Density Of Micropores Research Articles

A New Stent Graft

The use of stents improves the result after balloon coronary angioplasty. Restenosis due to neointimal hyperplasia and proliferation of smooth muscle cells are, however, a concern. In the present report, we studied the prevention of restenosis to allow endothelial cell migration and growth to proceed through micropores using our developed stent graft with micropored segmented polyurethane (SPU) thin film in a normal beagle model. Our developed stent graft was made from Palmaz stent and micropored SPU thin film. The SPU film was arranged into four different micropore densities around the circumference: no micropores, arrangement 4; micropores of 30mum in diameter with an orderly distance of 250mum; (arrangement 1), 500mum; (arrangement 2) and 125mum (arrangement 3) between the neighboring two pores. Micropores were made using the Excimer laser ablation technique. The Palmaz stent was wrapped with micropored film, sutured, and glued with DMF (dimethyl formamide) under aid of a microscope. These stents were placed in the common carotid arteries of beagles (n = 5). They were sacrificed at 1 month, and a histological study and scanning electron microscopy study were performed for evaluation of endoluminal endothelialization. In 10 arteries applied with stent grafts, there was no severe stenosis although it did occur to some extent. All stented arteries were patent. Endothelial cell migration and growth through micropores were observed histologically on micropored SPU thin film in this model, which did not affect the intraluminal diameter. In most non-porous regions, significant thrombi were found between the SPU film and the neointimal layer. On the other hand, in the porous region, little thrombosis was observed except in the lowest density region. In 125mum of distance between two neighboring pores, the neointimal layer was the thinnest, which was suitable for wide intraluminal space after placement of a stent graft. Endothelial cell migration and growth through micropores were confirmed in the animal model using our developed micropored stent graft. The proceeding of their migration was controlled by micropore density under a constant micropore diameter. The stent graft with micropored SPU thin film is promising for the prevention of restenosis due to neointimal hyperplasia.

The effect of microscopic inclusion locations and silicon segregation on fatigue lifetimes of aluminum alloy A356 castings

The microstructural heterogeneity of aluminum alloy A356 permanent mold castings impacts fatigue performance differently depending on location in the casting. Casting conditions, which include casting temperature, gradients in the mold temperature, plunger speed and casting pressure are variables which can affect solidification rates and therefore, the microstructure within the casting. In this paper an experimental investigation of fatigue performance of specimens cut from various locations in a test bar of cast aluminum alloy A356 is reported. The number of cycles to failure for each test was recorded and the fracture surfaces and local microstructure examined. It was observed that local fatigue resistance varied substantially along the solidification path while tensile strength was little affected. The amount of Al–Si eutectic and the density of micropores increases along the solidification path. Samples located near the surface and the farthest from the gate end demonstrated the longest lifetimes. Conversely, the samples taken from the centerline of the casting closest to the gate end demonstrated the shortest lifetimes.

Surface microarchitectural design in biomedical applications: in vivo analysis of tissue ingrowth in excimer laser-directed micropored scaffold for cardiovascular tissue engineering.

A micropatterned microporous segmented polyurethane film (20 x 12 mm in size, 30 micrometer thick) with four regions was prepared by excimer laser microprocessing to provide an in vivo model of transmural tissue ingrowth in an open cell-structured scaffold specially designed for cardiovascular tissue engineering. Three microporous regions had the same circular micropores (30 micrometer diameter) but different pore density arrangements (percentage of total pore area against unit area was 0.3%, 1.1%, and 4.5%), and the other region remained nonporous. The covered stent, prepared by wrapping the regionally different density-microporous film on an expandable metallic stent (approximately 3.1 mm in diameter), was delivered to the luminal surface of canine common carotid arteries and placed after expansion of the stent to a diameter of approximately 8 mm using a balloon catheter. At 4 weeks of implantation, all the covered stents (n = 10) were patent. The luminal surfaces of the covered stents were almost confluently endothelialized both in nonporous and microporous regions. Histological examination showed that the neointimal wall was formed by tissue ingrowth from host through micropores (transmural) and anastomotic sites. Thrombus formation occurred frequently in the lowest density porous region and nonporous region. With an increase in pore density, the thickness of the neointimal wall decreased. This study demonstrated how the micropore density of implanted devices influences tissue ingrowth in arterial implantation.

