Peculiar electrical and photoelectric behaviors in conducting multilayers: Insights into accumulative charge tunneling

Layer‐by‐layer self‐assembly is a versatile technique for the construction of well‐defined nanoarchitectures with outstanding electrical and photoelectric performances. The revelation of a potential charge‐transfer mechanism of extraordinary electrical and photoelectric behaviors is profound in the design of modern electrical and photoelectrical devices. With the aim of revealing the potential charge‐transfer mechanism in conducting multilayer films, in this study, we fabricated [poly(styrene sulfonate)/polyaniline]_n [(PSS/PANi)_n] multilayers with peculiar electrical and photoelectrical features. The fantastic increments in sheet conductivity and photoelectric response were believed to be the percolation reflection of accumulative electrons tunneled across the insulating PSS from the bottom to the top of the conjugated structure of PANi. These profound phenomena, along with simple fabrication and a well‐defined architecture, promise that the conducting multilayers will be good candidates for electronic and optoelectronic nanodevices. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40258.     

Variation of non‐isothermal crystallization behavior of isotactic polypropylene with varying β‐nucleating agent content

The non‐isothermal crystallization behavior, the crystallization kinetics, the crystallization activation energy and the morphology of isotactic polypropylene (iPP) with varying content of β‐nucleating agent were investigated using differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). The DSC results showed that the Avrami equation modified by Jeziorny and a method developed by Mo and co‐workers could be successfully used to describe the non‐isothermal crystallization process of the nucleated iPPs. The values of n showed that the non‐isothermal crystallization of α‐ and β‐nucleated iPPs corresponded to a tridimensional growth with homogeneous and heterogeneous nucleation, respectively. The values of crystallization rate constant showed that the rate of crystallization decreased for iPPs with the addition of β‐nucleating agent. The crystallization activation energy increased with a small amount (less than 0.1 wt%) of β‐nucleating agent and decreased with higher concentration (more than 0.1 wt%). The changes of crystallization rate, crystallization time and crystallization activation energy of iPPs with varying contents of β‐nucleating agent were mainly determined by the ratio of the content of α‐ and β‐phase in iPP (α‐PP and β‐PP) from the DSC investigation, and the large size and many intercrossing lamellae between boundaries of β‐spherulites for iPPs with small amounts of β‐nucleating agent and the small size and few intercrossing bands among the boundaries of β‐spherulites for iPPs with large amounts of β‐nucleating agent from the SEM examination. Copyright © 2010 Society of Chemical Industry     

Mixed electron and proton conductivity of polyaniline films in aqueous solutions of acids: beyond the 1000 S cm−1 limit

BACKGROUND: The application potential of conducting polymers depends on their conductivity. It is generally assumed that the conductivity determined in the dry state is a parameter that unambiguously characterizes them. RESULTS: The conductivity of polyaniline (PANI) films immersed in aqueous solutions of sulfuric acid may be more than 1000 times higher compared with that obtained by measurement of dry films in air, and is estimated to reach a value exceeding 3300 S cm^?1 in 1 mol L^?1 sulfuric acid. This is explained by the reduction of conductivity barriers between conducting PANI islands. CONCLUSION: The organized polymer chains in the conducting islands of a PANI film are separated by disordered regions of low conductivity in the dry state. The penetration of sulfuric acid solution into the disordered areas increases the overall conductivity of the PANI film by improving the electrical contact between the islands through ionic charge transport. The electronic conductivity of the PANI film in the dry state thus converts to mixed electron–proton conduction in acidic aqueous solutions, electron conductivity being dominant in ordered regions and ionic conductivity in disordered regions separating them. Weakly bound protons are the most important ionic charge carriers hopping along the PANI chains. Copyright © 2009 Society of Chemical Industry     

Isothermal crystallization of pure and glass fiber reinforced poly(phenylene sulfide) composites

The isothermal crystallization of poly(phenylene sulfide) (PPS) and its glass fiber (GF) reinforced composite has been investigated by differential scanning calorimetry (DSC) and polarized optical microscope (POM) equipped with a hot stage. During the isothermal crystallization process of PPS the Avrami exponent value decreases with time, indicating the change of crystallization mechanism and presence of mixed growth mechanism (bidimensional and three‐dimensional growth of crystals). The presence of GF greatly accelerates the bulk crystallization rate of PPS and changes the crystalline morphology of PPS from the spherulites to transcrystallization. And the reason for development of transcrystallization is considered to be relevant with the compatibility of the interface between GF and PPS. POLYM. COMPOS., 2009. © 2009 Society of Plastics Engineers     

