Optical and structural characterization of inhomogeneities in a-Si:H TO μc-Si transition

The a-Si:H films with different thickness and microstructure have been deposited with rf-PECVD using a plasma of silane diluted with hydrogen. The structure and optical analysis were carried out by X-ray diffraction, UV-VIS and Raman spectroscopy. Spectral refractive indices, optical energy band gaps, extinction coefficients, phases ratio and grain size were determined as a function of the hydrogen dilution (R = H₂/SiH₄). Hydrogen dilution of silane results in an inhomogeneous growth during which the material evolves from amorphous hydrogenated silicon (a-Si:H) to micro-crystalline hydrogenated silicon (μc-Si:H). XRD analysis indicated that films with R = 0 and R = 20 were amorphous and homogeneous, while films with R = 40 and higher were micro-crystalline consisting medium range ordered silicon hydride (Si₄H) and μc-Si phases with different size of crystallites, which was confirmed also by Raman spectroscopy.     

Synthesis and characterization of TiO<Subscript>2</Subscript>-incorporated silica foams

Titania-incorporated silica (TiO₂–SiO₂) porous materials have great applications in diverse areas. In this work, TiO₂–SiO₂ porous materials with tunable Si/Ti molar ratio (R) have been successfully prepared through a one-pot method under a near-neutral condition. With decreasing Si/Ti R, a phase transition from a macroporous foam-like structure to mesostructure is observed. The resultant TiO₂–SiO₂ porous materials possess large surface areas and high pore volumes. In addition, the titania species are homogenously dispersed in silica matrix when Si/Ti R ≥ 10. Our contribution provides a convenient method to synthesize TiO₂/SiO₂ porous materials with very large pore size, high pore volume, and relatively high titania content well dispersed in the silica wall framework.     

Crystal structure analysis of Y2Cu2O5

Single crystals of Y₂Cu₂O₅ were obtained in the flux growth process by controlled heating of a mixture of Y₂O₃, BaO, and CuO in a molar ratio of 1820. These crystals were analyzed by a single-crystal X-ray diffraction analysis. The crystal contains polymeric chains of Cu₂O₅ interspersed by yttrium ions surrounded by octahedral arrangements of oxygen atoms. Crystal data: space group=Pna2₁,a=10.799(2) Å,b=3.4990(5) Å,c=12.459(2) Å,Z=4, 380 reflections,R=0.026,R _w=0.030.     

High temperature magnetic properties of mechanically alloyed Fe–Zr powder

We report the preparation and characterization of amorphous/non-equilibrium solid solution Fe_100 − xZr_x (x = 20–35) alloys by mechanical alloying process. The microstructure and magnetic properties of milled powders have been studied as a function of Zr substitution. The effective magnetic moment of as-milled powders decreases as concentration of Zr is increased. Thermomagnetization measurements confirmed that the Fe₈₀Zr₂₀ sample exhibits two clear magnetic phase transitions due to the co-existence of an amorphous phase and a Fe rich non-equilibrium solid solution. All the other samples exhibiting an amorphous structure showed a single magnetic phase transition with Curie temperature of ~ 570 °C,which did not vary much with different composition. The Curie temperature of the mechanically alloyed powders is noticeably higher than those of melt-spun amorphous ribbons.     

Polysaccharide-based nanoparticles formation by polyeletrolyte complexation of carboxymethylated cashew gum and chitosan

Polyelectrolyte complex nanoparticles of chitosan and carboxymethyl cashew gum (CMCG) were prepared with CMCG with two different degrees of substitution (DS = 0.16 and 0.36). The effects of polymer concentration, molar mixing ratio (n ⁺/n ⁻) and mixing order of reactants on particle size distribution and zeta potential were investigated. Nanoparticle structure was elucidated by Fourier transform spectroscopy. Particle size of CH/CMCG DS = 0.16 dispersions was smaller than with DS 0.36 for all n ⁺/n ⁻ ratio investigated. Particle size smaller than 200 nm was obtained when CMCG with DS = 0.16 was used for particle formation. The polydispersity index values were small when CMCG DS 0.36 was used. Increasing the concentration of CMCG led to larger particle size. Zeta potential values for almost all molar mixing ratios were found to be positive (10–32 mV).     

