Absorption and emission properties of Bi-doped Mg–Al–Si oxide glass system

A new Bi-doped Mg–Al–silicate glass is suggested and investigated. It can be fabricated by moderate-temperature routine technology. The characteristic relaxation time of 300–800 μs in combination with the high quantum yield (up to 85%) and wide excitation spectrum makes this glass a promising laser material. The obtained quadratic dependence of the visible absorption intensity is an argument in favor of the hypothesis that the absorption and infrared luminescence in Bi-doped glasses are caused by Bi₂ dimers.     

Effect of chain length and donor–acceptor substitution on the electrical responsive properties of conjugated biphenyls: a DFT-based computational study

The effect of donor–acceptor (D-A) substituent and chain length on the electrical polarisabilities and first hyper polarisability of cis and trans biphenyl oligomeric compounds have been investigated using density functional theory-based hybrid functional CAM-B3LYP with 6-311G (2d,2p) basis set. Our extensive computational study reveals that both average polarisability and first hyper polarisability of the studied compounds increase with the increasing ethylene spacer chain length. Again the substitution of donor (NMe₂) and acceptor (C≡N) at the para position of the phenyl rings to each oligomer shows order of magnitude increase of both α_av and β_av value compared to the unsubstituted one. This increased α_av and β_av values have been explained due to increasing charge transfer contribution resulting from decreasing optical energy gap (ΔE?=?S₁???S₀) upon D-A substitution. It is also observed that the charge transfer contribution to first hyperpolarisability (β_CT) is more than the polarisability (α_CT) for the studied molecules. The electronic spatial extent (<R²>) which is a measure of electron density volume around the molecule is found to play a major role along with the intramolecular charge transfer character to explain the non-linear variation of first hyperpolarisability (β_av) as a function of ethylene spacer chain length (n) and D-A substitution.     

Sulfur substitution and pressure effect on superconductivity of α-FeSe

In this paper, sulfur substitution and pressure effect on superconductivity of α-FeSe has been investigated in Fe(Se_1 ? xS_x)_0.88 (x = 0.1, 0.2). For x = 0.1, the critical temperature T_c is slightly larger than that of non-substituted sample, in consistent with the pressure effect on the superconductivity of α-FeSe. However, with further increasing S content to x = 0.2, T_c decreases. Temperature dependent of specific heat showed that the structural transition seems to be suppressed. T_c for x = 0.2 can be further decreased by applying pressure of 5 kbar, in contrary to the pressure effect on α-FeSe. We suggest that, in addition to the suppression of structural transition, other factors like the increase of carrier concentration should be considered for understanding the pressure effect on the superconductivity of α-FeSe.     

Counter anion effect on structural,opto-electronic and charge transport properties of fused π-conjugated imidazolium compound

The structure–activity relationship of fused π-conjugated imidazolium cation with three counter anion molecules, BF₄^?, CF₃SO₃^? and (CF₃SO₂)₂N^?, was studied using electronic structure calculations. The structural, opto-electronic and charge transport properties of these complexes were studied. The charge transfer from π-conjugated imidazolium(I) to counter anion was confirmed in all the studied complexes. Interaction energy varies significantly depending on the counter anion and the stability was found higher for I-BF₄ complex than both I–CF₃SO₃ and I–(CF₃SO₂)₂N complexes. The strong (C–H)⁺···F^? hydrogen bond of length 1.95 Å between fused π-conjugated imidazolium and BF^?₄ anion is the driving force for the strongest interaction energy in I–BF₄ complex. The energy decomposition analysis confirms that the interaction between imidazolium and counter anion is mainly driven by electrostatic and orbital interaction. It has been observed that the absorption spectra of the complex are independent of anion nature but the influence of anion character is observed on frontier molecular orbital pattern. The charge transport property of I–BF₄ complex was studied by using tight-binding Hamiltonian approach and found that the hole mobility in I–BF₄ is 1.13 × 10^?4 cm² V^?1 s^?1.     

The Chiral Bag Model and properties of the πNΔ system

The effective Hamiltonian for theN system is proposed in the framework of the Chiral Bag Model (CBM). The taking into account of gluonic and pionic correctinos to the pion-nucleon coupling constantg _N and the-decay axial constantg _A leads tog _N 13·5,g _A 1·28 which are in good agreement with experimental data.Presented at the symposium Mesons and Light Nuclei, Liblice, Czechoslovakia, June 1981.I wish to thank Gulamov K. G., Shelest V. P. and Zinoviev G. M. for useful discussions.     