Space charge distribution and crystalline structure in low density polyethylene (LDPE) blended with high density polyethylene (HDPE)

The space charge distribution in polyethylene blends under direct current electrical field was measured by a pulsed electro‐acoustic method. It was found that blending LDPE with 0.5 wt% HDPE decreased the amount of accumulated space charges and improved their distribution. Small‐angle light scattering and differential scanning calorimetry showed that crystallization of LDPE/HDPE started at higher temperature than virgin LDPE, and the sizes of LDPE/HDPE spherulites were smaller than that of LDPE. HDPE plays a role on nucleation during the crystallization process. Crystalline form was investigated by wide‐angle X‐ray diffraction and the results indicate that the crystal form did not change after blending. The reduction of space charges in the blended sample can be explained as the result of the dissipation of charges through boundary regions of smaller spherulites. Copyright © 2004 Society of Chemical Industry     

Design and electrical conductivity of poly(acrylic acid‐g‐gelatin)/graphite conducting gel

A novel poly(acrylic acid‐g‐gelatin)/graphite composite is synthesized by aqueous solution polymerization. Based on the electrical conductivity of graphite nanoplatelets and the absorbency of poly(acrylic acid‐g‐gelatin)/graphite, a novel conducting gel with a conductivity of 3.18 mS cm^?1 is prepared. The effects of synthesis parameters on the electrical conductivity of the gels are investigated in detail. An appended network structure model of the poly(acrylic acid‐g‐gelatin)/graphite conducting gel is proposed. The conducting gel presents a high mechanical strength in cyclohexane, which is important for the gel applications. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Confined crystallization morphology of poly(ϵ‐caprolactone) block within poly(ϵ‐caprolactone)–poly(l‐lactide) copolymers

The confined crystallization of poly(?‐caprolactone) (PCL) block in poly(?‐caprolactone)–poly(l ‐lactide) (PCL‐PLLA) copolymers was investigated using differential scanning calorimetry, polarized optical microscopy, scanning electronic microscopy and atomic force microscopy. To study the effect of crystallization and molecular chain motion state of PLLA blocks in PCL‐PLLA copolymers on PCL crystallization morphology, high‐temperature annealing (180 °C) and low‐temperature annealing (80 °C) were applied to treat the samples. It was found that the crystallization morphology of PCL block in PCL‐PLLA copolymers is not only related to the ratio of block components, but also related to the thermal history. After annealing PCL‐PLLA copolymers at 180 °C, the molten PCL blocks are rejected from the front of PLLA crystal growth into the amorphous regions, which will lead to PCL and PLLA blocks exhibiting obvious fractionated crystallization and forming various morphologies depending on the length of PLLA segment. On the contrary, PCL blocks more easily form banded spherulites after PCL‐PLLA copolymers are annealed at 80 °C because the preexisting PLLA crystal template and the dangling amorphous PLLA chains on PCL segments more easily cause unequal stresses at opposite fold surfaces of PCL lamellae during the growth process. Also, it was found that the growth rate of banded spherulites is less than that of classical spherulites and the growth rate of banded spherulites decreases with decreasing band spacing. © 2019 Society of Chemical Industry     

Evaluation of mixed‐conducting lanthanum‐strontium‐cobaltite ceramic membrane for oxygen separation

In this study, La_0.4Sr_0.6CoO_3‐δ (LSC) oxide was synthesized via an EDTA‐citrate complexing process and its application as a mixed‐conducting ceramic membrane for oxygen separation was systematically investigated. The phase structure of the powder and microstructure of the membrane were characterized by XRD and SEM, respectively. The optimum condition for membrane sintering was developed based on SEM and four‐probe DC electrical conductivity characterizations. The oxygen permeation fluxes at various temperatures and oxygen partial pressure gradients were measured by gas chromatography method. Fundamental equations of oxygen permeation and transport resistance through mixed conducting membrane were developed. The oxygen bulk diffusion coefficient (D_v) and surface exchange coefficient (K_ex) for LSC membrane were derived by model regression. The importance of surface exchange kinetics at each side of the membrane on oxygen permeation flux under different oxygen partial pressure gradients and temperatures were quantitatively distinguished from the oxygen bulk diffusion. The maximum oxygen flux achieved based on 1.6‐mm‐thick La_0.4Sr_0.6CoO_3‐δ membrane was ～4.0 × 10^?7 mol cm^?2 s^?1at 950°C. However, calculation results show theoretical oxygen fluxes as high as 2.98 × 10^?5 mol cm^?2 s^?1 through a 5‐μm‐thick LSC membrane with ideal surface modification when operating at 950°C for air separation. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Influence of the thin‐film thickness and crystallization temperature on the spherulitic structure of polymer thin films

