Compositional Gradient Films Research Articles

Preparation and characterization of multi‐walled carbon nanotubes/polymer gradient composite films

AbstractBACKGROUND: Recently, much work has focused on the efficient dispersion of carbon nanotubes (CNTs) throughout a polymer matrix for mechanical and/or electrical enhancement. However, there are still only few reports about gradient distribution of CNTs in polymer matrices. In the work reported here, CNTs embedded in a polymer film with a gradient distribution were successfully obtained and studied.RESULTS: For composite films with gradient distributions of CNTs, the upper surface behaves as an intrinsic insulator, while the lower one behaves as a semiconductor, or even as a conductor. It is also found that with an increase of 1 wt% CNTs, the resistance of the bottom surface decreases by 2–3 orders of magnitude, as compared with pure polyarylene ether nitrile; furthermore, when the proportion of CNTs increases up to 5 wt%, the resistance of the bottom surface shows only very little change. As a result, sufficient matrix conductivity of the bottom surface could be achieved at a lower filler concentration with CNTs in a gradient distribution. Meanwhile, the thermal stability, glass transition temperature and tensile properties of the matrix are maintained.CONCLUSION: There is considerable interest in such gradient composite films, which could be applied in the electrical engineering, electronics and aerospace fields, for their excellent mechanical properties, thermal stability and novel electrical properties. Copyright © 2008 Society of Chemical Industry

Properties of Zr–ZrC–ZrC/DLC gradient films on TiNi alloy by the PIIID technique combined with PECVD

The Zr–ZrC–ZrC/DLC gradient composite films were prepared on TiNi alloy by the techniques combined plasma immersion ion implantation and deposition (PIIID) and plasma enhanced chemical vapor deposition (PECVD). With this method, the Zr–ZrC intermixed layers can be obtained by the ion implantation and deposition before the deposition of the ZrC/DLC composite film. In our study, an optimal gradient composite film has been deposited on the NiTi alloys by optimizing the process parameters for implantation and deposition. The surface topography was observed through AFM and the influence of the deposition voltage on the surface topography of the film was investigated. XPS results indicate that on the outmost layer, the Zr ions are mixed with the DLC film and form ZrC phase, the binding energy of C 1s and the composition concentration of ZrC depend heavily on the bias voltage. With the increase of bias voltage, the content of ZrC and the ratio of sp 3/sp 2 firstly increases, reaching a maximum value at 200 V, and then decreases. The nano-indentation and friction experiments indicate that the gradient composite film at 200 V has a higher hardness and lower friction coefficient compared with that of the bare NiTi alloy. The microscratch curve tests indicate that gradient composite films have an excellent bonding property comparing to undoped DLC film. Based on the electrochemical measurement and ion releasing tests, we have found that the gradient composite films exhibit better corrosion resistance property and higher depression ability for the Ni ion releasing from the NiTi substrate in the Hank's solution at 37°C.

Generation and characterization of compositional gradient structure in the biodegradable chitosan/poly(ethylene oxide) blend

AbstractBio‐inspired gradient materials based on the biodegradable polymer blend are expected to be indispensable to the new approaches for biomedical applications. In this study, multi‐step casting self‐organization on the aluminum surface are used to control the compositional distribution in the blend films of biodegradable chitosan and poly(ethylene oxide). By this way, three kinds of compositional gradient, i.e., pseudobilayer (PG), gradient (G) and semigradient (SG) blend films, along the film thickness were prepared. Their compositional morphologies were evaluated by means of ATR‐FTIR and FTIR mapping measurements. In addition, the moisture absorption behavior and the mechanical properties of the gradient film were characterized with referring their gradient morphologies. It is found that the compositional G films has better mechanical properties compared with those of the PG and SG films in both the dry and wet states. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007

Optical and electrical properties of polymer metal nanocomposites prepared by magnetron co-sputtering

Polymer-metal nanocomposite films were prepared by co-sputtering from two independent magnetron sources as well as from a silver-polytetrafluoroethylene (PTFE) composite target. By sputtering from two magnetrons we prepared both, samples with a metal gradient and homogenous composite films. The dielectric constant depends on the metal filling factor and increases approaching the percolation threshold. The resistivity drops from 10 7 to 10 − 3 Ω cm over a narrow range of metal content. The thin composite films show a strong optical absorption in the visible region and a red shift with increasing silver filling. We have produced Bragg reflectors using multilayer systems of composites and pure sputtered PTFE.

