Density, DSC, X-ray and NMR measurements through the gel and lamellar phase transitions of 1-myristoyl-2-stearoyl-sn-glycero-3-phosphatidylcholine (MSPC) and 1-stearoyl-2-myristoyl-sn-glycero-3-phosphatidylcholine (SMPC): observation of slow relaxation processes and mechanisms of phase transitions

Dialkyl lecithin dispersions in water exhibit two phase transitions upon cooling from the lamellar phase (L(α)). At the main transition (T(M)) the L(α) phase changes to a ripple (gel) phase (P(β')) which then transforms to a second gel phase (L(β')) at the "pretransition" (T(P)). We have made accurate density measurements through the various phases for two lecithins having unequal chains: 1-myristoyl-2-stearoyl-sn-glycero-3-phosphatidylcholine (MSPC) and 1-stearoyl-2-myristoyl-sn-glycero-3-phosphatidylcholine (SMPC). The measurements were carried out over five heat/cool cycles from 5 to 55 °C, followed by cooling back to 5 °C. The samples were then held at 50 °C for 24 hours, followed by a further three cool/heat cycles. For SMPC we observe an increase in density of the gel phases over the first 5 cycles, followed by much smaller changes after incubation at 50 °C. The lamellar phase also shows an increase in density, albeit much smaller. This parallels the behaviour of 1,2-di-palmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) and 1,2-di-myristoyl-sn-glycero-3-phosphatidylcholine (DMPC) reported earlier (Jones et al., Liquid Crystals 32, 1465 (2005)). For MSPC we observe a decrease in density within the gel phases while T(P) almost disappears after the first cycle. The lamellar phase shows little evidence of any change with each cycle. Within the lamellar phases there is a marked reduction in density on approaching T(M), which is attributed to the formation of transitory gel phase domains. Additional measurements by DSC and X-ray diffraction show that the changes in densities are not accompanied by large changes in transition enthalpies or phase structures. NMR data indicate that the pretransitional event within the L(α) phase is accompanied by ordering of the alkyl chains. The results indicate that the exact nature of the lipid alkyl chains could play a key role in the formation of gel phase patches within membrane bilayers. Their detailed chemical structures merit more attention than by simply assuming a uniform "bending energy" to describe the behaviour.     

Thermal transport properties in the normal state of CaAlxSi2−x superconductors

Normal state electrical and thermal properties, including electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (κ) of the CaAl_xSi_2−x (x=0.9-1.2) system were investigated. It is found that the electrical resistivity and Seebeck coefficient exhibit a typical metallic character throughout the temperature range investigated, and the metallicity of this series is enhanced with increase in Al/Si ratio. On the other hand, the thermal conductivity shows a weak temperature variation at low temperatures, whereas κ follows a T²-dependence for T>150 K. Analysis of the electronic thermal resistivity based on Klemen’s model reveals that the scattering of electrons from the defects and static imperfections becomes dominant as the temperature approaches T_c. These results are discussed in the light of simultaneous existence of various crystal structures and development of ultra-soft phonon mode recently observed in the CaAlSi system.     

Deposition and characterization of YBCO/CeO2/YSZ/CeO2 multilayers on biaxially textured Ni substrates

CeO2/YSZ/CeO2 buffer layers were deposited on biaxially textured Ni substrates by pulsed laser deposition. The influence of the processing parameters on the texture development of the seed layer CeO2 was investigated. Epitaxial films of YBCO were then grown in situ on the CeO2/YSZ （yttria-stabilized ZrO2）/CeO2-buffered Ni substrates. The resulting YBCO conductors exhibited self-fleld critical current density Jc of more than 1 MA/cm^2 at 77K and superconducting transition temperature Tc of about 91K.     