The spherulitic structure and morphology of poly(3‐hydroxybutyrate) (PHB) thin films crystallized from the melt were observed with a polarizing optical microscope. Depending on the thickness of the PHB thin film and crystallization temperature, banded and nonbanded spherulites could form. Reducing the thin‐film thickness and crystallization temperature was favorable for the formation of the banded structure. The morphology transition from banded spherulites to nonbanded spherulites was related to the ratio of the crystallization rate to the diffusion rate. The formation mechanism of the banded structure was examined with the discontinuity growth theory. A depletion zone was considered to appear periodically at the crystal growth front because of the slow diffusion rate, and this may have resulted in the banded spherulites. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Spherulitic formation and characterization of partially fluorinated copolymers and their nanohybrids with functional fillers

Poly[vinylidenefluoride‐co‐(tetrafluoroethylene)] (P(VDF‐TeFE)) exhibited clear spherulitic texture with negative birefringence. The number and growth rates of the spherulites decreased at high crystallization temperature than at low crystallization temperature. Nonetheless, overall larger spherulites were found at high crystallization temperature and the brightness of the spherulites increased very fast at low crystallization temperature, thereafter it seemed as diminution of birefringence. AFM was used to investigate the impact of organo modified nanodiamond(ND) on spherulitic textures, lamellar thickness, and thickness distribution of P(VDF‐TeFE) copolymer. Poly[ethylene‐co‐(tetrafluoroethylene)] (ETFE) also confirmed spherulites structure and the boundaries could be clearly observed. By incorporation of the organo modified nanodiamond (ND) and organo‐modified montmorillonite (MMT) in fluropolymer matrix, it was found that spherulitic texture was seriously disordered and their nanohybrids was found only to have poorly developed spherulite structure. Both of the nanohybrids samples show better crystallization temperature as compared to their neat copolymer samples. Furthermore, the incorporation of nanoparticles decreased the size of the spherulites. POLYM. ENG. SCI., 57:161–171, 2017. © 2016 Society of Plastics Engineers     

Crystallization behavior and crystallization kinetic studies of isotactic polypropylene modified by long‐chain branching polypropylene

In this article, we discuss the crystallization behavior and crystallization kinetics of isotactic polypropylene (iPP) modified by long‐chain‐branching (LCB) high‐melt‐strength iPP over a wide composition range, that is, LCB‐iPP from 10 to 50 wt %. Over the entire range we investigated, the presence of LCB‐iPP accelerated crystallization in both the isothermal crystallization process and nonisothermal crystallization process, even when the LCB‐iPP content was as low as 10%, and both crystallization processes were enhanced more significantly as the LCB‐iPP content increased. Hoffman–Lauritzen theory analysis revealed that the fold‐free energy decreased effectively with the occurrence of the LCB structure, although the growth rate of spherulites was depressed, as shown by polarized optical microscopy. Meanwhile, the regime III–regime II transition temperature was about 15° higher for all of the LCB‐iPP compositions than that of iPP because the LCB structure reduced the mobility of the polypropylene chains. Furthermore, the γ‐form crystal structure was favored by LCB compared to the β form, which was supported by wide‐angle X‐ray diffraction. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Electric Field‐Enhanced Solid‐State Conversion of Ceramic Sr5(PO4)3F to Crystals

The single crystal solid‐state conversion of fluorapatite‐type Sr₅(PO₄)₃F (Sr‐FAP) has been achieved by spark plasma sintering with the assistance of NaF additive. NaF was determined to act as both a sintering aid and impurity solute along the grain boundaries (GBs), controlling both the space charge and GB migration rate. Postsintering isothermal annealing was performed to examine the effect of DC electric field on grain growth. From the space charge potential determined from impedance spectra measurements, in combination with the theoretical contribution of space charge to grain‐boundary energy reduction, it was concluded that the magnitude of the space charge in Sr‐FAP is temperature dependent. As such, a moderate decrease in polycrystalline GB driving force is the main cause for the increased single crystal migration length that was observed in this study.     