Mixed heavy metal effect on emission properties of Er~(3+)-doped borosilicate glasses

Er3+-doped heavy metal borosilicate glasses were prepared using conventional melting and quenching method. The emission spectra of 4I13/2 → 4I15/2 transition were observed upon excitation at 974 nm and the lifetime of 4I13/2 level of Er3+ was measured. Based on these data, the fluorescence properties of Er3+ are investigated on the emission and gain characteristics at the 1.5 μm bands. In particular, the effect of relative heavy metal content on fluorescence properties is discussed.     

Study of impact properties and morphology of unidirectional bamboo strips–polyester composites: effect of mercerization

Composites were fabricated with alkali (10%, 15% and 20%) treated bamboo fibers incorporated to the extent of 50% (by volume) and polyester resin. Impact test was made on bamboo strips and composites samples. It was observed that the fracture energy undergoes an increase from untreated to alkali treated bamboo strips. Alkali treated bamboo fiber composites show higher fracture energy value than the untreated bamboo fiber composites. The phenomenal change was quite evident from scanning electron micrographs of the fractured surface. Considering the effects of all factors, the work of fracture shows maximum value in the case of 20% alkali treated bamboo strip composites. It was also observed that the rate of increase of work of fracture value is much higher in the case of composite samples than the bamboo sample. It was explained on the basis of synergism obtained by improved interfacial adhesion between fiber and matrix, and removal of hemicellulose from the bamboo itself, which was evident from the intermolecular H-bonding formation studied by FT-IR.     

Effects of La–Zn substitution on microstructure and magnetic properties of strontium ferrite nanofibers

Sr_1−x La _x Zn _x Fe_12−x O₁₉/poly(vinylpyrrolidone) (PVP) (0.0≤x≤0.5) precursor nanofibers were prepared by the sol–gel assisted electrospinning method from starting reagents of metal salts and PVP. Subsequently, the Sr_1−x La _x Zn _x Fe_12−x O₁₉ nanofibers with diameters of around 100 nm were obtained by calcination of the precursor at 800 to 1000°C for 2 h. The precursor and resultant Sr_1−x La _x Zn _x Fe_12−x O₁₉ nanofibers were characterized by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectrometer and vibrating sample magnetometer. The grain sizes of Sr_0.8La_0.2Zn_0.2Fe_11.8O₁₉ nanofibers are in a nanoscale from 40 to 48 nm corresponding to the calcination temperature from 800 to 1000°C. With La–Zn substitution content increase from 0 to 0.5, the grain size and lattice constants for the Sr_1−x La _x Zn _x Fe_12−x O₁₉ nanofibers obtained at 900°C show a steady reduction trend. With variations of the ferrite particle size arising from the La–Zn substitution, the nanofiber morphology changes from the necklace-like structure linking by single elongated plate-like particles to the structure building of multi-particles on the nanofiber cross-section. The specific saturation magnetization of Sr_1−x La _x Zn _x Fe_12−x O₁₉ nanofibers initially increases with the La–Zn content, reaching a maximum value 72 A m² kg⁻¹ at x=0.2, and then decreases with a further La–Zn content increase up to x=0.5, while the coercivity exhibits a continuous reduction from 413 (x=0) to 219 kA m⁻¹ (x=0.5). The mechanism for the La–Zn substitution and the nanofiber magnetic property are analyzed.     

Magnetic and microstructural properties of M-type strontium hexaferrites with Pr–ZnAl substitution

We have investigated the influence of Pr–ZnAl substitution on the magnetic and microstructural properties of M-type strontium hexaferrites Sr_1.0-xPr_xFe_12.0-x(Zn_0.5Al_0.5)_xO₁₉ (0.0 ≤ x ≤ 0.5) synthesized by the standard ceramic method. X-ray diffraction (XRD) was carried out to determine the crystal structure and the phase identification of the hexaferrites showed that a single magnetoplumbite phase was exhibited in the hexaferrites with Pr–ZnAl content (x) from 0.0 to 0.4 and impurity phase α-Fe₂O₃ was observed in the structure when Pr–ZnAl content (x)?=?0.5. The morphology of the hexaferrites was analyzed by a field emission scanning electron microscopy (FE-SEM). The representative FE-SEM micrographs showed that the particles were regular hexagonal platelets and the average grain size basically kept unchanged with increasing Pr–ZnAl content (x). A magnetic property measurement system was used to measure the magnetic properties of the hexaferrites. The remanence (B_r), maximum energy product [(BH)_max] and H_k/H_cj ratio decreased with increasing Pr–ZnAl content (x) from 0.0 to 0.5. The intrinsic coercivity (H_cj) and magnetic induction coercivity (H_cb) first increased with increasing Pr–ZnAl content (x) from 0.0 to 0.1, and decreased with increasing Pr–ZnAl content (x) from 0.1 to 0.2, and then increased when Pr–ZnAl content (x) ≥ 0.2.     