XPS investigation of a-Si : H thin films after light soaking

Amorphous hydrogenated silicon thin films prepared by homogeneous chemical vapour deposition have been studied. The Si2p, O1s and C1s electron spectra have been recorded after different light soaking times using repeated 100 mW/cm 2 white light illumination and X-ray photoelectron spectroscopy (XPS) measurements. The change of the position and intensity of the Si2p peak has been observed after light soaking and is explained by the transformation of the Si–H bonds. The correlation between the micropore density in a-Si : H film and the binding energy of Si2p electrons is demonstrated.

Modeling and microstructure analysis of fatigue initiation life extension by reductions in microporosity

Fatigue initiation lifetimes were demonstrated to be related to the size and density of microporosity in the midplate region for three differently processed variations of aluminum alloy 7050-T7451. Metallographic and fractographic examination of double edge notched fatigue specimens was performed to characterize microstructural inhomogeneities. A greater size and density of micropores were found for those materials that had failed at shorter fatigue lifetimes. The specific crack-initiating feature was identified for each of the known cracks. Linear elastic fracture mechanics modeling of crack initiation from each of the micropore-induced cracks showed that this could be adequately accounted for by a stress intensity factor analysis proposed by Trantina and Barishpolsky.

Hydrogenated amorphous silicon on soda-lime glass substrate: Correlation between sodium contamination and porosity of the films

Abstract electrical and optical properties of a-Si: H films deposited on Coming 7059 and soda-lime glass substrates have been studied. The obtained difference in the values of the measured parameters implies that the latter samples are contaminated with sodium. It has been established that the amount of micropores affects the penetration of sodium from the soda-lime glass substrate as a higher micropore density leads to a lower sodium contamination of the a-Si: H films.

Morphology of anodically etched Si(111) surfaces: A structural comparison of NH4F versus HF etching

We present a comparative scanning tunneling microscopy (STM) study on the porous layer formation in two different fluoride containing solutions, HF/ethanol and concentrated NH4F solution. After etching in dilute HF solution the samples display a high density of micropores with typical diameters ranging from 5 to 25 nm, while NH4F treated surfaces display shallow macropores of several hundred nm in diameter. These structural differences are discussed by comparing the different activity of both solutions for chemical etching of Si in the absence of an external potential, which provides an additional reaction channel also under anodic conditions.

Chemical properties of Central African carbonado and its genetic implications

Carbonado diamonds from Central African Republic exhibit very intense laser-induced photoluminescence due to the presence of the radiation damages. From the spectral profiles of photoluminescence obtained for these samples, the carbonado stones were grouped into three groups depending on a variation in the structure of radiation-damage products; Group-A carbonado with 504 nm band (3H band), Group-B carbonado with 575 nm band in addition to weak background peaks centered at 512 and 580 nm, and the Group-AB carbonado with features intermediate between Group-A and Group-B. When heating Group-A carbonado, Group-A was found to transform to Group-B at temperatures higher than 400°C. Thermogravimetry and Differential Thermal Analysis (TG-DTA) clarified the difference in thermal properties between Group-A and Group-B. While Group-A carbonado yields one intense and narrow exothermic peak at 800°C, Group-B carbonado exhibits a broad exothermic peak at 785°C. The distinct difference in the oxidation rate could be attributed to the difference in the density of micropores in carbonado. Chondrite-normalized rare earth element (REE) patterns for the bulk carbonado samples show negative Eu anomalies, and the inclinations of the patterns are similar to those of kimberlites and much steeper than that of continental crust as represented by continental shales. In the chondrite-normalized value, La/Lu ratios were of the order of 10 2 and the light REE abundances were as high as 10 3 relative to chondrite. Further, 1384 cm −1 absorption band was observed in IR (infrared) spectra of both Group-A and Group-B, which suggests the presence of nitrogen platelet in carbonado. These REE and IR results imply that the seed crystal of carbonado was formed at high pressure and high temperature and was subjected to the annealing process to generate nitrogen platelets. The genesis of Central African carbonado is discussed as a reconciliation of these experimental results.

