Matrix assisted pulsed laser evaporation of poly(d,l-lactide) thin films for controlled-release drug systems

We report the successful deposition of the porous polymer poly(d,l-lactide) by matrix assisted pulsed laser evaporation (MAPLE) using a KrF* excimer laser (248 nm, τ = 7 ns) operated at 2 Hz repetition rate. The chemical structure of the starting materials was preserved in the resulting thin films. Fluence played a key role in optimizing our depositions of the polymer. We demonstrated MAPLE was able to improve current approaches to grow high quality thin films of poly(d,l-lactide), including a porosity control highly required in targeted drug delivery.     

Influence of experimental parameters on physical properties of porous silicon and oxidized porous silicon layers

This paper reports physical properties of porous silicon and oxidized porous silicon, manufactured by anodisation from heavily p-type doped silicon wafers as a function of experimental parameters. The growth rate and refractive index of the layers were studied at different applied current densities and glycerol concentrations in electrolyte. When the current density varied from 5 to 100 mA/cm², the refractive index was between 1.2 and 2.4 which corresponded to a porosity range from 42 to 85%. After oxidation, the porosity decreased and was between 2 and 45% for a refractive index range from 1.22 to 1.46. The thermal processing also induced an increase in thickness which was dependent on the initial porosity. This increase in thickness was more important for the lowest porosities. Lastly, the roughness of the porous layer/silicon substrate interface was studied at different applied current densities and glycerol concentrations in solution. Roughness decreased when the current density or glycerol concentration increased. Moreover, roughness was also reduced by thermal oxidation.     

Stability analysis of a Maxwell fluid in a porous medium heated from below

Based on a modified-Darcy–Brinkman–Maxwell model, stability analysis of a horizontal layer of Maxwell fluid in a porous medium heated from below is performed. By solving the eigenvalue problems, the critical Rayleigh number, wave number and frequency for overstability are determined. It is found that the critical Rayleigh number for overstability decreases as the relaxation time increases and the elasticity of a Maxwell fluid has a destabilizing effect on the fluid layer in porous media. On the other hand, the critical Rayleigh number for overstability increases by increasing the porous parameter which acts to stabilize the system. In limiting cases, some previous results for viscoelastic fluids in nonporous media are recovered from our results.     

Microstructure characterization of porous silicon as studied by positron annihilation measurements at low temperatures and high vacuum

Atomic scale properties of thin porous silicon (PSi) layers, characterized by the formation of positronium, are investigated using positron annihilation lifetime spectroscopy in the temperature range 20-300 K under 10⁻⁷ Torr vacuum. The longest orthopositronium as well as the shortest parapositronium components are found to have quite low intensities in the thin layer at room temperature. It is also found that at temperatures ≤240 K, these two components do not show up in the spectrum. The reason for this absence of the longest lifetime component is suggested.     

Study of photoconductivity and photoluminescence of organic/porous silicon complexes

In this paper, time-varying photoconductivity (PC) and the photoluminescence (PL) of different complexes were studied. Due to thick polymer layer hindering light penetrating into porous silicon (PS) layer, intrinsic PS luminescence in polymer/PS system disappeared. The physical origin of PL may be related to the recombination mechanisms involving surface defect states such as silicon oxide, siloxene. Due to carrier transfer controlled by different energy barrier, different devices prepared from different doped Si wafer showed opposite current-voltage characteristic.     

Pyroelectric nano-rods grown inside alumina nano-pores

This paper reports the formation and properties of nano-composite pyroelectric thin films. They consist of pyroelectric triglycine sulfate (TGS) single-crystal nano-rods grown inside a highly dense array of alumina pores (about 65 nm diameter and density of 1011 cm⁻²). The nucleation and growth of the TGS single crystals are obtained by precipitation from a supersaturated aqueous solution. Nucleation is preferred only at the bottom of the pores due to a tight control of temperature, composition and pore diameter. Growth of single crystals with preferred crystallographic orientation is obtained with the aid of an applied electric field. Various crystallographic orientations (1 0 0) (−1 1 0) (−1 2 0) are obtained separately as a single preferred orientation by changing the amplitude of the electric field during crystal growth. The films exhibit ferroelectric behavior.     

Conditional methods for continuum reacting flows in porous media

A special version of conditional moment closure—PCMC—is suggested for modeling reacting flows in porous media. The model involves conditioning on a special tracer scalar, which is introduced to characterize scalar transport in the gaseous phase. (i.e., for the flow in the interparticle space or in the pores). The model accounts for interparticle variations of species concentrations and emulates diffusion in the interparticle space. Special boundary conditions that are consistent with conventional conditions at the phase interface are obtained for the PCMC model. The model is tested against complete direct simulation of a reacting flow in porous media with favourable results.     

Optical inter- and intra-band transitions in silicon nanocrystals: The role of surface vibrations

Silicon nanocrystals (ncs) belong to an interesting class of semiconductor nanostructures that manifest size dependent electronic properties. This well known effect of quantum confinement can explain many properties of silicon ncs. However, with decreasing size and dimension of the ncs, the role of surface phenomena becomes substantial. For example, we have shown recently that the strong luminescence from these ncs should be assigned to the exclusion of nonradiative channels rather than to the enhancement of radiative inter-band transitions. In addition, using infrared intra-band transitions spectroscopy, we were able to resolve the quantized electronic sublevels of small silicon ncs. We have found that under appropriate conditions, these electronic sublevels are resonantly coupled to surface vibrations. We suggest that this coupling mechanism is responsible for the exclusion of nonradiative channels in silicon ncs.     

电极界面浓差极化对锂金属沉积的影响

锂金属作为下一代高能量密度电池的理想负极材料受到研究人员广泛关注。然而,锂枝晶生长引起的安全隐患和循环寿命短等问题严重影响了锂金属电池的实用化进程。本文以电化学现象和理论为依据,从浓差极化角度详细分析锂金属电沉积过程中枝晶生长、死锂形成和全电池失效机制,并对目前研究较多的多孔宿主电极中的浓差极化及枝晶抑制进行分析,提出锂金属界面浓差电池现象。本文得到的结论为研究人员更深入地探究锂金属保护策略提供了理论依据。     

Direct synthesis of porous NiO nanowall arrays on conductive substrates for supercapacitor application

Porous NiO nanowall arrays (NWAs) grown on flexible Fe-Co-Ni alloy have been successfully synthesized by using nullaginite (Ni₂(OH)₂CO₃) as precursor and investigated as supercapacitor electrodes. In details, we adopted a simple hydrothermal method to realize Ni₂(OH)₂CO₃ NWAs and examined their robust mechanical adhesion to substrate via a long-time ultrasonication test. Porous NiO NWAs were then obtained by a post-calcination towards precursors at 500 °C in nitrogen atmosphere. Electrochemical properties of as-synthesized NiO NWAs were evaluated by cyclic voltammetry and galvanostatic charge/discharge; porous NiO NWAs electrode delivered a specific capacitance of 270 F/g (0.67 A/g); even at high current densities, the electrode could still deliver a high capacitance up to 236 F/g (13.35 A/g). Meanwhile, it exhibited excellent cycle lifetime with ∼93% specific capacitance kept after 4000 cycles. These results suggest that as-made porous NiO NWAs electrode is a promising candidate for future thin-film supercapacitors and other microelectronic systems.