Self‐organization of functional gradient structure in the biodegradable chitosan/poly(vinyl alcohol) blend film

AbstractBiologically inspired optimal structures combining the bioresorbable and bioactive properties are expected for the next generation of biomaterials. A compositional gradient structure was found to be spontaneously formed in the film of biodegradable chitosan/poly(vinyl alcohol) blend by casting aqueous solution on an aluminum dish. The formation of compositional gradient structure was confirmed by FTIR mapping measurement, DMTA measurement, and SEM observation on the freeze‐fractured cross section. In DMTA measurement, a broadening of the α‐relaxation curve corresponding to the glass transition was observed for the compositional gradient film, while a composition‐dependent single glass transition was observed for the homogeneous blend films. The resulted film with stable self‐organized compositional gradient exhibits novel physical properties inaccessible for the film of homogeneous blends obtained by casting from the same solution on a Teflon dish. The compositional gradient films present a unique combination of stronger stress and higher yield strain when compared with those of the homogeneous films at both dry and wet states. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3069–3076, 2005

THE MICROSTRUCTURE CHARACTERISTICS AND DIELECTRIC BEHAVIORS OF THE COMPOSITIONALLY GRADED Ba(Ti,Sn)O3 THIN FILMS

ABSTRACT The compositionally graded Ba(SnxTi1 - x)O3 (BTS) films with a compositional gradient from BaTiO3 to BaSn0.15Ti0.85O3 were fabricated on LNO-buffered Pt/Ti/SiO2/Si substrates by a sol-gel deposition method. The graded films crystallized into a pure perovskite structure after post-deposition annealing. The gradient of increasing Ti and decreasing Sn content from the surface of the film to the substrate was nicely demonstrated by the XPS data. Dielectric properties were investigated as a function of both temperature and frequency. It was observed that leakage currents increased asymmetrically for the negative and positive bias voltage with compositional gradient films. Dielectric constant peaks, common to a ferroelectric transition, were not observed in the temperature range from −25°C to 150°C. Generally, ferroelectric material is tunable in only a narrow temperature range near a phase transition. Ba(SnxTi1 - x)O3 compositionally graded films with weak temperature dependence of tunability in the temperature range between −25°C and 30°C could make attractive materials for situations in which precise control of temperature would be either impossible or too expensive.

Quantitative Analysis of Compositional Distribution in Biodegradable Poly(ethylene oxide)/Poly(3‐hydroxybutyrate) Blend Film with Compositional Gradient by FT‐IR Microspectroscopy

AbstractSummary: FT‐IR microspectroscopy was used to map the compositional distribution along the cross‐section on the gradient films of fully biodegradable poly(ethylene oxide) (PEO)/poly(3‐hydroxybutyrate) (PHB) blend system. First, a linear fitting line for a calibration, related the absorbance ratio of two peaks to the fraction of PEO in the blend, was established. During linear fitting, a new equation was deduced and the influence of the different crystallinities on the absorption peaks at the wavenumber 962 and 1 342 cm−1 for PEO, 980 and 1 380 cm−1 for PHB, have been taken into account. For the PEO/PHB blend system, it was found that the crystallinity has little effect on the absorbance ratios of A962/A1 380 and A1 342/A1 380. Based on the results from the linear fitting, which comes from the relation of the absorbance ratios of A962/A1 380 or A1 342/A1 380 and the combination of the weight fraction of PEO (WPEO/(1 − WPEO)), the compositional distributions on the cross‐section of three kinds of compositional gradient films of the PEO/PHB blend prepared by different technologies have been successfully estimated. Furthermore, the better method for preparing the PEO/PHB blend film with compositional gradient was found based on the result of the quantitative analysis of the compositional distribution.The compositional distribution along the cross‐section of the gradient films based on PEO/PHB binary system for the Type I gradient film.magnified imageThe compositional distribution along the cross‐section of the gradient films based on PEO/PHB binary system for the Type I gradient film.