Structural and optical properties of Ti oxide cluster-assembled films prepared by plasma-gas-condensation

Ti and Ti oxide cluster-assembled films have been prepared using a plasma-gas-condensation apparatus. Transmission electron microscopy and electron diffraction measurement indicates that their structures vary from a face-centered-cubic (fcc) Ti phase, via an NaCl-type TiO_x phase, to an amorphous and rutile-type TiO₂ mixture phase with increasing the O₂ flow rate in the Ar/He gas mixture. Cluster shapes are spherical for the fcc phase, cubic for the NaCl-type, and spherical for the amorphous and rutile-type mixture, while the cluster size monotonically decreases with increasing O₂ flow rate. The Ti and Ti oxide cluster-assembled films have a sooty appearance, showing a very porous morphology in the scanning electron microscopy images. The electrical resistivity dramatically increases between R_O2 = 0.065 and 0.075 sccm, while the optical transmittance spectra in the visible wavelength range rapidly increases between R_O2 = 0.1 and 0.12 sccm, well reflecting the structure variations in these Ti and Ti oxide cluster-assembled films.     

Impact of Tuning Molar Ratio in the Starting Materials: <Emphasis Type="Italic">Magneto</Emphasis>-Transport Features of Hybrid YSr<Subscript>2</Subscript>Ru<Subscript>0.9</Subscript>Cu<Subscript>2.1</Subscript>O<Subscript>7.9</Subscript>

The impact of slightly tuning molar ratio in the starting materials on the physical properties of 1212-type rutheno-cuprate, YSr₂Ru_0.9Cu_2.1O_7.9 (nominal) samples prepared under four synthesis approaches are reported. Interestingly, all samples clearly show the differences in the physical properties of the samples prepared under different synthetic protocols. However, neither XRD nor EDX reveal any notable differences in the crystal structure or sample composition. All the samples exhibit magneto-superconducting properties (H _ext=5 Oe) which are slightly varied with synthetic approaches. The high field (H _ext=10 kOe) temperature dependence of magnetization data shows a sharp ferromagnetic transition around 150 K and all the samples obey Curie–Weiss linear behavior above 180 K. The experimental effective paramagnetic moment for the various samples is in the range of 2.5 and 2.7μ _B/Ru which are in line with the literature report. The magnetization, M(H) isotherm curves measured at 5 K and −10 kOe≤H≤10 kOe conditions reveal weak ferromagnetic-like hysteresis loops for all samples with returning moment (M _r) and coercive field (H _c), whereas the high field M(H) loops indicate soft ferromagnetic behaviors with magnetic saturation. The saturation moment of the samples is slightly varied with the synthesis approaches. None of the samples showed bulk superconductivity (T_c^{R = 0})T_{\mathrm{c}}^{R = 0}) down to 2 K, while all samples show onset transitions (T_c^onsetT_{\mathrm{c}}^{\mathrm{onset}}) except the sample prepared by approach-3. The latter approach sample shows semiconducting behavior down to 2 K. The T_c^onsetT_{\mathrm{c}}^{\mathrm{onset}} noticed at 34 K, 12 K, and 6 K for the sample prepared by approach-1, 2, and 4, respectively. The nearly linear dependence suggests that hopping conduction is dominant in certain temperature range for all samples. The magneto-transport features of these samples exhibit maximum magnetoresistance (MR) at low temperatures. Remarkably, the sample prepared by approach-1 shows largest −MR about 77% at low temperature 2 K and H=90 kOe which stimulates for further investigations. Among the four synthesis approaches employed in the present study, we can probably suggest that the approach-1 (0.5Y₂O₃+0.5SrO₂+1.5SrCuO₂+0.9RuO₂+0.6CuO) is the preferable method to achieve the best sample (in terms of magneto-transport features).     