Magnetic behavior in the incommensurate phase ∼LaCrS3:: effect of the partial substitution of Cr by Ti on the magnetic properties

Dilution of the magnetic interactions between Cr³⁺ ions by Ti³⁺ ions was observed in the CrS₂ layer of the misfit-layer compound ∼LaCrS₃. Pure ∼LaCrS₃ has complex magnetic properties which are reminiscent of spin glass behavior. This magnetic behavior comes from both the modulated character of the structure and the magnetic frustration of the planar-antiferromagnetic-triangular network of Cr³⁺ ions. Thus, there is a large hysteresis between the zero field cooled and the field cooled magnetic susceptibility curves below the transition temperature (≈75 K). Formation of a solid solution ∼LaCr_1−xTi_xS₃ by the addition of Ti³⁺ ions results in the decrease of transition temperature up to a doping level of x≈0.5, where the transition is no longer observed. The magnetic behavior of the phase with x≈0.5 is similar to that of several random exchange antiferromagnetic compounds.     

Magnetocaloric effect and multifunctional properties of Ni–Mn-based Heusler alloys

The studies of magnetocaloric properties, phase transitions, and phenomena related to magnetic heterogeneity in the vicinity of the martensitic transition (MT) in Ni–Mn–In and Ni–Mn–Ga off-stoichiometric Heusler alloys are summarized. The crystal structure, magnetocaloric effect (MCE), and magnetotransport properties were studied for the following alloys: Ni₅₀Mn_50−xIn_x, Ni_50−xCo_xMn₃₅In₁₅, Ni₅₀Mn_35−xCo_xIn₁₅, Ni₅₀Mn₃₅In₁₄Z (Z=Al, Ge), Ni₅₀Mn₃₅In_15−xSi_x, Ni_50−xCo_xMn_25+yGa_25−y, and Ni_50–xCo_xMn_32−yFeyGa₁₈. It was found that the magnetic entropy change, ΔS, associated with the inverse MCE in the vicinity of the temperature of the magneto-structural transition, TM, persists in a range of (125-5) J/(kg K) for a magnetic field change ΔH=5 T. The corresponding temperature varies with composition from 143 to 400 K. The MT in Ni₅₀Mn_50−xIn_x (x=13.5) results in a transition between two paramagnetic states. Associated with the paramagnetic austenite-paramagnetic martensite transition ΔS=24 J/(kg K) was detected for ΔH=5 T at T=350 K. The variation in composition of Ni₂MnGa can drastically change the magnetic state of the martensitic phase below and in the vicinity of TM. The presence of the martensitic phase with magnetic moment much smaller than that in the austenitic phase above TM leads to the large inverse MCE in the Ni₄₂Co₈Mn_32−yFeyGa₁₈ system. The adiabatic change of temperature (ΔT_ad) in the vicinity of TC and TM of Ni₅₀Mn₃₅In₁₅ and Ni₅₀Mn₃₅In₁₄Z (Z=Al, Ge) was found to be ΔT_ad=−2 K and 2 K for ΔH=1.8 T, respectively. It was observed that |ΔT_ad|≈1 K for ΔH=1 T for both types of transitions. The results on resistivity, magnetoresistance, Hall resistivity in some In-based alloys are discussed.     

Synthesis,thermionic emission and magnetic properties of （NdxGdl-x）B6

Polycrystalline rare-earth hexaborides （NdxGdl-x）B6 （x = 0, 0.2, 0.6, 0.8, 1） were prepared by the reactive spark plasma sintering （SPS） method using mixed powder of GdH2, NdH2 and B. The effects of Nd doping on the crystal structure, the grain orientation, the thermionie emission and the magnetic properties of the hexaboride were investigated by X-ray diffraction, electron backscattered diffraction and magnetic measurements. It is found that all the samples sintered by the SPS method exhibit high densities （〉95%） and high values of Vickers hardness （2319 kg/mm2）. The values are much higher than those obtained in the traditional method. With the increase of Nd content,the thermionic emission current density increases from 11 to 16.30 A/cm2 and the magnetic phase transition temperature increases from 5.85 to 7.95 K. Thus, the SPS technique is a suitable method to synthesize the dense rare-earth hexaborides with excellent properties.     

