Transient transport measurements

Transient-transport measurements are a powerful tool for evaluating the validity of any sorption-transport model. The ability of a model to predict diffusion time lags is a test for its validity, as all the parameters are fixed by the equilibrium sorption and steady state transport, and because the time lag depends on the specific form of the concentration and diffusion gradients developed during the transient-state experiments. 

Carbon Dioxide Transport. Measuring the permeation of carbon dioxide occurs far less often than measuring the permeation of oxygen or water. A variety of methods ate used however, the simplest method uses the Permatran-C instmment (Modem Controls, Inc.). In this method, air is circulated past a test film in a loop that includes an infrared detector. Carbon dioxide is appHed to the other side of the film. AH the carbon dioxide that permeates through the film is captured in the loop. As the experiment progresses, the carbon dioxide concentration increases. First, there is a transient period before the steady-state rate is achieved. The steady-state rate is achieved when the concentration of carbon dioxide increases at a constant rate. This rate is used to calculate the permeabiUty. Figure 18 shows how the diffusion coefficient can be deterrnined in this type of experiment. The time lag is substituted into equation 21. The solubiUty coefficient can be calculated with equation 2. 

Nanosecond transient absorption measurements provided a further indication of efficient mediator transport within the nanopores of the gel, showing basically no difference in dye regeneration using the liquid iodide/iodine precursor and the quasi-solid-state polymeric electrolyte. 

Figure 42 Photogeneration efficiencies for a double-layer photoreceptor comprised of a MSQ generation layer determined by photoacoustic and transient photocurrent measurements. The transport layer was a hydrazone doped polymer.

Brohede U, Bramer T, Edsman K, et al. Electrodynamic investigations of ion transport and stractural properties in drug-containing gels dielectric spectroscopy and transient current measurements on catanionic carbopol systems. J Phys Chem B 2005 109 15250-15255. 

Ostroverkhova, O. et al., Bandhke transport in pentacene and functionalized pentacene thin films revealed by subpicosecond transient photoconductivity measurements, Phys. Rev. B, 71, 035204, 2005. 

For the transient photocurrent measurements a Dye laser pumped by 500 ps pulsed Nitrogen laser was used for the optical excitation. The incident light was linearly polarized parallel and perpendicular to the stretch-direction. The bias field was applied parallel to the c-axis, therefore the photocurrent reflects the transport properties along the chains. Details of the photoconductivity apparatus can be found in Ref. 5. 

The photoconductivity in polyacetylene, the simplest conjugated polymer, has been the subject of intense investigations [103 106]. Transient photoconductivity measurements on a picosecond time scale have been carried out [107-112]. These ultrafast methods are a powerful tools to investigate the transport properties as well as the recombination kinetics of charged excitations. It was found [107] that the photocurrent in trans-polyacetylene consists of two components a fast component which relaxes on a picosecond time scale and for which a carrier mobility of about 1 cm V s was reported [110,111] and a slow component with earner lifetimes up to seconds. 

The characterisation of a DSSC device or the study of partial processes that occurs at such cells uses a series of optical and electrochemical techniques, either stationary or time-resolved. The studies cover a wide range of timescales, accompanying the wide time span of phenomena occurring in a DSSC (from fs/ps for electron injection to ms for electron transport). Optical transient absorption techniques (see Chaps. 8, 14, 15) are used in combination with transient electrical measurements to follow the appearance and disappearance of chemical species and charges on a DSSC [27]. 

Current vs. time transients Membranes with 1.77 cm area were placed between two aqueous solution in an electrochemical cell designed for ionic transport measurements (76,78). Desired voltage steps were applied by a Solatron 1250 FRA through an EG G 363 potentiostat controlled by a nith XT computer. Current vs. time data were collected by the same computer using a Data Translation DT 2801 board. 

Finaliy, photoconductivity (PC) studies have been performed by Moses et al. [121] on a-8T single crystals to study the intrinsic properties of photoexcitation and transport in such a model molecular crystal systems and to determine the role of structural defects on these properties. Picosecond transient PC measurements over a wide range of temperatures (10-300K) demonstrate that the dependence of the transient photocurrent on hght intensity and electric field in the single crystal a-8T is radically different from that in vacuum-deposited polycrystalUne films. The photo-current lifetime in the a-8T crystal is of the order of a nanosecond whereas in the film it is less than 100 ps. These observations indicate a bi-molecular carrier recombination component prevailing in the a-8T single crystals, whereas a mono-molecular mechanism operates in polycrystalline films. 

A sample thermal response from transient thermoreflectance measurement of 30 nm A1 film on a sapphire substrate is shown in Figure 11.20. The experimental data can be compared with the best-fit equation from modeling the thermal transport in the sample. The unknown parameters, the thermal conductivity (k) of the sample and the thermal boundary conductance (a), can be obtained from the best-fit parameters. 

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