Multiaxial fatigue of a short glass fibre reinforced polyamide 6.6 – Fatigue and fracture behaviour

The multiaxial fatigue behaviour of a short glass fibre reinforced polyamide 6.6 (PA66-GF35) is investigated on hollow tubular specimens in the range of fatigue lives between 10² and 10⁷ cycles. Fatigue experiments included pure tension, pure torsion, combined tension–torsion at different biaxiality ratios and phase shifting angles between the stress components. Tests were carried out with load ratio R = 0 and R = −1 at room temperature as well as at 130 °C. The influence of biaxiality ratio, phase angle between load components and load ratio is discussed.An extensive analysis of the fracture behaviour is performed on the specimens to recognise the crack nucleation and propagation mechanisms; failure modes were evaluated via optical and scanning electron microscopy.     

Synthesis and photoluminescence of Eu-doped Zn/Al layered double hydroxides

Eu-doped ZnAl-layered double hydroxides (ZnAl–LDHs) with various Zn²⁺/(Al³⁺+Eu³⁺) molar ratios from 1:1, 2:1, 3:1, to 4:1 were first synthesized by the coprecipitation method at room temperature and the Eu³⁺/Al³⁺ molar ratio of 0.06 was almost maintained. The obtained solids were characterized by powder X-ray diffraction (XRD), photoluminescent spectrum (PL), scanning electron microscope (SEM), infrared spectroscopy (IR), and thermogravimetric (TG) analysis. XRD results show that the crystallinity of the Eu-doped products gradually was improved when the Zn²⁺/(Al³⁺+Eu³⁺) molar ratio was higher than 2. The photoluminescent spectra of the Eu-doped ZnAl–LDHs are described by the well-known ⁵ D ₀–⁷ F _J transition (J = 1, 2, 3, 4) of Eu³⁺ ions with the strongest emission for J = 2.     

Processing effect on structural changes of high acid-catalysed silica gel

The structure of silica gels prepared by hydrolysis of tetraethoxysilane (TEOS) with high acid content has been investigated. The gels were obtained by varying the nitric acid contents at a constant H₂O/TEOS molar ratio of 10. The molar compositions were TEOS: H₂OHNO₃ =110: x(x=0.1–1.0). The effects of acid content and drying temperature on structural changes in the gels were examined. In addition, the surface area, pore volume, pore size distribution, and the microstructure were determined. The results showed that the pore structure changed to larger pore sizes and broader size distributions as a result of increasing the drying temperature. However, at a constant drying temperature, a higher acid content yielded a higher pore volume. The gels dried at lower temperatures had slit-shaped micropores and a narrow pore size distribution; whereas, the gels dried at higher temperatures had ink bottle-shaped pores and a relatively broad size distribution.     

Mean stress and plasticity effect prediction on notch fatigue and crack growth threshold,combining the theory of critical distances and multiaxial fatigue criteria

In the present work, we propose a robust calibration of some bi‐parametric multiaxial fatigue criteria applied in conjunction with the theory of critical distances (TCD). This is based on least‐square fitting fatigue data generated using plain and sharp‐notched specimens tested at two different load ratios and allows for the estimation of the critical distance according to the point and line method formulation of TCD. It is shown that this combination permits to incorporate the mean stress effect into the fatigue strength calculation, which is not accounted for in the classical formulation of TCD based on the range of the maximum principal stress. It is also shown that for those materials exhibiting a low fatigue‐strength‐to‐yield‐stress ratio σ_fl,R = ?1/σ_YS, such as 7075‐T6 (σ_fl,R = ?1/σ_YS = 0.30), satisfactorily accurate predictions are obtained assuming a linear‐elastic stress distribution, even at the tip of sharp notches and cracks. Conversely, for any materials characterized by higher values of this ratio, as quenched and tempered 42CrMo4 (σ_fl,R = ?1/σ_YS = 0.54), it is recommended to consider the stabilized elastic‐plastic stress/strain distribution, also for plain and blunt‐notched samples and even in the high cycle fatigue regime still with the application of the TCD.     