Effect of hopping rate and jump length of hopping electrons on the conductivity and dielectric properties of Co–Cd ferrite

The cobalt cadmium ferrite has an n-type conductivity. As the Cd addition (x) increases, the hopping rate increases and so the conductivity increases up to x=0.5. A sudden decrease in the hopping rate occurs above x=0.5 due to the decrease of Co²⁺ ions on octahedral sites. Above x=0.7, the hopping rate increases again and the conductivity increases. As the hopping length increases the conductivity decreases, i.e. conductivity is inversely proportional to hopping length, but is directly proportional to the hopping rate of electrons. As the ferrous ions increase on octahedral sites, the conductivity increases and the jumping length decreases leading to the increase of dielectric constant.     

Solvent effects on structure and optical properties of a D- π-A azobenzene dye

Time-dependent hybrid density functional theory in combination with Onsager reaction field model and super-molecular model has been applied to study solvent effects on the geometrical and electronic structures, as well as one/two-photon absorption properties, of 4-（N-（2-hydroxyethyl）-N-methyl）-amino-4＇-nitroazobenzene. It is found that the short-range interaction has a large effect on the electronic structure of the solute molecule, namely, large red-shift of the maximum one-photon absorption is induced by hydrogen bonding. The solute molecule has a large two-photon absorption cross section, which is enhanced by the solvent effect. The computational results are in good agreement with measurements.[第一段]     

Influence of manganese substitution and annealing temperature on the formation,microstructure and magnetic properties of Mn–Zn ferrites

The effects of annealing temperature and manganese substitution on the formation, microstructure and magnetic properties of Mn_xZn_1−xFe₂O₄ (with x varying from 0.3 to 0.9) through a solid-state method have been investigated. The correlation of the microstructure and the grain size with the magnetic properties of Mn–Zn ferrite powders was also reported. X-ray diffraction (XRD), a scanning electron microscope (SEM) and a vibrating sample magnetometer (VSM) were utilized in order to study the effect of variation of manganese substitution and its impact on crystal structure, crystalline size, microstructure and magnetic properties of the ferrite powders formed. The XRD analysis showed that pure single phases of Mn–Zn ferrites were obtained by increasing the annealing temperature to 1200–1300 °C. Increasing the annealing temperature to ?1300 °C led to abnormal grain growth with inter-granular pores and this led to a decrease in the saturation magnetization. Moreover, an increase in the Mn²⁺ ion substitution up to x=0.8 increased the lattice parameter of the formed powders due to the high ionic radii of the Mn²⁺ ion. Mn–Zn ferrites phases were formed and the positions of peaks were shifted by substituting manganese. The average crystalline size was increased by increasing the annealing temperature and decreased by increasing the substitution by manganese up to 0.8. The average crystalline size was in the range 95–137.3 nm. The saturation magnetization of the Mn–Zn-substituted ferrite powders increased continuously with an increase in the Mn concentration up to 0.8 at annealing temperatures of 1200–1300 °C. Further increase of Mn substitution up to 0.9 led to a decrease of saturation magnetization. The saturation magnetization increased from 17.3 emu/g for the Mn_0.3Zn_0.7Fe₂O₄ phase particles produced to 59.08 emu/g for Mn_0.8Mn_0.2Fe₂O₄ particles.     

Growth and field emission properties of nanotip arrays of amorphous carbon with embedded hexagonal diamond nanoparticles

Nanotip arrays of amorphous carbon with embedded hexagonal diamond nanoparticles were prepared at room temperature for use as excellent field emitters by a unique combination of anodic aluminum oxide (AAO) template and filtered cathodic arc plasma (FCAP) technology. In order to avoid nanopore array formation on the AAO surface, an effective multi-step treatment employing anodization and pore-widening processes alternately was adopted. The nanotips were about 100 nm in width at the bottom and 150 nm in height with density up to 10¹⁰ cm⁻². Transmission electron microscopy investigation indicates that many nanoparticles with diameters of about 10 nm were embedded in the amorphous carbon matrix, which was proved to be hexagonal diamond phase by Raman spectrum and selected-area electron diffraction. There is no previous literature report on the field emission properties of hexagonal diamond and its preparation at room temperature under high-vacuum condition. The nanotip arrays with hexagonal diamond phase exhibit a low turn-on field of 0.5 V/μm and a threshold field of 3.5 V/μm at 10 mA/cm². It is believed that the existence of hexagonal diamond phase has improved the field emission properties.     

