Dezhen Wu

Silica encapsulation of n-octadecane via sol-gel process: a novel microencapsulated phase-change material with enhanced thermal conductivity and performance.

A novel microencapsulated phase-change material (PCM) based on an n-octadecane core and an inorganic silica shell was designed to enhance thermal conductivity and phase-change performance. These silica microcapsules were synthesized by using TEOS as an inorganic source through a sol-gel process. Fourier transform infrared spectra confirm that the silica shell material was successfully fabricated onto the surface of the n-octadecane core. Scanning electronic microscopy images suggest that the silica microcapsules exhibit a spherical morphology with a well-defined core-shell microstructure. Furthermore, the silica microcapsules synthesized at pH 2.45 display a smooth and compact surface. These microcapsules also present a large particle size range of 7-16 microm. Wide-angle X-ray scattering patterns indicate that the n-octadecane inside the silica microcapsules still retains a good crystallinity. Thermogravimetric analysis shows that these silica microcapsules are degraded in two distinct steps, and have good thermal stability. The silica-microencapsulated n-octadecane can achieve good phase-change performance, high encapsulation efficiency, and good antiosmosis property by controlling the loading of core material and acidity of the reaction solution during the sol-gel process. The thermal conductivity of the microencapsulated n-octadecane is also significantly enhanced due to the presence of the high thermal conductive silica shell.

Dynamic random access memory effect and memory device derived from a functional polyimide containing electron donor-acceptor pairs in the main chain.

A functional polyimide, hexafluoroisopropyl bis(phthalic dianhydride)/3,6-diaminocarbazole (6FDA/DAC), in which DAC serves as electron donor and 6FDA as electron acceptor, has been synthesized in our present work. Electrical characterization results on the sandwiched polyimide memory device (ITO/Thin polyimide Layer/Au) indicate that the polyimide possesses electrical bistability and the device exhibits two accessible conductivity states, which can be reversibly switched from the low-conductivity (OFF) state to the high-conductivity (ON) state with an ON/OFF current ratio of about 10(4). Different from the widely reported write-once-read-many-times (WORM) effects, the device with the 6FDA/DAC polyimide as the active layer shows dynamic random access memory (DRAM) behavior. The ON state of the device was lost immediately after removal of the applied voltage, while by applying a constant bias (e.g., 3 V) the ON state can be electrically sustained. The roles of donor and acceptor components in the polyimide main chain were elucidated through molecular simulation.

Synthesis, characterization and curing properties of a novel cyclolinear phosphazene-based epoxy resin for halogen-free flame retardancy and high performance

A novel cyclolinear phosphazene-based epoxy resin has been synthesized through a four-step synthetic route. The curing behaviors of this epoxy resin with methyl tetrahydrophthalic anhydride, 4,4′-diaminodiphenylmethane, and novolak as hardeners were investigated by differential scanning calorimetry (DSC). The thermal behaviors and stabilities were also evaluated with DSC and thermogravimetric analysis. These thermosets achieved high glass transition temperatures over 150 °C and also gained good thermal stabilities with high char yields. The flammability characteristics of the cyclolinear phosphazene-based epoxy thermosets were investigated by limiting oxygen index (LOI) and UL-94 vertical burning experiments. The high LOI values and UL-94 V-0 classification of these epoxy thermosets indicate that the incorporation of phosphazene rings into the molecular backbone imparts non-flammability to the epoxy resin as a result of the unique combination of phosphorus and nitrogen following by a synergistic effect on flame retardancy. The analysis of the residual chars collected from the UL-94 test demonstrates that cyclotriphosphazene moieties of this epoxy resin can enhance char formation during combustion serving as a barrier against heat and oxygen diffusion, and consequently the flame retardancy of the thermosets is improved significantly.

Nonvolatile memory effect of a functional polyimide containing ferrocene as the electroactive moiety

A functional polyimide, hexafluoroisopropyl bis(phthalic dianhydride)/4-(bis(4-aminophenyl) methyl)phenol grafted with ferrocene, was synthesized. Electrical characterization results indicate that the sandwiched devices using our synthesized polyimide as the active layer possess electrical bistability and exhibit nonvolatile memory behavior with an ON/OFF current ratio of about 103. Molecular orbitals and electronic properties are investigated by molecular simulation and cyclic voltammetry characterization. The charge transfer mechanisms in the OFF and ON states of the memory device were reasonably interpreted by using the thermionic emission and space-charge-limited-current model, respectively.

Preparation of highly reflective and conductive metallized polyimide films through surface modification: processing, morphology and properties

Silvered polyimide films have been prepared by potassium hydroxide hydroxylation of polyimide film surfaces and incorporation of silver ions through subsequent ion exchange. Thermal curing not only recycloimidized the poly(amic acid) into polyimide, but also reduced silver ions into silver atoms and near-atomic silver clusters, which diffused and aggregated to give reflective and conductive surfaces without need of the addition of reducing agents. Films were characterized by X-ray photoelectron spectrometer, transmission electron microscopy, scanning electron microscopy and tapping mode atomic force microscopy. The thickness of the silvered layers and the reflectivity and conductivity of the silvered films can be controlled. By this method, the double-sided silvered polyimide films with excellent reflective (reflectivity > 97%) and conductive surfaces (surface resistivity = 0.02 Ω cm−2) could be easily fabricated. Their essential mechanical properties could be maintained, and the silver–polymer adhesion was outstanding.