Detailed Physical Characterizations of Cu-rich and Ru-poor (Ru0.9Cu0.1)Sr2YCu2O7.9

The optimized nominal composition, (Ru_0.9Cu_0.1) Sr₂YCu₂O_7.9 sample, has been prepared through high-pressure and high-temperature solid-state densification method. The obtained material has been studied by X-ray (laboratory) diffraction powder technique, magnetization and detailed magneto-transport measurements. The title compound indicates bulk magneto-superconducting properties under field strengths of H=10, 100, 500 and 1000 Oe. It shows diamagnetic transition at T _d=54, 38, 20 and 8 K for H=10, 100, 500 and 1000 Oe, respectively, in the zero-field-cooled susceptibility measurements. The high-field (H=5 and 10 kOe) molar susceptibility measurements show sharp ferromagnetic transition at ∼150 K with reduced molar susceptibility values. The various field dependence of magnetization, M(H), isotherm curves recorded at constant temperatures (5, 10, 25, 50, 100 and 150 K) indicate ferromagnetic saturation, whereas the MH curves measured at 200 and 300 K conditions reveal the paramagnetic state of the compound. Though the sample showed onset transition temperature, T_c^onsetT_{\mathrm{c}}^{\mathrm{onset}}, at ∼34 K under different field strengths (H=0, 10, 30, 50, 70 and 90 kOe), no T_c^R=0T_{\mathrm{c}}^{R=0} is seen down to 2 K. Even under relatively low applied field (ΔH=10 kOe) the title compound shows large negative magnetoresistance (MR) of about 68% at 2 K and increases with increasing the field strength up to ΔH=90 kOe (MR=77% at 2 K). This value is amazing and probably higher than other 1212 type ruthenocuprates. The title compound which shows little negative MR (about 1%) in the high temperature regions (125–300 K) is not affected much by different field strengths. Among the different fixed temperature MR(H) isotherms, the MR(H) curve measured at 5 K shows maximum negative MR of about 47% at 90 kOe compared to other four (T=50, 100, 200 and 300 K) MR(H) curves.     

Electrical-thermal switching effect in high-density polyethylene/graphite nanosheets conducting composites

The nonlinear response in high-density polyethylene/graphite nanosheets conducting composites under increasing applied voltage is investigated. Under sufficient applied constant voltage, the resistance increases initially due to Joule heating effect and then eventually reaches a steady value with a characteristic thermal relaxation time τ _h , which decreases as the applied field increases. The switch value, namely the ratio of the resistance under steady condition to the resistance of sample in linear regime, gradually increases with the increasing applied field. The threshold voltage (V ₀) at which the resistance start to increase with time scales with linear regime resistance (R ₀) of the sample as with the exponent x = 0.78 ± 0.05. All the curves of R/R ₀ vs. V/V ₀ collapse to a similar curve with the function R/R ₀ = 1 + α(V/V ₀) ^θ . The results reveal that the threshold voltage value decreases with increasing graphite nanosheets content in the composites.     

Nanocrystalline spinel NixCu0.8?xZn0.2Fe2O4: a novel material for humidity sensing

The humidity sensing property of the ferrite systems Ni _x Cu_0.8−x Zn_0.2Fe₂O₄ with 0.0 ≤ x ≤ 0.8 (x = 0, 0.2, 0.4, 0.6, 0.8) was studied using the standard ceramic technique. The spinel structure of the compounds was confirmed by X-ray diffraction studies, BET study and surface morphology by scanning electron microscopy. The compounds were subjected to dc electrical conductivity studies at room temperature. The resistance measurements as a function of relative humidity (RH) in the range of 5–98% were done and the humidity sensing factors (S _f = R _5%/R _95%) were calculated. The composite NiCuZn-3 (x = 0.4) possessed the highest humidity sensing factor of 3051.9 ± 500, whilst CuZn–1 (x = 0) possessed the lowest humidity sensing factor of 45.3 ± 12. The other compounds possessed lower humidity sensing factors of 116.7 ± 35, 783.4 ± 160 and 416.2 ± 65 for x = 0.2, 0.6 and 0.8, respectively.     

