Annealing dependence of optical properties of Ga20S75Sb5 and Ga20S40Sb40 thin films

Amorphous Ga₂₀S₇₅Sb₅ and Ga₂₀S₄₀Sb₄₀ thin films were prepared onto glass substrates by using thermal evaporation method. The effect of annealing (under vacuum) at different temperatures on the optical parameters was investigated in the temperature range 373-593 K. The optical absorption coefficient (α) for the as-deposited and annealed films were calculated from the reflectance and transmittance measurements in the range 190-900 nm. X-Ray diffraction indicates that the as-deposited films and those annealed up to the glass transition temperature (T_g) exhibit amorphous state. On annealing above the glass transition temperature these films show a polycrystalline structure. Analysis of the optical absorption data indicates that the optical band gap E_g^opt of these films obeys Tauc's relation for the allowed non-direct transition. It was found that the optical band gap E_g^opt increases with annealing temperature up to T_g, whereas above T_g there is a remarkable decrease. The obtained results were interpreted on the basis of amorphous- crystalline transformation.     

Enhancing the grafting of poly(2-hydroxyethyl methacrylate) on silica nanoparticles (SiO2-g-PHEMA) by the sequential UV-induced graft polymerization with a multiple-UV irradiation

Grafting of poly(2-hydroxyethyl methacrylate) on silica nanoparticles was accomplished via the sequential UV-induced graft polymerization. Under UV-irradiation, the silica was functionalized with the surface initiator, benzophenone (BP) and subsequently graft-polymerized with 2-hydroxyethyl methacrylate (HEMA). The grafting on the silica particles was confirmed by DSC analysis which revealed a shift of the glass transition temperature (T_g) of grafted PHEMA to higher temperature than T_g of ungrafted PHEMA. A significant improvement in the grafting efficiency and the grafting percentage was achieved when a sequential grafting approach was taken, employing multiple UV exposures. Using this approach, the efficient chain extension from the grafted-PHEMA was possible without producing significant amounts of ungrafted PHEMA when low HEMA concentrations were used during each UV-exposure.     

Solidification pathway and phase transformation behavior in a beta-solidified gamma-TiAl based alloy

The phase transformation behavior of an as-cast Ti-42Al-5 Mn(at.%) alloy after subsequent quenching from 1380 ℃ to 1000 ℃ was investigated based on the differential thermal analysis(DTA),electron probe micro analyzer-backscattered electrons(EPMA-BSE),transmission electron microscope(TEM) and X-ray diffraction(XRD).The results show that,the solidification path can be summarized as follows:Liquid→Liquid+β→β→β→α→β+α+γ→β_o+α_2+γ→β_o+γ+α_2/γ→β_o+γ+α_2/γ+β_(o,sec),with the phase transformation α→β temperature(T_β)=1311℃,phase transformation γ→β temperature of(T_(γsolv))=1231℃,phase transformation α_2→α or β_o→β temperature(T_(α2→α)/T_(β_o→β))=1168 C,eutectoid temperature(T_(eut))=1132 ℃ and T_(α_2/γ→βo,sec)≈1 120℃.In comparison with Ti-42 Al alloy,the T_(eut) and T_(γsolv)are slightly increased while both the Tp is decreased obviously by 5% Mn addition.When quenched from the temperature of 1380-1260 ℃,the martensitic transformation β→α' could occur to form the needlelike martensite structure in β area.This kind of martensitic structure is much obvious with the increase of temperature from 1260℃ to 1380 ℃.When the temperature is below T_(γsolv)(1231℃),the γ grains would nucleate directly from the β phase.For the temperature slightly lower than T_(eut)(1132℃),the dotted β_(o,sec) phases could nucleate in the lamellar colonies besides the γ lamellae precipitated withinα_2 phase.Finally,at room-temperature(RT),the alloy exhibits(p_o+α_2+γ) triple phase with microstructure of β_o+lamellae+γ,of which the lamellar structure consists of α_2,γ and β_(o,sec) phases.The phase transformation mechanisms in this alloy,involving β→α',β→γ,α_2→α_2/γ and α_2→β_(o,sec) were discussed.     

