junctions doped by boron (B) or phosphorus (P) on different sites are investigated by using the non-equilibrium Green’s function method combined with the density functional theory. Results show that B or P doping can decrease the resistances of alkanedithiol molecular junctions obviously and the direct-current conductance becomes stronger gradually when doped sites vary from side to center. Namely, the B or P doped effects are not uniform which are sensitive to its doped sites. Interestingly, significant negative differential resistance behaviors are only found in B-doped molecular junctions and peak-to-valley ratios are also sensitive to the B doped sites.     

Electrosynthesis of organic–inorganic compounds (p–n heterojunction)

An electrochemical deposition procedure by cyclic voltammetry, in an electrolyte solution was adopted for the preparation of thin films of polypyrrole–gallium arsenide composite materials. The properties of the composite layers were studied by cyclic voltammetry, electrochemical impedance spectroscopy and photoelectrochemical measurements. The p- and n-type semiconductor behaviour of the polypyrrole (PPy) and gallium arsenide (GaAs) were studied by photocurrent measurements. It was found that the composite material (PPy–GaAs) had a (p–n) heterojunction behaviour.     

Triangular representation of Fornasini–Marchesini systems

We further develop the study of Fornasini–Marchesini linear systems with upper triangular state operators, addressing the problem of constructing a triangular Fornasini–Marchesini model equivalent (under a proper definition) to a given system. In particular, we are interested in the problem of determining when such a system can be constructed, without losing the information about the state of the original system.

Electron Microscopy Study of Ferroelectric Memory Based on Si–SiO2–Ti–Pt–PZT Multilayer Structure

Model Si–SiO₂–Ti–Pt–PZT multilayer structures obtained by chemical solution deposition at excessive (relative to the stoichiometric composition) amounts of lead in the starting film-forming solution (x= 0–30 mol %) were studied by TEM and X-ray microanalysis techniques. In the absence of excessive lead, the films crystallize largely into the metastable phase of pyrochlore, which does not have ferroelectric properties. With an excessive amount of lead added, PZT ceramic crystallizes into the ferroelectric phase of perovskite and has columnar grains 0.2 m in size. The thermal stability of the metallization system during the formation of the ferroelectric film was investigated. A 180-nm-thick transition layer between the Pt electrode and the adhesive titanium film was discovered. This layer, resulting from high-temperature synthesis, consists of fine-grain platinum, as well as metal oxides and silicides.     

Limiting amplitude–frequency characteristics of the output cavities of klystrons

An analytical ratio determining the maximum possible amplitude of the interaction impedance of the output cavity of the klystron generally connected to N passive cavities forming a filter system is obtained.