Unseeded Ones Research Articles

DEUTERIUM RETENTION IN N-SEEDED AND UNSEEDED TUNGSTEN LAYERS OBTAINED WITH DCMS AND M-HIPIMS TECHNIQUE

The retention of hydrogen isotopes in tungsten co-deposited layers is a subject severely understudied in the literature compared to hydrogen isotope implantation in bulk tungsten specimens. However, impurity seeding in future next-generation fusion devices can reduce the sputtering threshold of tungsten and potentially increase the contribution of the tungsten layers to the global tritium retention problem. This paper reports on the development of fusion-relevant tungsten-deuterium and nitrogen-seeded tungsten deuterium layers using direct current (DCMS) and multipulse high-power impulse magnetron sputtering techniques (HiPIMS). The four layers resulting from co-deposition in argon-deuterium and argon-deuterium-nitrogen atmosphere were investigated from the deuterium retention and release point of view. Supplementary the crystalline structure of all samples and particularly the composition of the nitrogen-seeded layers were also studied. The tungsten-deuterium layers produced using m-HiPIMS technology showed a higher crystalline coherence compared to layers deposited with DCMS in part owned to the enhanced ion bombardment of the forming layers during deposition. On the other hand, the nitrogen-seeded layers showed without exception the formation of the tungsten-nitride compound. Here on the contrary the crystalline coherence as revealed by X-ray diffraction measurements measurement showed that nitride formation is favored for DCMS compared with m-HiPIMS. The elemental areal densities of the layer constituents were analyzed using Rutherford Backscattering Spectrometry. In this case, only the two nitrogen-seeded layers were analyzed revealing that a higher nitrogen content by a factor of ~2 is retained in DCMS deposited layer compared to HiPIMS.The release of deuterium was higher in the case of m-HiPIMS layers and it was shown that despite the deposition method the release behavior occurs predominantly at temperatures above 800 K suggesting a high energy trapping of deuterium in tungsten layers. Nitrogen seeding influences both the retention mechanisms and increases the deuterium retention by two orders of magnitude in seeded layers compared to unseeded ones.

Synthesis of zinc oxide nanowires on seeded and unseeded gold substrates: Role of seed nucleation and precursor concentration

ZnO nanowires were synthesized on gold-coated glass substrates using the hydrothermal method. The effects of precursor concentration, substrate annealing and seeding on the morphology, dimension, and distribution of resultant nanowires were investigated. We found that the density of nanowires on substrates pre-seeded with ZnO nanoparticles is about two orders of magnitude greater than unseeded ones, while the dimension of ZnO nanowires for pre-seeded samples is much smaller than unseeded samples. In addition, we found that the fraction of substrate area covered by nanowires for unseeded samples is proportional to the precursor concentration, and proposed a simple nucleation model to explain this behavior. For pre-seeded substrates, ZnO nanowire density first increases with concentration and decreases as concentration exceeds 20mM. We attribute this behavior to the competition for ions among the dense wires of varying length as well as the fusion of neighboring wires into larger ones.

Reassessment of rain enhancement experiments and operations in Israel including synoptic considerations

A re-analysis of the results from cloud seeding in northern Israel is described. The analysis covers the period of the randomized Israel II experiment (1969–1975) and the subsequent period (1975 to the present) where operational seeding is being conducted. The evaluation is carried out using the double ratio method, as was done in the past. We analyzed the precipitation data in the north of Israel before and during the seeding period, stratified the data based on synoptic conditions and compared the results with an area to the south that had been unseeded during the seeded days in the north. The results show that during Israel II the rain enhancement in the target area in the north of Israel was about 12%, similar to the results reported previously. However, these results have two major problems: 1) During Israel II the rainfall ratio between the inland areas (target) and the coast (control) was unusually higher in comparison with the unseeded periods prior to the initiation of the experiment and also, in comparison with the seeded period following Israel II. 2) Comparison of the double ratio in the north during the Israel II experiment was found to be slightly lower than that in an equivalent unseeded area to the south on the same days (1.15 vs. 1.13). This implies that the high double ratio values in the north may not be a consequence of successful cloud seeding but of preferred synoptic conditions during seeding in which inland rainfall is relatively higher, i.e. deep cyclones over the E. Mediterranean. The analysis also shows that during Israel II the frequency of deep lows that accompanied the rainy days was higher on seeded days than during unseeded ones with stronger westerly winds. This can explain why on seeded days, rain clouds penetrated more efficiently towards inland areas resulting in higher rainfall ratios of target (inland mountainous)/control (coastal strip) on seeded days both inside and outside the seeding project area. Furthermore, the double ratio obtained for the whole study period (1.00) also strengthens the point that the high double ratio value during Israel II experiment is not the result of cloud seeding.