Designing amorphous formulations of polyphenols with naringin by spray-drying for enhanced solubility and permeability

Naringin (NAR), a major flavanone (FVA) glycoside, is a component of food mainly obtained from grapefruit. We used NAR as a food additive to improve the solubility and permeability of hydrophobic polyphenols used as supplements in the food industry. The spray-dried particles (SDPs) of NAR alone show an amorphous state with a glass transition temperature (T_g) at 93.2 °C. SDPs of hydrophobic polyphenols, such as flavone (FVO), quercetin (QCT), naringenin (NRG), and resveratrol (RVT) were prepared by adding varying amounts of NAR. All SDPs of hydrophobic polyphenols with added NAR were in an amorphous state with a single T_g, but SDPs of hydrophobic polyphenols without added NAR showed diffraction peaks derived from each crystal. The SDPs with NAR could keep an amorphous state after storage at a high humidity condition for one month, except for SDPs of RVT/NAR. SDPs with NAR enhanced the solubility of hydrophobic polyphenols, especially NRG solubility, which was enhanced more than 9 times compared to NRG crystal. The enhanced solubility resulted in the increased membrane permeability of NRG. The antioxidant effect of the hydrophobic NRG was also enhanced by the synergetic effect of NAR. The findings demonstrated that NAR could be used as a food additive to enhance the solubility and membrane permeability of hydrophobic polyphenols.     

Nonlinear optical materials: Synthesis,characterizations, thermal stability and electro-optical properties

Two series of thermal stable polyimides and polyimide/silica hybrid materials have been synthesized. The silica content in the hybrid materials was varied from 0 to 22.5 wt.% and SiO₂ phase was well dispersed in the polymer matrix. The glass transition temperature (T_g) and the decomposition temperature (T_d) were in the range 226–382 °C and 345–499 °C, respectively, for materials. The polymer solutions could be spin coated on the indium–tin–oxide (ITO) glass or other substrates to form optical quality thin films. The electro-optic coefficients (γ₃₃) at the wavelength of 832 nm for polymer thin films poled were in the range of 12–25 pm/V and the values remained well (retained > 85% for more than 100 h). The heat capacities of some materials were in the range of 0.9755–1.1821 J K^− 1 g^− 1 for the temperature 273 and 363 K. No thermal anomaly was found in this temperature range. Most of them showed high thermal stability.     

Compositional effects on the optical and thermal properties of potassium aluminophosphate glasses

The K₂O–Al₂O₃–P₂O₅ glass system has been examined and various compositions have been melted. Their optical and thermal properties have been measured to assess their potential for athermalisation. The addition of alumina (Al₂O₃) increases the refractive index (n) and glass transition temperature (T_g) and decreases the thermal expansion coefficient (α), consequently leading to positive thermo-optic coefficient (dn/dT). In addition to thermal expansion, polarisability of the glass also affects dn/dT. Generally, glasses must exhibit negative dn/dT to counter thermal expansion, in order to have potential application in athermalisation.     

Dynamic mechanical analysis of carbon/epoxy composites for structural pipeline repair

The viscoelastic behavior of a carbon fiber/epoxy matrix composite material system used for pipeline repair has been evaluated though dynamic mechanical analysis. The effects of the heating rate, frequency, and measurement method on the glass transition temperature (T_g) were studied. The increase in T_g with frequency was related to the activation energy of the glass transition relaxation. The activation energy can be used for prediction of long term performance. The measured tan delta peak T_g’s of room temperature cured and post-cured composite specimens ranged from 60 to 129 °C. Analysis of T_g data at various cure states was used to determine use temperature limits for the composite repair system.     