A-Si:H thin film microstructure influenced by HOMOCVD-process parameters

Abstract Hydrogenated amorphous silicon films prepared by homogeneous chemical vapor deposition (HOMOCVD) process were studied with respect to their microstructure by field-assisted ion exchange (FAIE) techniques. Results concerning the influence of HOMOCVD process parameters such as temperature, gas pressure, flow and substrate temperature are presented. Two series of deposition experiments were carried out: at constant gas phase conditions and at the variable ones. The obtained results showed that the amount of micropores was not determined by the growth rate. This inference is in contrast with the results obtained for plasma CVD a-Si:H. On the basis of the presented results it is possible to choose the deposition conditions, which yield a-Si:H films with the lowest micropore density.

Raman scattering and electrical conductivity in highly disordered activated carbon fibers

Because of their unusually large specific surface area (SSA), Activated Carbon Fibers (ACF's) have a huge density of micropores and defects. The Raman scattering technique and low-temperature dc electrical conductivity measurements were used as characterization tools to study the disorder in ACF's with SSA ranging from 1000 m2/g to 3000 m2/g. Two peaks were observed in every Raman spectrum for ACF's and they could be identified with the disorder-induced peak near ∼1360 cm−1 and the Breit–Wigner–Fano peak near ∼1610 cm−1 associated with the Raman-active E2g2 mode of graphite. The graphitic nature of the ACF's is shown by the presence of a well-defined graphitic structure with La values of 20–30 Å. We observed that the Raman scattering showed more sensitivity to the precursor materials than to the SSA of the ACF's. From 4 K to room temperature, the dc electrical resistivity in ACF's is observed to follow the exp [(T0/T)1/2] functional form and it can be accounted for by a charge-energy-limited tunneling conduction mechanism. Coulomb-gap conduction and n-dimensional (n ≤ 3) variable-range hopping conduction models were also considered but they were found to give unphysical values for their parameters.

Ultrasonic characterization of microstructure in powder metal alloy

The ultrasonic wave propagation characteristics were measured for IN-100, a powder metallurgy alloy used for aircraft engine components. This material was as a model system for testing the feasibility of characterizing the microstructure of a variety of inhomogeneous media including powder metals, ceramics, castings and components. The data were obtained for a frequency range from about 2 to 20 MHz and were statistically averaged over numerous volume elements of the samples. Micrographical examination provided size and number distributions for grain and pore structure. The results showed that the predominant source for the ultrasonic attenuation and backscatter was a dense (approx. 100/cubic mm) distribution of small micropores (approx. 10 micron radius). Two samples with different micropore densities were studied in detail to test the feasibility of calculating from observed microstructural parameters the frequency dependence of the microstructural backscatter in the regime for which the wavelength is much larger than the size of the individual scattering centers. Excellent agreement was found between predicted and observed values so as to demonstrate the feasibility of solving the forward problem. The results suggest a way towards the nondestructive detection and characterization of anomalous distributions of micropores when conventional ultrasonic imaging is difficult. The findings are potentially significant toward the application of the early detection of porosity during the materials fabrication process and after manufacturing of potential sites for stress induced void coalescence leading to crack initiation and subsequent failure.

Effect of Five Years of No-tillage and Mulch on Soil Properties and Tuber Yield of Cassava on an Acid Ultisol in South-eastern Nigeria

SUMMARYThe effect of no-tillage and mulch on soil properties and tuber yield of cassava was studied over a five year period on an infertile, acid Ultisol in south-eastern Nigeria. Mulch was applied at a rate of 12 t ha−1as a split application once at planting and once six months after planting. No chemical fertilizers were used during the trial. Bulk density and proportion of micropores (pore radius, r, <1.4 µm) were greater, and proportion of macropores (r>14.4 µm) lower, with no-tillage. Water infiltration was increased by mulching, being greatest in tilled, mulched plots. Soil chemical properties were not significantly affected by tillage system, although levels of exchangeable calcium, magnesium and potassium were higher and acid saturation lower with mulching. Soil physical and chemical properties were best with tillage and mulching, and poorest when neither tillage nor mulch was used. Cassava tuber yield was unaffected by tillage system but increased by mulching.