Relationship between the morphological structure and mechanical properties of copper‐in‐hydrophilic polymer gradient composite films

AbstractThis investigation was focused on the influence of polymer hydrophilicity on the morphological structure and mechanical properties of copper‐in‐polymer gradient composite films (CPGCFs). The ion motion and reduction in swelling cathode films under an electric field were the core of the gradient morphology formation of a metal region in the polymer matrix. The morphological study of CPGCFs revealed that the hydrophilicity of poly(vinyl alcohol) was helpful in forming a continuously deposited layer. The nanoclusters (40 nm) aligned into a branchlike form in the copper‐rich region in the poly(vinyl alcohol) matrix. On the basis of the fuzzy interface between the poly(vinyl alcohol) matrix and copper nanoclusters, a complex interaction between them was inferred. The reduced copper affected the mechanical properties of CPGCFs. The maximum load of CPGCFs could be enhanced by 25% to 167.0 N with optimal electrochemical reduction, but the elongation was depressed. An excess of reduced copper in the polymer matrix reduced both the strength and elongation of CPGCFs. The moduli of related samples showed trends similar to those of the strength. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 95: 539–545, 2005

Deposition of Ti/C compositional gradient film onto superplastic Ti‐alloy by magnetron sputtering

AbstractDeposition of Ti/C compositional gradient film onto superplastic titanium‐based alloy such as Ti6Al4V or Ti4.5Al3V2Fe2Mo was performed by magnetron sputtering, in order to improve not only the biocompatibility and the abrasion resistance of the alloy but also the adhesion between the deposited film and the alloy substrate while preserving the high hardness of such ceramic coatings. The Ti/C compositional gradient films were deposited by co‐sputtering of two sputter cathodes which had pure titanium and titanium carbide targets, respectively, and their compositional gradient was realized by varying continuously the electric power supplied to each sputter cathode. Under visual observation, the obtained film appeared to be uniform and adhesive. According to AES in‐depth profiles, the carbon (C) concentration in the film gradually decreased in depth direction from the surface toward the substrate, confirming that Ti/C compositional gradient film had formed on the alloy substrate. On the basis of XRD, it was found that titanium carbide (TiC) and α‐titanium phases were formed in the films. Furthermore, the Vickers hardness of the films reached over HV = 1800. Therefore not only the biocompatibility and the abrasion resistance of the alloy but also the adhesion of the deposited hard coatings were expected to be improved by this method. Copyright © 2004 John Wiley & Sons, Ltd.

Influences of co-monomers and electrolyte acidity on morphological structure of copper-in-copolymer gradient film

In this paper, the influences of composition of copolymers and acidity of electrolyte in an electrochemical reactor on morphological structure of copper-in-polymer gradient composite film were investigated. For binary copolymers, poly(acrylonitrile-co-methyl acrylate) [P(AN-co-MA)] and poly(acrylonitrile-co-sodium allyl sulfonate) [P(AN-co-SAS)], the charged group SO in P(AN-co-SAS) improves the swelling of the copolymer phase and copper reduction to form gradient morphology; the carboxylic ester group in P(AN-co-MA) is not effective because of its poor hydrophilicity, but it is a cooperating component with P(AN-co-SAS) to avoid excess of counterion (i.e., Na+) in SCF, which might severely interrupt Cu2+ coexistence. The swelling of the polymer phase is helpful to decrease the energy of the transfer ions in SCF and to enhance copper deposition and gradient formation. The increase of surface energy because of cluster growth raises the surface energy level of deposited Cu0 clusters. The conteraction between these two energy factors allows the size of clusters to be 50–100 nm. The appropriate H+ concentration improves active Cu2+ reduction and thus deposited gradient copper phase in the copolymer matrix. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 373–380, 2004

Adhesion properties of functionally gradient diamond composite films on medical and tool alloys

We have investigated the adhesion properties of microcrystalline diamond thin films on Ti-Al-V alloy, Co-Cr-Mo alloy and steel. Microcrystalline diamond possesses high hardness, a low coefficient of friction, extreme chemical inertness and biocompatibility; these properties can enhance the performance of metal alloys used in medical implants and in machine tools. We have adopted three methods for improving the adhesion of microcrystalline diamond to commonly used metal alloys: (1) by alloying the substrate surface to minimize graphitization; (2) by employing appropriate buffer layers between the diamond film and the substrate; and (3) by creating functionally gradient diamond-(titanium carbide, tungsten carbide, titanium nitride and aluminum nitride) composites. We have demonstrated that functionally gradient discontinuous buffer layers of titanium carbide, titanium nitride, aluminum nitride and tungsten carbide are able to control stress and graphitization in microcrystalline diamond thin films. This work on buffer layers and functionally gradient coatings should allow the development of more adherent crystalline diamond films for medical and tribological applications.