Heterogeneity of spiral wear patterns produced by local heating on amorphous polymers

We report on spiral wear patterns produced at constant angular velocity by hot tip atomic force microscopy (HT-AFM) on surfaces of two common amorphous polymers: polystyrene (PS) and polymethylmethacrylate (PMMA). Topography of these patterns is obtained with regular AFM cantilevers. Topography cross-sections taken from a center of each spiral at a given azimuthal angle Θ relate changes of surface corrugation h_corr with tangential velocity v of a thermal cantilever. Polymer wear is characterized by a power law h_corr(v) = α(v/v_max)^−β, which yields a pre-factor α and an exponent β. Below the glass transition temperature T_g, α is polymer specific and β varies weakly between similar conditions and samples. Variations of β are hypothesized to reflect polymer relaxation processes, which are expected to vary only weakly between amorphous polymers. At and above T_g, α approaches initial thermal tip indentation depth within a polymer, β plummets, and a power law relation of h_corr with v diverges. These results are explained by heterogeneous wear around T_g due to a local nature of glass transition. At all studied temperatures, additional wear heterogeneities are found as due to position on the polymer and Θ. Variations of α and β with position on the polymer are found to be only marginally larger then uncertainties of the thermal tip–polymer interface temperature. Variations of α and β with Θ are found to be largely influenced by buckling of thermal cantilevers traveling in a spiral pattern.     

Dielectric relaxation and ionic conduction in 66%Silica/CW229-3/HW229-1 microcomposite polymer

Composites of epoxy resin with high percentage of silica fillers (66%) are designed to improve mechanical and electrical properties of transformers used in railway application. FTIR, (DRX and FE-SEM) and absorption/desorption phenomena are used to investigate the structure, the morphology and the diffusion of water in the microcomposites, respectively. Good dispersion of silica filler with size less than 10 μm was assessed by SEM, although some clustering (agglomerates) of greater than 1 μm was observed. The absorption water in the microcomposite obeys to the first Fickian law and shows saturation water of 0.6%. The calculation of the diffusion coefficient of water leads to a value of 2.9*10⁻¹² m² s⁻¹ in the studied system. Relaxation times of α-relaxation and ionic conduction relaxation processes are determined. A correlation is observed between the ionic conductivity and dielectric relaxation processes. The dc-current behavior shows a change in the conduction mechanism from electronic conduction below the T_g to ionic conduction above the T_g. Shallow traps of 0.54 eV and deep traps of 2.21 eV are determined below and above the T_g, respectively. The TSC analysis confirms the VFT behavior of the α-relaxation of the microcomposite as obtained by the dielectric spectroscopy.     

Woven hybrid biocomposites: Dynamic mechanical and thermal properties

The dynamic mechanical and thermal analysis of oil palm empty fruit bunch (EFB)/woven jute fibre (J_w) reinforced epoxy hybrid composites were carried out. The storage modulus (E′) was found to decrease with temperature in all cases, and hybrid composites had showed better values of E′ at glass transition temperature (T_g) compared to EFB and epoxy. Loss modulus showed shifts in the T_g of the polymer matrix with the addition of fibre as reinforcing phase, which indicate that fibre plays an important role in case of T_g. The Tan δ peak height was minimum for jute composites and maximum for epoxy matrix. Complex modulus variations and phase behaviour of the hybrid composites was studied by Cole-Cole analysis. Thermal analysis result indicates an increase in thermal stability of EFB composite with the incorporation of woven jute fibres. Hybridization of EFB composite with J_w fibres enhanced the dynamic mechanical and thermal properties.     

Effect of matrix on the dry friction coefficient of unidirectional fiber-reinforced composite

The coefficients of dry rest (μ₀) and sliding (μ _s ) friction on a polished disk made of quenched steel have been measured for various polymer matrices and fibers and for a composite unidirectionally reinforced with poly(amidobenzimidazole) (PABI) fibers. It is established that μ₀ > μ _s for matrices and fibers with glass transition temperatures T _g below room temperature T _R , otherwise μ₀ ≈ μ _s . This effect is explained by a sharp growth in the plasticity of polymers at T _g , which leads to an increase in the polymer-steel contact area. For a composite with T _g > T _R , the dry friction coefficients obey the relation μ₀ ≈ μ _s ≈ μ _e /C, where μ _e is the coefficients of sliding friction of PABI fibers and C is their concentration. For T _g < T _R (plastic matrix), the friction coefficients of the composite and matrix are close because the latter cannot hold the fiber ends during friction. As a result, they are bent and aligned along the matrix surface or embedded in the surface layer.     