Ultrasonic Characterization of Microstructure in Powder Metal Alloy

Abstract The ultrasonic wave propagation characteristics were measured for IN-100, a powder metallurgy alloy used for aircraft engine components. This material was selected as a model system for testing the feasibility of characterizing the microstructure of a variety of inhomogeneous media including powder metals, ceramics, castings, and composites. The data were obtained for a frequency range from about 2–20 MHz and were statistically averaged over numerous volume elements of the samples. Micrographical examination provided size and number distributions for grain and pore structure. The results showed that the predominant source for the ultrasonic attenuation and backscatter was a dense (∼100/mm3) distribution of small micropores (∼10 μm radius). Two samples with different micropore densities were studied in detail to test the feasibility of calculating from observed microstructural parameters the frequency dependence of the microstructural backscatter in the regime for which the wavelength is much larger than the size of the individual scattering centers. Excellent agreement was found between predicted and observed values so as to demonstrate the feasibility of solving the forward problem. The results suggest a way towards the nondestructive detection and characterization of anomalous distributions of micropores when conventional ultrasonics imaging is difficult. The findings are potentially significant toward the application of the early detection of porosity during the materials fabrication process and after manufacturing of potential sites for stress-induced void coalescence leading to crack initiation and subsequent failure.

Dependence of glow discharge Si:H:Cl film morphology on diluent gas and SiCl4 partial pressure

TEM, THEED and field-assisted silver ion exchange have been employed to study the glow discharge (GD) Si∶H∶Cl films deposited from SiH4-SiCl4 gas mixtures. The character of the THEED patterns shows that the films are rather amorphous, and that their structure does not alter with the change in the gas mixture. The honeycomb-like morphology of the films is strongly affected by the type of gas in which SiH4 is diluted (Ar or H2). An increase in the SiCl4 partial pressure leads to the uniformity and to the decrease of the island dimension only for the films deposited from SiH4(H2)-SiCl4. A possible correlation between the film morphology and the micropore density is proposed.

Revelation of micropores in a-Si:H films

Glow discharge a-Si:H has been studied regarding micropore density in the films. These defects have been visualized by means of field-assisted silver ion exchange. It has been shown that a 5 × 10 3 cm −2 micropore density is an intrinsic feature of the studied films and is not affected by the thermal and field treatment during the ion diffusion.

Micropore density in a-Si:H and a-Si:H:Cl films

a-Si:H films deposited from various silane-containing gas mixtures have been studied regarding their porosity. Micropore densities of 2. O × 10 2 cm -2, 5.0 × 10 4 cm -2 and 5.0 × 10 4 cm -2 were determined for a-Si:H:C1, a-Si:H(H 2) and a-Si:H(Ar) films, respectively. It is suggested that these values correlate with the structural properties of the films, so that a-Si:H:Cl films seem to be the most uniform on the microstructural scale.

Ultrasonic characterization of microstructure in powder metal alloy

The ultrasonic wave propagation characteristics were measured for IN‐100, a powder metallurgy alloy used for aircraft engine components. Attenuation, velocity, and backscattered microstructural noise were measured on several research samples. The data were obtained with a variety of different transducers placed either in a waterbath or in sample contact for a frequency range from about 2 to 18 MHz and statistically averaged over numerous volume elements of the samples. Micrographical examination provided size and number distributions for grain and pore structure. The results showed that the predominant source for the ultrasonic attenuation and backscatter was a dense (∼ 100/mm3) distribution of small micropores (∼ 10 μm radius). Two samples with different micropore densities were studied in detail to test the feasibility of calculating microstructural parameters from ultrasonic backscatter data in the regime for which the wavelength is much larger than the size of the individual scattering centers. The re...