Phase transformation and oxygen permeation properties of partially substituted strontium cobaltite — Influences of B-site partial substitution —

A group of B-site partially substituted strontium cobaltites Sr_0.9Ca_0.1Co_0.9B'_0.1O_2.5+δ (B'=Mn, Co, Ni, In, Fe and Al) were examined for phase transformation and oxygen permeation properties. As revealed by XRD analysis, these oxides had 2H-BaNiO₃-type structure (B'=Mn, Co, Ni and In) or brownmillerite-type structure (B'=Fe and Al) and transformed into perovskite-type structure at elevated temperatures (750 °C or above). The transformation temperature was dependent on the B-site substituent and agreed well with the switching-on temperature of oxygen permeation for the same oxide. Due to a lowering in transformation temperature, the Fe- or Al-doped oxide was made oxygen permeable in the lower temperature range 750–850 °C where the undoped oxide remained non-permeable. It was also found that for a series of Fe-partially substituted samples Sr_0.9Ca_0.1Co_1−xFe_xO_2.5+δ the transformation temperature lowered with increasing x up to 0.1, while, for x≥0.2, perovskite-type structure tended to be stabilized at room temperature. TGDTA analysis revealed that phase transformation was accompanied by absorption or desorption of oxygen. Based on the correlation between the oxygen permeability and the membrane thickness, the rate-determining step of oxygen permeation in the present system was assumed to be of the bulk diffusion of oxide ion vacancies.     

Broadband 2 μm emission and energy-transfer properties of thulium-doped oxyfluoride germanate glass fiber

High near-infrared transparency Tm³⁺-doped germanate bulk glass and fiber have been fabricated and 2 μm emission properties demonstrated. Energy-transfer processes in the germanate glass and fiber are discussed. The emission spectra are obtained from both bulk glass and fiber with the excitation of a 794 nm laser diode. The results indicate that the line width of the Tm³⁺:³F₄→³H₆ emission spectra measured in fibers is narrower than that of the bulk glass sample and shifts to longer wavelengths with increment of fiber length. The extended overlap integral method is used to calculate the microparameters of energy transfer and critical distance. A model is derived to better understand of the energy-transfer process of thulium ions in the germanate glasses responsible for emission at 2 μm. The study indicates that Tm³⁺-doped germanate fibers with a large core diameter has proved to be promising infrared optical and high-power level laser materials.     

Changes of thermo-morphologic and thermotropic properties of liquid crystals at direct and reverse nematic mesophase ↔ isotropic liquid phase transitions: surface-induced effect

In this work, the effect of thin films on the thermo-morphologic and thermotropic properties of the phase transitions between the nematic mesophase and isotropic liquid has been investigated. Investigations have been carried out for both the heating and cooling processes. The temperature and linear widths of the biphasic regions of the direct and reverse phase transitions in nematic liquid crystals versus thickness of the thin films have been calculated with a high accuracy. The shift of the nematic mesophase–isotropic liquid and the isotropic liquid–nematic mesophase phase transition temperatures to higher temperatures and the enlargement of the temperature and linear widths of the biphasic regions as the effect of surfaces have been found.     

Beneficial effect of Co substitution on the structure and magnetic properties of mechanically alloyed (Pr,Tb)–Fe–(C,B) magnets

The effect of Co substitution on the structure and magnetic properties of mechanically alloyed Pr₁₄Tb₂Fe_76−xCo_xC₆B₂ and Pr₁₆Fe_76−xCo_xC₆B₂ (x=0–20$x=0–20$) alloys has been studied systematically. The main phase in the alloys is Pr₂Fe₁₄C-type carbide, coexisting with a small amount of α-Fe and rare-earth-rich phases. In addition to the increasing of the Curie temperature of the Pr₂Fe₁₄C-type phase, Co substitution can affect the magnetic properties by adjusting the α-Fe fraction of the alloys. The increase of both coercivity and remanence has been realized in a certain composition range. This increase may be attributed mainly to the enhancement of the effective anisotropy constant _Keff$K_{\mathrm{eff}}$ of the magnets due to the reduced α-Fe fraction with a small Co addition. The highest coercivity _iH_c of 20.3 kOe and the optimum energy product (BH)_max of 10.3 MG Oe have been obtained for the Pr₁₄Tb₂Fe_69.5Co_6.5C₆B₂ alloy.