Phase formation kinetics of nanoparticle-seeded strontium bismuth tantalate powder

Strontium bismuth tantalate (SBT) having composition of Sr0.7Bi2.3Ta2O9 has been prepared through sol-gel method using their corresponding metal alkoxides as precursors. Seeded SBT powder was prepared by the addition of 5 wt% of nanometer-sized SBT particles to the sol followed by pyrolysis. Differential thermal analyses (DTA) were performed on the unseeded and seeded powder and Aurivillius phase formation temperatures were found to be reduced by ∼60°C in the seeded ones. Non-isothermal kinetic analyses were applied to the DTA results to obtain activation energy and Avrami exponent values for the Aurivillius phase formation in unseeded and seeded samples. The activation energy for the Aurivillius phase formation was found to be ∼268 kJ/mol for the seeded ones, while ∼375 kJ/mol for the unseeded ones, which plays a major role for the enhanced kinetics in the seeded ones. The Avrami exponent values for the Aurivillius phase formation in unseeded and seeded ones were determined as ∼2.80 and ∼1.15, respectively, revealing different nucleation and crystal growth mechanisms.

Structural and electrical properties of seeded lead zirconate titanate thin films

Films of lead zirconate titanate (PZT) (52/48) on platinum passivated silicon substrates were prepared from sol precursors with the addition of perovskite PZT seeds (52/48, 0–5 mol.%). The phase evolution and microstructure of the films were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis. The effect of seeds on the crystallization kinetics is discussed. For the different films X-ray rocking curves were constructed and the crystallographic orientation of the films is discussed, stressing the influence of seeds on the orientation of the films. Ferroelectric properties were measured and compared between films prepared with and without seeds. The addition of perovskite seeds to the PZT precursor sols induced the formation of the pure perovskite phase at lower temperatures in thin films. Although seeded films show less intense preferred orientation, they show superior ferroelectric properties over unseeded ones.

ENDOTHELIAL CELL CULTURE AND SEEDING OF PROSTHETIC VASCULAR GRAFTS: AN EXPERIMENTAL STUDY

Current synthetic vascular prostheses do not acquire lining of vascular endothelium in humans or dogs. Endothelial seeding of vascular grafts has been proposed as a means of reducing the thrombogenicity of these grafts. We examined feasibility of cultivating endothelial cells (EC) by tissue culture technique and their subsequent seeding onto small diameter polytetra fluoroethylene (PTFE) grafts. Twenty adult dogs underwent common carotid artery interposition with 4 mm PTFE grafts. Ten dogs received seeded and the remaining ten received unseeded grafts. Grafts were removed at 4 and 12 weeks and their gross/morphological features compared. Cumulative patency rates for seeded grafts were 70% as compared to unseeded ones 30%. Seeded grafts were completely surfaced with a mono-layer of endothelium by 4 weeks. Small graft patency appears to be related to the establishment of an endothelial surface, the development of which is clearly facilitated by seeding with autogenous endothelium.

An independent evaluation of a South African hygroscopic cloud seeding experiment, 1991–1995

An independent statistical evaluation of a randomised hygroscopic cloud seeding experiment carried out in South Africa from 1991–1995 is presented, based on the original radar data for each storm. It is found that seeded storms clearly lasted longer than unseeded ones. It is suggested that the initiation of precipitation at a lower height in seeded than in unseeded clouds (demonstrated through microphysical measurements by the experimenters) led to a reduced dispersion by windshear of the precipitation falling from the updraft in the seeded clouds. A more concentrated downdraft region resulted, closer to the updraft. The surface gust front was thereby intensified and its interaction with the storm inflow or other gust fronts enhanced convection. Some consequences of such a mechanism are explored. It is concluded that this experiment offers a real hope of increasing precipitation from convective storms and represents an important advance in weather modification.