Mixed Conduction in Ceramic Hydrogen/Steam Electrodes by Hebb–Wagner Polarization in the Frequency Domain

The unique mixed conduction aspect has much to be explored for ceramic fuel/steam electrodes suggested for higher chemical stability than cermets in SOFC/SOEC applications. Typical symmetric cell configuration of the thin ceramic electrodes on the electrolytes was shown to be a versatile configuration of the electron‐blocking Hebb–Wagner polarization cell for the characterization of the mass and charge transport in mixed conducting oxides by impedance spectroscopy. The Jamnik–Maier transmission line model for the mixed conductors was generalized by replacing the capacitors with constant‐phase elements to describe the extremely non‐ideal impedance behavior of porous but negligibly catalytic La₂Ti₂O₇ layers at various humidity levels up to 90% in hydrogen at 850 °C. Modeling allowed the deconvolution of the parameter set for the evaluation of chemical diffusivity and non‐stoichiometry as well as partial ionic and electronic conductivity of La₂Ti₂O₇. Electrostatic double layer capacitance was found to be orders of magnitude higher than the theoretical despite the correct oxygen activity dependence, which appears generic and thus requires a theoretical clarification. Overlapping interfacial and chemical capacitance as well as the strongly non‐ideal behavior of the latter are mainly responsible for the less clearly distinguished diffusion‐limited feature of the Hebb–Wagner polarization cell.     

In situ studies on the temperature‐related deformation behavior of isotactic polypropylene spherulites with uniaxial stretching: The effect of crystallization conditions

The deformation behavior of isotactic polypropylene (iPP) spherulites with uniaxial stretching was investigated at different drawing temperatures via in situ polarized optical microscope (POM) observation. The iPP spherulites were prepared by two procedures: cooled to the room temperature from melt and annealed at 135, 140, and 145°C for 3 h. It was found that the crystallization conditions dominate the crystalline morphology and even the tensile properties of iPP. For iPP which crystallized during cooling progress, the spherulites were imperfect and the boundaries of the spherulites were diffuse, displaying good toughness at various drawing temperatures. For iPP annealed at high temperatures displayed the brittle fracture‐modes and the crack happened between spherulites, which due to the large and perfective spherulites have thick lamellas and weak connection at interspherulitic boundary. The shape and size of the iPP spherulites formed at 140 and 145°C are affected with uniaxial stretching till to the fracture of the samples at different drawing temperatures. The spherulites obtained at 135°C are deformed along the drawing direction at 100°C but not affected at low drawing temperatures, indicating the toughness increased with the increase of the drawing temperatures. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

Fragment Velocity Distribution of Cylindrical Rings Under Eccentric Point Initiation

A design mode, in which a casing is filled with a charge initiated off‐centre (eccentric point initiation), is common in the field of explosion and structural protection. The fragment velocity distribution along the circumference of the casing is distinctly non‐uniform due to the difference in blast loading around the circumference of the casing. To quantify the fragment velocity distribution, two kinds of cylindrical rings with different structural parameters were adopted. Static explosive experiments with three eccentric coefficients (0, 0.5, 1.0) were conducted with pulsed X‐ray diagnostics. Using coefficients derived from experimental data and calculating the effects of both the eccentricity of initiation and angle around the circumference, an angle‐dependent ratio β_θ of charge mass to casing mass has been derived as a mean to modify the fragment velocity formula of Gurney for this application. The derived formula was shown to correctly predict the fragment velocity distribution around the circumference of the cylindrical ring. The calculated velocity distributions show good agreement with the experimental results.     

Investigation of electrical conduction mechanism in double‐layered polymeric system

The electrical conduction in solution‐grown polymethylmethacrylate (PMMA), polyvinylidenefluoride (PVDF) and PMMA‐PVDF double‐layered samples in the sandwich configuration (metal‐polymer‐metal) was investigated at different fields in the range 100–120 kV/cm as a function of temperature in the range 293–423 K for samples of constant thickness of about 50 μm. Certain effects which lead to a large burst of current immediately after the application of field were observed in double‐layered samples. An attempt was made to identify the nature of the current by comparing the observed dependence on electric field, electrode material and temperature with the respective characteristic features of the existing theories on electrical conduction. The observed linear I‐V characteristics show that the electrical conduction follows Pool‐Frenkel mechanism in PMMA and PVDF samples. Whereas, the non‐linear behavior of current‐voltage measurements in PMMA‐PVDF double‐layered samples have been interpreted on the basis of space charge limited conduction (SCLC) mechanism. The conductivity of the polymer films increased on formation of their double‐layer laminates. The polymer‐polymer interface act as charge carrier trapping centres and provides links between the polymer molecules in the amorphous region. The interfacial phenomenon in polymer‐polymer heterogeneous system has been interpreted in terms of Maxwell‐Wagner model. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Alternative concept for SOFC with direct internal reforming operation: Benefits from inserting catalyst rod

