Excimer laser ablation of silicon carbide ceramic targets

The ablation mechanisms of ceramic silicon carbide targets were investigated for laser fluences up to 12 J/cm². Two different regimes were identified: the vaporization was the dominant process but, above approximately 3 J/cm², the plasma shielding of the laser beam took place and decreased the efficiency of the process. Moreover, after a few laser shots, the decomposition of the target material took place with the segregation of a carbon phase, as resulted from Raman microscopy measurements. Laser induced periodic structures were also evidenced on the inner laser spots at different fluences. The effects of the ablation mechanisms were correlated with the morphological properties of the films, in particular with the size and the number of the particulates.     

Excimer laser reduction and patterning of graphite oxide

A successful approach and the operational parameters necessary for reduction of graphite oxide (GO) to multilayer graphene using 248 nm excimer laser irradiation in both vacuum and ultrahigh purity N₂ background environments is described. The utility of excimer laser reduction is demonstrated by production of simple line and logo patterns using standard microscale lithographic patterning strategies. Multilayer graphene formation is confirmed with Raman and X-ray photoelectron spectroscopies, and the morphology of the processed GO sample is evaluated with scanning electron microscopy. Four-point probe measurements of the excimer laser reduced GO indicate typical sheet resistances of ∼100–500 Ω/sq, which is a significant improvement over other values reported in the literature for other laser-based GO reduction methods.     

Excimer laser projection patterning with and without resists

Projection imaging with the deep-UV (193 nm) and VUV (157 nm) output of an excimer laser has been applied to submicrometer patterning of thin films by injected-defect, surface-chemical, and solid-transformation processing. The methods have been designed to take advantage of the short-wavelength, high-peak-intensity pulsed radiation from these sources. Examples are described of pattern definition by exposure of multilayer organic resists, by maskless etching and doping of solids in reactive vapors, and by solid-state chemical transformations in inorganic Al/0 films. Well-resolved 0.4-μm lines and spaces have been achieved. Required doses, between 0.04 and 1 J/cm², are compatible with single- or multi-pulse step-and-repeat projection patterning with a small excimer laser.     

Rheology of solvent-cast polymer films

This paper describes and analyzes the results of an experiment where various thin polymeric films are continuously sheared between smooth glass substrates. The shear force per unit area has been measured as a function of mean uniaxial stress and temperature using representative “good” and “poor” casting solvents followed by a range of heat treatments. The polymers studied include high density polyethylene, polybisphenol-A–carbonate, poly(ethylene terephthalate), atactic polystyrene, isotactic polystyrene, atactic poly(methyl methacrylate), isotactic poly(methyl methacrylate), poly(vinyl acetate), poly(vinyl alcohol), poly(vinyl pyrrolidone), poly(vinyl chloride), and polytetrafluoroethylene. The results indicate that the casting solvent has a very pronounced influence upon the rheology of the film. The casting solvents may apparently confer either ductile or brittle failure in the film and also influence the nature of the temperature and pressure dependence of the shear stress. The data have been analyzed using Eyring theory and also by reference to relevant published literature on the influence of solvent and thermal treatments on the morphology and deformation behavior of polymers. “Good” solvents generally tend to promote a brittle mode of failure with little temperature dependence. The same type of solvents also produced films which have higher shear strengths and show greater increases in shear strength with pressure. These data are adequately rationalized using free volume and entanglement notions.     

Photophysical studies of polyvinylcarbazole polymer films

Polyvinylcarbazole (PVK) films have been prepared by solution method. Their absorption spectra, photoluminescence, and electroluminescence have been studied. Absorption in pure PVK is found to be high at lower wavelengths in UV region and decreases in visible range. In case of I₂ doped PVK, absorption is observed in visible region also. The photoluminescence is obtained in pure PVK when exited by violet or blue light but in case of I₂ doped PVK, similar excitation gives low intensity peaks in yellow–green region. The results indicate that I₂ levels in PVK causes absorption in 400–600 nm wavelength range and makes the deexcitation of the molecules nonradiative. Light emission by application of ac voltage from mains up to 100 V has been studied. Nonlinear increase in emission intensity has been observed with increasing voltage. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 722–726, 2007     

Excluded volume effect on polymer films

Using Monte Carlo methods we have investigated the excluded volume effect on polymer melts in two dimensions. Investigating chain lengths of up to N = 59 at concentration $\text{c =}\text{5}\text{6}$, our results strongly support predictions made recently by de Gennes: (1) end-to-end distance and gyration radius exhibit Gaussian behaviour ∝ √N; (2) but their prefactors differ significantly from their random flight values; (3) in contrast to three-dimensional melts the chains are strongly segregated.     

Singlet oxygen lifetime in polymer films

Singlet oxygen lifetimes [τ(¹O₂)] in polymer matrices were calculated using the strong exponential correlation between the incremental rate constant (K_xy) of energy transfer from ¹O₂ to terminal oscillators X—Y contained in solvent molecules and the energy (E_xy) of the highest fundamental vibration of the oscillator X—Y. The lifetimes in polymers correspond very well to experimental data in the literature. They depend only on the nature and quantity of molecular oscillators in the medium but not on the state of aggregation. The computed lifetimes in common polymers vary only from 10 to 50 μs. Therefore the lifetime of ¹O₂ in singlet oxygen reactions in polymers is of secondary importance in comparison to the diffusion coefficient.     