Endothelial Cell Seeding to the Dog Vena Caval Prostheses Made of Different Materials and Configurations

Producing an early and complete prosthetic endothelialization is one of the best methods for improving performance of venous grafts. This study investigated the effect of graft material and configuration on the prosthetic endothelialization. Forty-two mongrel dogs were randomly divided into six groups. Four animals from each group were implanted with endothelial cell (EC)-seeded prostheses and 3 with unseeded ones in the inferior vena cava (IVC). ECs were harvested from autogenous omentum by enzymatic method yielding 1.65 ±0.43x106 ECs per gram of omentum. A 5-cm-long segment of IVC was resected, and an equal-dimension prosthesis was interposed. A femoral arteriovenous fistula (AVF) was created. Prostheses were explanted at ten days following surgery. All EC-seeded knitted Dacron prostheses and 3 seeded polytetrafluoroethylene (PTFE) ones were patent with complete endothelialization. All seeded woven Dacron prostheses were patent with partial endothelialization. Only 2 seeded and one control Silk prostheses were patent, and no EC lining could be detected. Light microscopy revealed that the neointima was thinner in the patent seeded PTFE grafts than that in controls (p<0.001) and in seeded knitted Dacrons (p < 0.02) . The neointima of seeded knitted Dacron grafts was thinner than that of controls (p<0.01). No marked difference was seen among the neointima formed on Meadox porous, Vascutek porous grafts (VP1200), or Vascutek sealed Gelsoft grafts. It was concluded that: (1) Excellent and early endothelialization of EC-seeded dog caval implants can be established with Vascutek VP1200, Gelsoft, Meadox knitted Dacron, and indigenous ePTFE. (2) Knitted Dacron is superior to woven Dacron in regard to EC seeding. (3) The gelatin-impregnated Gelsoft does not interfere with prosthetic endothelialization. (4) The woven Dacron, and especially the Silk prosthesis, seem unsuitable for EC seeding in the venous circuit.

Crystalline perfection of melt-grown CdTe

Single crystals of CdTe have been grown by the vertical gradient freeze method using both spontaneous nucleation and seeding in the [111] direction. It is shown that almost single grain 50x40 mm crystals containing no twins can be obtained. The interface shapes have been analyzed by the electron microprobe profiling of Zn distribution in CdZnTe crystals and found nearly planar during the major part of growth. The [111] seeded crystals appear to be of clearly superior crystalline perfection compared to the unseeded ones. The difference in quality of the two types of crystals is related to higher thermomechanical stresses imposed on the solidified material when the (111) crystallographic plane is not parallel to the growth interface, as it is in the case of the unseeded grown crystals.

Hydration and Strength Development in Tricalcium Silicate Pastes Seeded with Afwillite

The formation of afwillite in tricalcium silicate pastes initiated by ding with crystals was studied to determine the effects on development of tensile strength. Afwillite forms in pastes only when fresh surfaces of both tricalcium silicate and afwillite seed are exposed by initial grinding. Higher early strengths reflect increased rates of hydration in seeded pastes, but strengths at later ages are lower than those of untreated pastes. At all ages tensile strength is lower at a given degree of hydration in seeded pastes than in unseeded ones; this behavior is considered to result from changes in pore‐size distribution which occur as afwillite forms. Afwillite initially forms concomitantly with calcium silicate hydrate gel but later arises from conversion of the gel.

Growth of Seeded Copper Crystals of Perfection Comparable to Unseeded Crystals

A graphite mold of a new design was made for growing better seeded crystals from the melt. The main features were (1) a narrow seed constriction, (2) a split mold, (3) high density graphite, (4) equal cross sections of seed and crystal, and (5) high ratio of sample to mass crucible mass. The resulting crystals were compared by two complementary techniques with those grown in a conventional mold. An improvement of at least a factor of ten was obtained. The best crystals thus far have 1−2×104 etch-pit sites/cm2 intersecting the {100} surfaces, and are free from subboundaries. This brings the level of perfection of as-grown seeded crystals up to that of unseeded ones thereby making them useful in dislocation etch-pit studies.

Factors influencing friction & wear with solid lubricants : M. B. Peterson and R. L. Johnson. Lubrication engineering, v. 11, Sept.–Oct. 1955, p. 325–330

Current synthetic vascular prostheses do not acquire lining of vascular endothelium in humans or dogs. Endothelial seeding of vascular grafts has been proposed as a means of reducing the thrombogenicity of these grafts. We examined feasibility of cultivating endothelial cells (EC) by tissue culture technique and their subsequent seeding onto small diameter polytetra fluoroethylene (PTFE) grafts. Twenty adult dogs underwent common carotid artery interposition with 4 mm PTFE grafts. Ten dogs received seeded and the remaining ten received unseeded grafts. Grafts were removed at 4 and 12 weeks and their gross/morphological features compared. Cumulative patency rates for seeded grafts were 70% as compared to unseeded ones 30%. Seeded grafts were completely surfaced with a mono-layer of endothelium by 4 weeks. Small graft patency appears to be related to the establishment of an endothelial surface, the development of which is clearly facilitated by seeding with autogenous endothelium.