Positron annihilation in boron-implanted n-type silicon

The ion damaged effect and subsequent isothermal annealing in boron-implanted Si was studied by positron annihilation lifetime measurements. The mean positron lifetime in preimplanted n-type Si is 243 psec. The variation of mean lifetime is detectable when the implanted boron dose is greater than 1.0x10¹⁵/cm². The saturated mean positron lifetime (247 psec) occurs when the implantation dose reaches 2.5x10¹⁵/cm². The mean electron density of the positron sensitive defects is estimated to be about 85% less than that in the perfect parts of the crystal. Isothermal annealing was held in every 5-minute step at 1000°C. In the first step, the positron lifetime in the implanted sample increases slightly and then decreases completely to its initial state in the 3rd step. Sheet resistance of the sample monitored by 4-point probe method has been found closely related to the positron lifetime.     

Asymptotic behaviour of some nonlocal diffusion problems

We deal with a parabolic equation having a diffusion coefficient depending on a nonlocal quantity. We investigate the convergence of the solution towards a steady state, extending previous results obtained by [M. Chipot and B. Lovat (1999). On the asympotic behaviour of some nonlocal problems. Positivity, 3, 65-81]. Using the dynamical systems point of view, we are able to treat the case of a continuum of steady states     

Applications of ion chromatography in the semiconductor industry II. Determination of basic airborne contaminants in a cleanroom

Since the geometry of semiconductors and integrated circuits has been shrunk to well below sub-micron dimensions, there is a great demand for precise and reliable analytical techniques to measure and monitor the contaminants in all areas related to the fabrication process. Special concerns about the air cleanliness in a Fab lead to the necessity for developing analytical techniques to perform this task. In this research, basic airborne contaminants in a cleanroom were adsorbed onto a collection tube, subsequently extracted with deionized water and analyzed by ion chromatography. Such a method is capable of simultaneously measuring the concentrations of ammonia and cations (e.g., sodium, potassium, calcium, magnesium, etc.) present in the cleanroom air samples. The optimal sample preparation method was determined and the analyte concentrations at various locations in the cleanroom were measured. The results showed significant variations from one location to another. The long-term fluctuations in the contaminant levels were also significant. Data obtained using this method compared well with data from inductively coupled plasma analysis of the same materials.     

Rheological Behaviors in the Regimes from Dilute to Concentrated in Cellulose Solutions Dissolved at Low Temperature

Cellulose was dissolved rapidly in 9.5 wt.‐% NaOH/4.5 wt.‐% thiourea aqueous solution pre‐cooled to ?5 °C to prepare cellulose solution with different concentrations. The rheological properties of the cellulose solutions in wide concentration regimes from dilute (0.008 wt.‐%) to concentrated (4.0 wt.‐%) at 25 °C were investigated. On the basis of data from the steady‐shear flow test, the critical overlap (c*), the entanglement (c_e) and the gel (c_g) concentrations of the cellulose solution at 25 °C were determined, respectively, to be 0.10 wt.‐%, 0.53 wt.‐% and 2.50 wt.‐%, in accordance with the results of storage modulus (G′) versus c by dynamic test. Moreover, the Cox‐Merz deviation at relatively low concentrations was in good agreement with the micro‐gel particles in dilute regime. As the cellulose concentration increased, a homogeneous 3‐dimensional network formed in the cellulose solution in the concentrated regime, and further increasing of the concentration led to micro‐phase separation as determined by the time‐temperature superposition (tTS). So far, this complex cellulose solution has been successfully described by the concentration regime theory for the first time, and the relatively molecular morphologies in each regime have been determined, providing useful information for the applications of the cellulose solution systems.

     

The phenomenology of Dvali–Gabadadze–Porrati cosmologies

Cosmologists today are confronted with the perplexing reality that the universe is currently accelerating in its expansion. Nevertheless, the nature of the fuel that drives today's cosmic acceleration is an open and tantalizing mystery. There exists the intriguing possibility that the acceleration is not the manifestation of yet another mysterious ingredient in the cosmic gas tank (dark energy), but rather our first real lack of understanding of gravity itself, and even possibly a signal that there might exist dimensions beyond that which we can currently observe. The braneworld model of Dvali, Gabadadze and Porrati (DGP) is a theory where gravity is altered at immense distances by the excruciatingly slow leakage of gravity off our three-dimensional Universe and, as a modified-gravity theory, has pioneered this line of investigation. I review the underlying structure of DGP gravity and those phenomenological developments relevant to cosmologists interested in a pedagogical treatment of this intriguing model.