Preparation of polyimide/zinc oxide nanocomposite films via an ion-exchange technique and their photoluminescence properties

Polyimide (PI) composite films with ZnO nanoparticles embedded in the surface layer were prepared by alkali hydrolyzation following ion exchange in Zn(NO3)2 solution and thermal treatment of the zinc ion-doped PI films in air atmosphere. The effect of alkali treatment, ion exchange, and thermal treatment conditions was investigated in relation to the amount of zinc atomic loading, morphology, photoluminescence (PL), and thermal properties of the PI/ZnO composite films using ICP, XPS, FE-SEM, TEM, Raman microscope, TGA, and DSC. ZnO nanoparticles were formed slowly and dispersed uniformly in the surface-modified layers of PI films with an average diameter of 20 nm. The PL spectra of all the PI/ZnO nanocomposite films obtained at 350°C/7 h possessed a weak ultraviolet emission peak and a broad and strong visible emission band. The PI/ZnO nanocomposite films maintained the excellent thermal property of the host PI films.

Fabrication of nickel oxide nanocomposite layer on a flexible polyimide substrate via ion exchange technique.

Continuous nickel oxide (NiO) nanocomposite layer on flexible polyimide (PI) substrate was prepared via an ion exchange technique. First, nickel(Pi) poly(amate) layers were formed on both surfaces of PI film through chemical surface modification of PI films in aqueous NaOH solution and then ion exchange in aqueous NiSO4 solution. Subsequently, hydrothermal treatment of the Ni2+-loaded PI films in an aqueous urea solution led to Ni(OH)2 formation in the surface-modified layers. Final thermal annealing in ambient air made Ni(OH)2 decompose to NiO, which diffused and aggregated to give continuous layers on both surfaces of PI film. The composite films were characterized by XRD, XPS, SEM, TEM, TGA, and DSC, respectively. Results from SEM and TEM measuring revealed that the NiO layers consisted of NiO nanoparticles with diameter ranging from 10 to 15 nm. Thermal properties of PI/NiO nanocomposite films were similar to those of host PI. This paper provides an effective methodology for the preparation of polymer/metal oxide nanocomposite films, which hold great promise toward the potential application in the areas of flexible microsensors and devices.

Nonvolatile write-once read-many-times memory device based on an aromatic hyperbranched polyimide bearing triphenylamine moieties

An aromatic hyperbranched polyimide, poly(N,N,N′,N′-tetrakis(4-aminophenyl)benzidine-N,N- 4,4′-hexafluoroisopropylidene-diphthalimide) (6F-TEAPBD PI), was synthesized. Semiconductor parameter analysis on the sandwich devices using the synthesized polyimide as the active layer indicates that the polymer possesses distinct electrical bi-stable states with an ON/OFF current ratio of about 300 and a switching voltage at around 2.0 V, which could be applied as nonvolatile write-once read-many-times (WORM) memory. Mechanisms associated with the electrical switching effect are discussed on the basis of the experimental and quantum simulation results. It is suggested that the electric-field-induced charge transport from triphenylamine moieties to hexafluoropropylidene phthalimide units and the subsequent formation of charge-transfer complexes are responsible for the observed electrical memory effect.

Synthesis and characterization of stimuli-responsive poly(acrylic acid) grafted silica nanoparticles

Silica-poly(acrylic acid) (PAAc) core–shell nanoparticles (NPs) were successfully prepared via graft copolymerization of acrylic acid (AAc) onto vinyl-bond-modified silica NPs. Transmission electron microscopy (TEM) results indicated that the obtained micropheres have a core–shell morphology. Fourier transform infrared (FTIR) analysis and x-ray photoelectron spectroscopy (XPS) measurements confirmed that the surface of the nanoparticles was polymer-rich, consistent with the core–shell morphology. The influence of the synthetic conditions, such as reaction time and AAc concentration on the graft yield of PAAc grafted on the silica NPs was investigated. Dynamic light scattering (DLS) analysis showed that the silica-PAAc core–shell nanoparticles possessed excellent response to pH and ion strength. Because of their pH-responsive behavior and small feature size, nanostructure devices designed from the smart silica nanoparticles have potential applications including sensors and membranes.

Tuning the electrical memory characteristics from WORM to flash by α- and β-substitution of the electron-donating naphthylamine moieties in functional polyimides

Two novel functional aromatic polyimides (PIs), 6F-αNA PI and 6F-βNA PI, in which the hexafluoroisopropylidene-diphthalic anhydride (6FDA) serves as the electron-accepting unit and the diphenylnaphthylamine (DPNA) functions as the electron-donating species, were synthesized for memory device applications. The 6F-αNA PI shows distinct electrical bistable states with an ON/OFF current ratio up to 106, and can be switched on bi-directionally with no polarity, which could be applied as the nonvolatile write-once read-many times (WORM) memory. Whereas, the 6F-βNA PI-based memory device exhibits flash type memory characteristics with a switching-on voltage at ca. 1.1 V and an ON/OFF ratio of 104. Both polyimides exhibit good long-term operation stability, survive up to 108 reading cycles with no current degradation, and show ultrafast switching with a response time less than 20 ns. Mechanisms associated with the electrical switching behaviors are discussed on the basis of the experimental and quantum simulation results. The electric-field-induced electronic transition from diphenylnaphthylamine units to hexafluoropropylidene phthalimide units and the subsequent formation of charge-transfer complexes are supposed to be responsible for the observed electrical memory effects. Molecular simulation suggests that α-tethering of the naphthyl group results in more non-coplanar conformation of the DPNA species in the 6F-αNA PI, as compared to that of the β-tethering in the 6F-βNA PI, therefore producing a higher energy barrier that prevents the back charge transfer processes, consequently leading to the WORM vs. flash memory behaviors. The 6F-αNA PI differs from 6F-βNA PI only in the substitution position of the naphthyl group, i.e., α-tethering vs. β-tethering. The distinct memory effects observed here suggest the significance of the electron-donating structures on the memory effects, and the tailorability of the memory characteristics through fine structure adjustment.