Characterization of polystyrene and doped polymethylmethacrylate thin layers

About 1 μm thick films of polystyrene (PS) and polymethylmethacrylate (PMMA) were prepared from solutions using spin-coating method. The PMMA films were doped with diphenylsulfoxide (DS) up to 45 wt%. Glass transition temperature (T _g ) of doped PMMA films was determined by DSC technique and relative permittivity (ε) as a function of the sample temperature was determined from capacitance measurement. The dependence of polarization (P) on electric field (E) and the temperature was measured using a standard Sawyer-Tower circuit. Spectral dependence of film refractive index was measured using a refractometer. The glass transition temperature T _g of PMMA/DS composite was found to be decreasing function of the DS concentration. Relative permittivity ε of unpolar PS is lower than that of polar PMMA. The PS permittivity does not depend on the sample temperature. For PMMA the permittivity is increasing function of both, DS dopant concentration and sample temperature. The dependence of the polarization on the electric field on PS film does not exhibit a hysteresis and indicate no polarization contrary to PMMA. PMMA/DS composites exhibit easier and larger polaribility and a permanent dipole moment. Resulting polarization is an increasing function of DS concentration. Refractive index of both pristine PS and PMMA decreases with increasing wave length. The refractive index of PMMA/DS composites depends on the DS concentration.     

Thermal stability and fire behavior of aluminum diethylphosphinate-epoxy resin nanocomposites

The flame retardancy of 2, 2-bis(4-glycidyloxyphenyl)propane (DGEBA)-aluminum diethylphosphinate (AlPi) nanocomposites (EP-AlPi/(P ? x), x = 1, 2, 3 %) was greatly enhanced by ultrasonic dispersion of nano-sized AlPi into epoxy resin. The UL 94 V-0 rating can be reached for EP-AlPi nanocomposites with a relatively low addition amount of AlPi (on the account of 8.4 wt% or phosphorus content of 2 wt%) as well as the LOI value over 37.2. The glass transition temperature (T _g) enhanced properties were investigated by DTA, which showed that: T _gs were about 5 °C higher than that of neat epoxy resin; T _g increased along with content increasing of AlPi. Based on TGA results under a non-isothermal condition, the thermal degradation kinetics of EP-AlPi/(P ? x) composites were studied by Kissinger’s, Ozawa’s, Flynn–Wall–Ozawa’s and Coast-Redfern’s methods, which suggested the conversion function f (α) = 1/2α ^?1 or f (α) = [?ln(1 ? α)]^?1 for EP-AlPi/(P ? 1 %); f (α) = [?ln(1 ? α)]^?1 for EP-AlPi/(P ? 2 %) and EP-AlPi/(P ? 3 %) during the investigated process. The epoxy resin nanocomposites obtained in this study are green functional polymers and will become flame retardant potential candidates in electronic fields such as printed wiring boards with high performance.     

Temperature shift of the optical gap in some PbO-ZnO-P2O5 glasses

The optical gap (E_g) between 4.54 eV-4.88 eV at room temperature was determined for PbO-ZnO-P₂O₅ glasses with the formal content of P₂O₅ in the region of 30 to 50 mol% and with the formal content of PbO in the region of 50 to 45 mol%, respectively. The temperature (T) dependence of the optical gap (E_g(T)) in the region 80 < T[K] < 600 was determined, and an electron-phonon interaction is suggested to be a major contribution to the temperature shift of the optical gap. In the temperature region of 300-600 K, the E_g(T) dependence can be approximated by a simple linear relation with the temperature coefficient (γ) of the optical gap in the region 6.04 ≤ γ × 10⁴ [eV/K] ≤ 7.39.     

Correlation between glass transition temperature and melting temperature in metallic glasses

We report that the glass transition temperature (T_g) of a variety of metallic glasses (MGs) correlates with the eutectic or peritectic temperature of two main components corresponding stoichiometric proportion in their binary phase diagram. The correlation suggests that the T_g of MGs is mainly determined by their solvent of two base components, which have composition close to the eutectic and peritectic points in the binary phase diagram and the weakest link in amorphous structure. The results have implications for understanding the structure and glass transition in MGs and for predicting and designing metallic glasses with a desirable T_g.     