Densification of (Na,K)NbO<Subscript>3</Subscript> piezoelectric ceramics by two-step mixing process

Lead-free Na_0.5K_0.5NbO₃ (NKN) piezoelectric ceramics were sintered with a new process, “two-step mixing process,” in which a part of alkali source powders was initially preserved and mixed with the rest matrix powders after the calcinations step. As a result, the sintering of NKN ceramics was improved, and the sample sintered at 1082 °C with the initial preservation ratio (R _A) of 5% demonstrated the highest density of ρ = 4.38 g/cm³ (97.1% of the theoretical density), compared with ρ = 4.36 g/cm³ (96.7% of the theoretical density) for the non-preservation specimen (R _A = 0%). The former sample showed the best piezoelectric constant of d ₃₃ = 125 pC/N and electromechanical coupling coefficient of k _p = 0.42, while the latter sample had d ₃₃ = 116 pC/N and k _p = 0.37. These results indicated that the two-step mixing process was effective for the sintering of lead-free NKN ceramics, despite no sintering additive and cold isostatic pressing were used.     

A novel approach for synthesis of M-type hexaferrites nanopowders via the co-precipitation method

M-type hexaferrites; barium hexaferrite BaFe₁₂O₁₉ and strontium hexaferrite SrFe₁₂O₁₉ powders have been successfully prepared via the co-precipitation method using 5 M sodium carbonate solution as alkali. Effects of the molar ratio and the annealing temperature on the crystal structure, crystallite size, microstructure and the magnetic properties of the produced powders were systematically studied. The results indicated that a single phase of barium hexaferrite was obtained at Fe³⁺/Ba²⁺ molar ratio 12 annealed at 800–1,200 °C for 2 h whereas the orthorhombic barium iron oxide BaFe₂O₄ phase was formed as a impurity phase with barium M-type ferrite at Fe³⁺/Ba²⁺ molar ratio 8. On the other hand, a single phase of strontium hexaferrite was produced with the Fe³⁺/Sr²⁺ molar ratio to 12 at the different annealing temperatures from 800 to 1,200 °C for 2 h whereas the orthorhombic strontium iron oxide Sr₄Fe₆O₁₃ phase was formed as a secondary phase with SrFe₁₂O₁₉ phase at Fe³⁺/Sr²⁺ molar ratio of 9.23. The crystallite sizes of the produced nanopowders were increased with increasing the annealing temperature and the molar ratios. The microstructure of the produced single phase M-type ferrites powders displayed as a hexagonal-platelet like structure. A saturation magnetization (53.8 emu/g) was achieved for the pure barium hexaferrite phase formed at low temperature 800 °C for 2 h. On the other hand, a higher saturation magnetization value (M _s = 85.4 emu/g) was obtained for the strontium hexaferrite powders from the precipitated precursors synthesized at Fe³⁺/Sr²⁺ molar ratio 12 and thermally treated at 1,000 °C for 2 h.     

Heterogeneous crystallization and composition dependence of optical parameters in Sn–Sb–Bi–Se chalcogenides

Bulk samples of Sn₁₀Sb_20−x Bi _x Se₇₀ (0 ≤ x ≤ 8) chalcogenide alloys were prepared by the conventional melt quenching technique. Thin films were prepared on well-cleaned glass substrates by thermal evaporation technique. X-ray diffraction studies revealed that the alloys with x = 0 and 2 at.% of Bi were amorphous, whereas the alloys with x = 4, 6, 8 at.% were crystalline. The crystalline phases are identified as due to the formation of Bi₂Se₃ and BiSe₂ phases. The microstructural and differential scanning calorimetric studies show the presence of these phases. A simple, straight forward procedure suggested by Swanepoel has been used to calculate the optical parameters, refractive index, and extinction coefficient. The optical gap for all the samples has been obtained from the Tauc plots. The variation in optical parameters for different Bi concentration has been explained on the basis of presence of defect states and the change in stoichiometry with the change in Bi concentration.     