Mathematical models of direct internal reforming solid oxide fuel cell (DIR‐SOFC) fueled by methane are developed using COMSOL® software. The benefits of inserting Ni‐catalyst rod in the middle of tubular‐SOFC are simulated and compared to conventional DIR‐SOFC. It reveals that DIR‐SOFC with inserted catalyst provides smoother temperature gradient along the system and gains higher power density and electrochemical efficiency with less carbon deposition. Sensitivity analyses are performed. By increasing inlet fuel flow rate, the temperature gradient and power density improve, but less electrical efficiency with higher carbon deposition is predicted. The feed with low inlet steam/carbon ratio enhances good system performances but also results in high potential for carbon formation; this gains great benefit of DIR‐SOFC with inserted catalyst because the rate of carbon deposition is remarkably low. Compared between counter‐ and co‐flow patterns, the latter provides smoother temperature distribution with higher efficiency; thus, it is the better option for practical applications. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

Crystallization behavior of spray‐dried and freeze‐dried graphene oxide/poly(trimethylene terephthalate) composites

Two types of graphene oxide (GO) powders were prepared by freeze‐drying or spray‐drying method, and their composites with poly(trimethylene terephthalate) (PTT) were prepared by melt blending. The influence of GO powders' type and content on crystallization behavior of PTT was investigated by differential scanning calorimeter (DSC) and polarized optical microscopy (POM). DSC results indicated that the overall crystallization rate of PTT was accelerated by well‐dispersed GO material which acts as a heterogeneous nucleation agent, since the Avrami parameter obtained is near 3. On the contrary, large GO aggregates which is in the minority will hinder the nucleation. Interestingly, large and well‐defined PTT spherulites instead of tremendous stunted spherulites were observed from POM, which means only a small portion of GO powders was acted as nucleation agent. Meanwhile, GO powders had no effect on PTT spherulites growth rate. In addition, the band spacing of PTT spherulites became weaker and wider with increasing GO content. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40332.     

Probing inhomogeneous doping in overlapped graphene grain boundaries by Raman spectroscopy

The effect of grain boundaries and wrinkles on the electrical properties of polycrystalline graphene is pronounced. Here we investigate the stitching between grains of polycrystalline graphene, specifically, overlapping of layers at the boundaries, grown by chemical vapor deposition (CVD) and subsequently doped by the oxidized Cu substrate. We analyze overlapped regions between 60 and 220 nm wide via Raman spectroscopy, and find that some of these overlapped boundaries contain AB–stacked bilayers. The Raman spectra from the overlapped grain boundaries are distinctly different from bilayer graphene and exhibit splitting of the G band peak. The degree of splitting, peak widths, as well as peak intensities depend on the width of the overlap. We attribute these features to inhomogeneous doping by charge carriers (holes) across the overlapped regions via the oxidized Cu substrate. As a result, the Fermi level at the overlapped grain boundaries lies between 0.3 and 0.4 eV below the charge neutrality point. Our results suggest an enhancement of electrical conductivity across overlapped grain boundaries, similar to previously observed measurements (Tsen et al., 2012). The dependence of charge distribution on the width of overlapping of grain boundaries may have strong implications for the growth of large-area graphene with enhanced conductivity.     

Crystallization and melting behaviour of polypropylene ‘catalloys’

The crystallization and melting behaviour of polypropylene ‘catalloys’ (PP‐cats) as well as pure polypropylene (PP) were investigated using differential scanning calorimetry. The results showed that, for PP‐cats and PP, a single melting peak of PP appeared under slow cooling rate. When the cooling rate is fast enough in the non‐isothermal case, or the crystallization temperature is relatively high in the isothermal case, a shoulder peak appears in front of the melting peak with increasing ethylene content in PP‐cats. It is believed that this shoulder is induced by recrystallization of crystals initially formed during non‐isothermal or isothermal crystallization. When the ethylene component in PP‐cats reached a certain level, there existed a melting peak of polyethylene (PE) crystallized during the cooling process. Polarized optical microscopy (POM) showed that the spherulites formed by PP‐cats were much smaller and had less perfect morphology compared with that formed by pure PP at the same cooling rate. And with the increase of the cooling rate, the spherulites could not be clearly observed. Copyright © 2004 Society of Chemical Industry