Deposition of diamond/β–SiC Gradient Composite Films by Microwave Plasma-assisted Chemical Vapor Deposition

Mixed-phase diamond/β–SiC composite films with compositional gradient were prepared by microwave plasma-assisted chemical vapor deposition using a gas mixture of hydrogen, methane and tetramethylsilane (TMS). Single-crystalline silicon wafers, pretreated with diamond nanoparticles before deposition, were used as substrates. The film characterization by scanning electron microscopy, electron probe microanalysis, and energy-dispersive x-ray analysis shows that the contents of diamond and silicon carbide in the films vary with TMS flow rate. Diamond/β–SiC composite films with compositional gradients are achievable by varying the TMS flow rate during the film growth process.

Micromorphological structure and its forming mechanisms of polyacrylonitrile‐based copper gradient composite film

In this article, the micromorphological structure of polyacrylonitrile-copper gradient composite film obtained from the electrochemical reduction of a swelling cathode film was investigated with a scanning electronic microscope and a transmittance electronic microscope. It was found that the micromorphological structure in deposited phase is constructed with copper particles and sheaves of polyacrylonitrile chains. The diameters of both the copper particles and the sheaves of macromolecules are about 50 nm. Thus the idea of the interpenetration between polymeric matrix and deposited metal is suggested. Under the enhanced power voltage and the reactor temperature, the maximums of current in the loop of reaction system shift to lower electrochemical reaction time. That means the higher value of them makes the higher activity in the two aspects of ion move and ion reduction in swelling cathode film. The dissolving experiment of gradient composite film showed that it does not dissolve in N,N-dimethylformamide, and this further confirmed the interpenetration between the polymer matrix and the deposited copper. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 1927–1932, 1999

Micromorphological structure and its forming mechanisms of polyacrylonitrile-based copper gradient composite film

In this article, the micromorphological structure of polyacrylonitrile-copper gradient composite film obtained from the electrochemical reduction of a swelling cathode film was investigated with a scanning electronic microscope and a transmittance electronic microscope. It was found that the micromorphological structure in deposited phase is constructed with copper particles and sheaves of polyacrylonitrile chains. The diameters of both the copper particles and the sheaves of macromolecules are about 50 nm. Thus the idea of the interpenetration between polymeric matrix and deposited metal is suggested. Under the enhanced power voltage and the reactor temperature, the maximums of current in the loop of reaction system shift to lower electrochemical reaction time. That means the higher value of them makes the higher activity in the two aspects of ion move and ion reduction in swelling cathode film. The dissolving experiment of gradient composite film showed that it does not dissolve in N,N-dimethylformamide, and this further confirmed the interpenetration between the polymer matrix and the deposited copper.

Relationship between structures of polyacrylonitrile (PAN)-copper gradient composite film and electrochemical-reaction conditions

In this article, the relationship between the morphological structure and electrochemical conditions, the crystal structure in the electrochemical deposited phase, and the thermal properties of polyacrylonitrile (PAN)-copper gradient composite films ( GCF ) were studied. The results showed that the morphology of the GCF depended very much on the electrochemical conditions used in the reactions. The percent area of the deposited phase on the cross section of the GCF decreases with increases of the predrying time of the solution coated on the cathode, the power voltage, and the temperature in the electrochemical reactor. According to the data of a differential scanning calorimeter and a scanning electronic microscope, PAN chains are physically crosslinked by a deposited phase. Under the electrochemical conditions used in this article, the components of PAN are not affected by the electrochemical reactions.