A new hydroxide-based synthesis method for inorganic polymers

A new synthesis method is described to produce aluminosilicate inorganic polymers by direct reaction of aluminium hydroxide with amorphous silica under alkaline conditions. XRD and ²⁷Al and ²⁹Si MAS NMR spectroscopy suggest that the reaction mechanism involves the dissolution of the aluminium hydroxide in the alkali. The best results are obtained with more reactive gibbsite of small particle size and X-ray amorphous ρ-alumina containing 5-fold coordinated Al, and with compositions in the previously reported optimum range (SiO₂: Al₂O₃ = 3.0, M₂O: SiO₂ = 0.34, H₂O: M₂O = 9.4). Unreacted aluminium hydroxide occurs where there is insufficient silica for complete inorganic polymer formation but the reaction is less sensitive to variations in the K₂O: Al₂O₃ and K₂O: SiO₂ ratios. Zeolite formation was not observed in any of the present samples. More complex alumina sources such as thermally activated hydrotalcite can also be used to form an inorganic polymer, but under the alkaline synthesis conditions the unused alumina reacts with the poorly crystalline MgO present to re-form hydrotalcite, presumably containing atmospheric carbonate as the interlayer anion. This synthesis method can also potentially be extended to hydroxides other than those of aluminium.     

Study the Influence of Drying Control Chemical Additives on the Physical and Optical Properties of Nanocrystalline Cadmium Sulfide–Doped Tetraethylorthosilicate Silica Xerogels

The experimental results of the influence of drying control chemical additives (DCCAs)—such as formamide (FA), N-methyl formamide (NMF), N-N-dimethyl formamide (DMF), acetamide (AA), glycerol (GLY), and oxalic acid (OXA)—on the physical and optical properties of nanocrystalline cadmium sulfide (CdS)–doped silica xerogels were reported in this paper. Tetraethylorthosilicate [TEOS, Si(OC₂H₅)₄] was used as a precursor for the three-dimensional silica network in which the CdS nanocrystallites were trapped. Silica alcosol was prepared by taking the mixture of ethanolic (EtOH) TEOS, water (H₂O), hydrochloric acid (HCI), and ammonium hydroxide (NH₄OH). Ethanolic cadmium acetate [Cd(CH₃COO)₂ 2H₂O] and thiourea [CS(NH₂)₂] were added to the alcosol for the formation of CdS crystallites. To study the effect of DCCAs on the physical and optical properties of nanocrystalline CdS-doped silica xerogels, the molar ratio of TEOS:EtOH:H₂O:HCl:NH₄OH:Cd(CH₃COO₂)₂ 2H₂O:CS(NH₂)₂ was kept constant at 1:5:7:0.01:0.0027:0.001:0.002, respectively, and the molar ratio of DCCA/TEOS was varied from 0.001 to 1. The addition of DCCAs (e.g., formamide) resulted in decreased gelation time (tg) from 240 to <20 hrs. Transparent CdS-doped alcogels and xerogels were obtained for the DCCA/TEOS molar ratios of <0.5, whereas turbid alcogels and translucent nanocrystalline CdS-doped silica xerogels were obtained for higher DCCA/TEOS molar ratios (>0.5). It has been found that the percentage of volume shrinkage of the samples, when heated at 300°C for 3 hr, was more (>15%) for OXA, GLY, and DMF and the shrinkage was less (<15%) for AA, NMF, and FA. The density of the CdS-doped silica xerogels with DCCAs was less than that without DCCAs. Red shift was observed in the optical absorption spectra of the samples with addition of the DCCAs. An increase in CdS crystallite size in the silica matrix with the incorporation of DCCAs was observed in the X-ray diffraction patterns in the order: OXA < GLY < DMF < AA < NMF < FA. From XRD studies, the structure of the CdS crystallites in the silica matrix was found to be hexagonal wurtzite.