Permeability studies on heterogeneous polymer films

Oxygen permeability data on heterogeneous films prepared from mixtures of different polymer latices were compared with predictions of a permeability theory of Higuchi based on an idealized barrier model. The theory attempts to evaluate the effect of perturbations of the diffusion process associated with the presence of a discontinuous internal-phase component. An approximate form of the theoretical permeability expression was found to render reliable predictions of the effective permeability of heterogeneous barrier systems. When the partition of the permeant between different phases of the barrier is sufficiently rapid, the theory becomes applicable to the early, nonsteadystate, stage of permeation. Implications of the general applicability of the Higuchi theory to heterogeneous systems are discussed with respect to the interaction between the constitutents and the associated departures from the ideal behavior assumed for the model.     

Spin coating of very thin polymer films

Very thin films of natural rubber, polystyrene, and poly(methylmethacrylate) have been spun-cast on silicon wafers from dilute solutions of toluene and cyclohexane. Layers were uniform across the wafers, ranging from 0.5 to 170 nm, as measured by ellipsometry. Their average thickness e increased with solution concentration and decreased with rotational rate w. Changing the volume of the solution pipetted onto the wafers did not affect the final thickness, whereas changing the solvent did. The previously reported empirical relation for much thicker films, e α ω^?1/2, held over the range investigated here.     

Experimental modeling of solvent-casting thin polymer films

An apparatus was designed and assembled to study the solvent removal from solution-cast thin polymer films. The computer interfacing of a thermogravimetric analyzer, spectrophotometer, electronic flowmeters, and control valves for the apparatus enabled the preprogramming of the carrier gas velocity, carrier gas solvent content, and temperature profiles to simulate the environment experienced in large parallel flow industrial driers. The apparatus has also been designed and operated to enable the visual observation of the drying film with an optical microscope. Initial experimental studies conducted with the apparatus involved the effect of temperature on solvent removal. The results indicate that high dryer gas temperatures can apparently cause skinning of the film surface resulting in slower solvent removal rates. The skin formation can be suppressed by higher solvent concentration in the carrier gas. The visual observations revealed the formation of standing waves in the film surface during drying at high gas velocities (>2OO cm/min). The wave formation at least partially overcomes the effect of skinning by increasing the surface area of the film, and may be the manifestation of flow instabilities involving circulation within the film.     

Enhancement of impurity removal from polymer films

The problem of the enhancement of the removal of a volatile impurity in a polymer film by the addition of a second solvent is analyzed. The introduction of a second solvent that diffuses faster than the impurity can increase the free volume of the system and can thus facilitate removal of the volatile impurity. Solution of the ternary diffusion problem indicates the role of the properties of the second solvent and of processing conditions on the devolatilization effectiveness. It is shown that more than a 10-fold decrease in the devolatilization time can be achieved using a second solvent and appropriate processing conditions.     

Oxygen plasma removal of thin polymer films

Relative rates of polymer removal in an oxygen plasma have been measured for 40 polymer samples. The rates of removal are correlated, with structural factors which enhance or retard removal. Strong backbone bonds, aromatic and polar functional groups, and metallic atoms decrease the removal rates. Weak bonds not attached to the- polymer backbone have little affect while weak bonds attached directly to the chain or in the chain greatly accelerate removal. Chlorine present in the polymer catalyzes removal. This can be mimicked by mixtures of CF₄, and O₂ for which much enhanced removal rates are observed.     

Ion, solvent and polymer dynamics in polyaniline conducting polymer films

We describe the application of a new model for the visualization of mobile species (ion and solvent) transfers accompanying redox switching of electroactive films. The system studied was polyaniline, for which the mobile species population changes were determined from redox-driven film mass changes using a thickness shear mode acoustic wave resonator. Acoustic admittance data were used to establish conditions under which the resonator frequency response could be interpreted gravimetrically. Charge and frequency changes accompanying the first redox transition of polyaniline films exposed to aqueous perchloric acid were then used to determine the film's ion and solvent populations. The data are best described by a mechanism in which the early stages of film oxidation are associated with proton transfer (exit) and the latter stages with perchlorate transfer (entry) to satisfy electroneutrality; solvent enters throughout film oxidation and exits throughout film reduction. The alternative of a single ion satisfying electroneutrality throughout would require the rather less likely situation of a non-monotonic solvent flux. Application of the model's diagnostic criteria indicates that the film solvent population is in equilibrium on the timescale of slow scan voltammetry, but shows thermodynamic non-idealities. Hysteresis in the film ion population signals failure of the redox state to maintain equilibrium with the applied potential.     

ETFE聚合工艺研究

ETFE由乙烯、四氟乙烯单体共聚生成,通过改变聚合压力来控制产品的参数,以适应不同的加工方式。     

Dynamics of ion beam modification of polymer films

Thin polymer films were irradiated in a high vacuum environment with energetic (～2 MeV) ions. The emitted molecular species were studied with a quadrupole mass spectrometer during bombardment. The emitted species are predominantly small molecules. The efficiency of emission depends strongly upon the electronic energy loss of the incident ions in the film and, hence, upon the velocity and atomic number of the incident ion. The emission efficiency of all species decreases with increasing damage in the film. By pulsing the ion beam, the time dependence of irradiation induced emission of molecular species from the films has been studied. Emission delays of hundreds of milliseconds are observed in some cases. These delays are found to depend upon the emitted species as well as the host film and seem to be associated with diffusion of the species in the films.