Relationship between structures of polyacrylonitrile (PAN)–copper gradient composite film and electrochemical‐reaction conditions

In this article, the relationship between the morphological structure and electrochemical conditions, the crystal structure in the electrochemical deposited phase, and the thermal properties of polyacrylonitrile (PAN)–copper gradient composite films (GCF) were studied. The results showed that the morphology of the GCF depended very much on the electrochemical conditions used in the reactions. The percent area of the deposited phase on the cross section of the GCF decreases with increases of the predrying time of the solution coated on the cathode, the power voltage, and the temperature in the electrochemical reactor. According to the data of a differential scanning calorimeter and a scanning electronic microscope, PAN chains are physically crosslinked by a deposited phase. Under the electrochemical conditions used in this article, the components of PAN are not affected by the electrochemical reactions. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 1159–1165, 1998

Formation of Ti/O compositional gradient film on Ti–6Al–4V alloy by reactive DC sputtering

Coating of Ti–6Al–4V alloy substrates with Ti/O compositional gradient films was carried out by reactive DC sputtering not only to improve the biocompatibility and the surface hardness of the alloy but also to relax the stress concentrated at the interface between the film and the alloy. The compositional gradient was realized by varying continuously the oxygen content in Ar–O 2 sputter gas during depositing. Under AES, it was confirmed that a Ti/O compositional gradient film had formed on the alloy, and it was concluded that the oxygen atoms which had concentrated at the surface of the alloy substrate diffused into the titanium layer in the compositional gradient film. Based on the results of XRD, it was found that α-titanium phase was formed in the compositional gradient film with (002) orientation dominant during the film growth. Furthermore, based on indentation–fracture test, it was concluded that this depositing method improved the adhesion of the deposited film to the alloy substrate and the toughness of the film.

Preparation of Ti–Al gradient composite films by sputtering

Titanium aluminide thin films have been deposited on quartz plates by DC magnetron sputtering with a solenoid coil. A round shaped Ti and Al complex disk was employed as a target. In our sputtering system, because the spatial position of plasma on the target surface can be controlled by the solenoid coil current, the in-depth concentration profile of Ti and Al can be controlled by changing the solenoid coil current. Highly oriented Ti 3Al, TiAl and TiAl 3 films were synthesized from one complex Ti and Al target at 400°C by changing the solenoid coil current. Gradient composite film of these titanium aluminides could also be fabricated by changing the solenoid coil current.

COATING OF Ti-6Al-4V ALLOY SUBSTRATE WITH Ti/N COMPOSITIONAL GRADIENT FILM BY REACTIVE DC SPUTTERING

Coating of Ti-6Al-4V alloy substrates with Ti/N compositional gradient films was carried out by reactive DC sputtering, to improve the blood compatibility of the alloy and to relax the stress concentrated at the interface. The compositional gradient was realized by varying continuously the nitrogen content in Ar-N 2 sputter gas during depositing. The deposited film appeared to be uniform and adhesive. TiN formation at the surface of the film was assumed, because of its yellow gold color and high hardness. Under SEM, it was found that the surface had fine particles dispersed on a smooth, accumulated deposit and that this depositing method improved not only the structural property of the film surface but also the adherence of the film with the substrate. Under AES, it was confirmed that nitrogen concentration in the film decreased gradually from the surface toward the substrate. © 1997 Elsevier Science Ltd

Al / Al-N / Ain Compositional Gradient Film Synthesized by Ion-Beam Assisted Deposition Method

AbstractAl / AI-N / AIN compositional gradient thin film was deposited on a Si(100) substrate at room temperature by ion-beam assisted deposition method, with a diminishing ion beam current from 1.4 to 0 mA at increments of 0.3 mA in order to gradually decrease the nitrogen to aluminum ratio at the substrate. The gradual Al and AIN variation in composition was shown by the change of the Al / N atomic ratio analysed by the energy dispersive X-ray spectroscopy(EDX) and the X-ray photoelectron spectroscopy (XPS) in the cross section of the film. The formation of crystalline Al metal and AIN ceramic layer on the Si substrate was revealed by X-ray diffraction(XRD). The cross sectional image taken by high resolution transmission electron microscope (HRTEM) showed a nano-sized crystalline AI-N ceramic material and the flat interface between the Si substrate and the AIN film.