Multi-step shear banding for bulk metallic glasses at ambient and cryogenic temperatures

Analysis of energy dissipation during shear-banding aids to understand plastic deformations of bulk metallic glasses (BMGs). For Zr₅₅Al₁₀Ni₅Cu₃₀ BMGs at 298 K, multi-step shearing is proposed, and the thermal energy during serrations cannot result in a temperature rise (ΔT) higher than T_m. At 77 K, N-step shearing is supposed. When N is in the range of 24–47, it is reasonable to deduce the resulting temperature from 0.8T_g to T_m, accompanied by a continuous plastic deformation.     

Effect of nanofiber on material properties of vapor-grown carbon nanofiber reinforced thermoplastic polyurethane (TPU/CNF) nanocomposites prepared by melt compounding

The present work deals with the preparation of the CNF based TPU nanocomposites by melt blending to explore the effect of state of dispersion and wt.% loading of CNF on material properties. In addition, the morphology, mechanical, thermal, rheological, and electrical properties of the nanocomposites have been evaluated through various characterization techniques with an aim to find the suitability of the nanocomposites for industrial applications. Transmission electron microscopy (TEM) study reveals that the CNFs exhibited a uniformly dispersed in TPU matrix. The thermal stability of the TPU evaluated by thermogravimetric analysis (TGA) showed significant increase with increased CNF content. It is observed that storage modulus (E′) and glass transition temperature (T_g) of the TPU matrix increases by the incorporation of CNF. The melting point (T_m) and the T_g of soft segments observed from the differential scanning calorimetry (DSC) were found to shift towards higher temperature with the inclusion of CNF.     

Non-stoichiometry and optical spectra of Nd(III) substituted PbTiO3

The non-stoichiometry of the perovskite (ABO₃)-type phase in the system PbOTiO₂Nd₂O₃ has been studied. Monophasic compounds of composition Pb_1?αxNd_xTiO_3+x(1.5?α) with x ? 0.21 and 0.09 ? α ? 1.5 were prepared. The ferroelectric Curie temperature (T_c) shows a decrease of 18.5 K/at% Nd with increasing value of x. T_c shows an increase of 3.5 K/mol % PbO with decreasing value of α (increasing content of PbO). The observed effect of α on optical spectra can be interpreted by assuming that Nd(III) ions partly occupy B sites in compounds with α < 1.5.     

Crystallization study of Te-Bi-Se glasses

Crystallization studies are carried out under non- isothermal conditions with samples heated at several uniform rates. The dependence of the glass transition temperature (T_g), the crystalline temperature (T_c) and the peak temperature of crystallization (T_p) on the composition and heating rate (β) has been studied. For a memory/switching material, the thermal stability and ease of glass formation are of crucial importance. The glass transition temperature, T_g, increases slightly with the variation of Bi content. From the heating rate dependence of T_g, the activation energy for glass transition (E_t) has been evaluated. The results are discussed on the basis of Kissinger’s approach and are interpreted using the chemically ordered network model (CONM).     

Relook on fitting of viscosity with undercooling of glassy liquids

The present approach is on the modification of viscosity fitting of undercooled liquid as a function of undercooling. The method consists of finding analytical solution of three arbitrary constants of the Vogel–Fulcher–Tamman (VFT) equation by choosing three viscosity data at three critical temperatures for an undercooled liquid. Three critical temperatures are liquidus temperature (T _l), crystallization onset temperature (T _x) and glass transition temperature (T _g). The experimental viscosity data at or very near to these three critical temperatures (depending on the availability in literature) have been utilized to achieve the analytical solution. The analytical solution of VFT equation is further examined by selecting the experimental data points away from the critical temperatures in order to check their (T _l,T _x and T _g) significance towards the solution. Total absolute error (TAE) and total squared error (TSE) values obtained from the present method with respect to the experimental viscosity data in the temperature range between T _l and T _g are very much comparable and in most of the cases lower than that of existing ‘best-fit’ cited in the literature for a number of glassy alloys. Moreover, this method interestingly enables us to find the fragility parameters for a number of glassy alloys and convincingly explain their true glass forming abilities (GFA).