Available and Waiting Times for Cognitive Radios

Cognitive radios (CRs) have been recently proposed for the problem of spectrum scarcity. The principle of CRs?? operation is based on the opportunistic access to the frequency spectrum mainly dedicated to primary users (PUs). The statistical time pattern of PUs?? channel usage and arrival can affect the usability of specific frequency bands for CRs. In this note, the effect of the arrival rate and channel holding time of PUs on the available times for CRs is analyzed. To this end, first, based on Poissonian arrivals, the available time for CRs is calculated. Then, assuming a gamma distribution for the inter-arrival times and a uniform distribution of channel holding time of PU in these intervals, the probability density function and moments of the available time for CRs are derived. Next, the effect of PUs statistical parameters on the average number of packets and the average symbol rate that a CR can transmit is analyzed. Also, taking that CR needs at least T seconds, the average waiting time is calculated.     

Microstructure and mechanical properties of a copper–stainless-steel dual-phase system prepared by spark plasma sintering

Spark plasma sintering (SPS) has been used to synthesise samples of a model dual-phase (DP) system, consisting of copper and AISI420 martensitic steel. Mixed powders were sintered at different temperatures in the range from 850°C to 1000°C using a loading pressure of 60?MPa. Tensile testing revealed dimpled fracture surfaces in samples sintered at higher temperatures, indicating ductile failure and good interfacial bonding between the phases, with the best results overall obtained for samples sintered from wet-mixed powders. The results show that by control of the mixing, sintering and post-sintering heat treatment conditions, the fabrication of dense samples covering a range of DP microstructures is possible using SPS, with independent control of the microstructure and properties of the two phases.     

Fabrication and characterization of silk/forsterite composites for tissue engineering applications

In this research, novel composite scaffolds consisting of silk fibroin and forsterite powder were prepared by a freeze-drying method. In addition, the effects of forsterite powder contents on the structure of the scaffolds were investigated to provide an appropriate composite for bone tissue engineering applications. The morphology studies using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques showed that the forsterite ceramic was well distributed throughout the structures of SF/forsterite scaffolds. Furthermore, the forsterite powder (up to 40 wt%) was homogenously distributed within the silk fibroin as a matrix.     

Effect of cerium ion addition on corrosion and wear characteristics of plasma electrolytic oxidation coating of CP-Ti

Cerium containing coatings are interested because of corrosion and wear resistance characteristics, in another point valve metals have the ability to be treated with plasma electrolytic oxidation (PEO). Different cerium sources, from base metal, pretreatment, post treatment and electrolyte are implemented for Mg alloys. In this research Ti substrates was treated via PEO in less studied, cerium chloride containing electrolytes. Effect of cerium ion concentration was studied on morphology, corrosion and wear properties using SEM, FTIR, polarization, EIS, pin on disk experiments. It was observed that cerium participates in coatings slightly and there is an optimum content for cerium ions in electrolytes which results in improved corrosion and wear properties, mostly from modifying coating roughness and morphology. Addition of cerium ions to electrolyte, first decreases and then increases coating roughness and coefficient of friction.     

Development of a new analytical tool to design hierarchical truss beams for natural frequency

A set of recursive equations are introduced for optimum design of a wide range of hierarchical truss beams to satisfy a minimum required natural frequency. The design equations are based on a general analytical solution, which are derived based on a critical dimensional analysis. A practical example is examined in both 2D and 3D spaces, which promises substantial mass reduction, as much as 99%, when using high order hierarchical truss beams. The results are verified by numerical evaluation of three case studies. The approach looks also very promising to design nanostructures when it comes to the problems associated with vibration.     

Stabilisation mechanism of various inulins and hydrocolloids: Milk–sour cherry juice mixture

Milk–fruit juice mixtures, such as the mainly acidic nutraceutical soft drinks, usually suffer from phase separation due to aggregation of caseins at low pH. In this study, short‐chain inulin (SCI), native inulin (NI), long‐chain inulin (LCI) and a combination of long‐ and short‐chain inulins (LCI:SCI) (MIX) in different ratios (20:80, 50:50 and 80:20) were added (up to 10% w/v) to a milk–sour cherry juice mixture and their stabilisation mechanisms investigated using rheological, microstructural and zeta potential observations. In addition, gum tragacanth (GT) and Persian gum (PG) as adsorbing and guar gum (GG) as nonadsorbing hydrocolloids were combined with inulin to enhance their stabilising properties. Finally, sensory analyses were carried out on the stabilised samples. According to our findings, LCI fully stabilised the mixture (8% w/v), while LCI: SCI and NI only reduced phase separation at very high concentrations, and SCI had no significant effect on the stabilisation. Moreover, no inulin aggregates and rheological changes were observed with SCI. However, LCI, LCI: SCI and NI formed inulin aggregates and the mixtures became even more viscous and thixotropic (LCI > LCI: SCL > NI). Based on these observations, it can be concluded that chain length and concentration are two important factors that affect the functionality of inulin. On the other hand, the combination of inulin with GT and PG did not have any pertinent effect on the stabilisation. However, the mixture of inulin and GG could stabilise the mixtures at certain ratios and concentrations. Furthermore, in mixtures containing GG and SCI, GG played the main role in the stabilisation by increasing the viscosity and forming gel network.     

Catalytic oxidative desulfurization of dibenzothiophene utilizing molybdenum and vanadium oxides supported on MCM-41

As a series of bimetallic nanocatalysts, molybdenum/vanadium oxides supported on the silica (MoO₃/V₂O₅/MCM-41) were prepared by the impregnation. Their catalytic activity in the oxidation of dibenzothiophene was investigated using H₂O₂ as an oxidant. Textures and surface properties of the prepared catalysts were characterized using FT-IR, XRD, FESEM-EDX and N₂ adsorption/desorption techniques. The effects of main process variables including H₂O₂/DBT molar ratio, catalyst dosage, reaction temperature and reaction time were analyzed by employing the response surface methodology (RSM). The effect of the MoO₃/V₂O₅ loading on the catalytic performance of the catalysts was also investigated. The results indicated that the catalytic activity of the catalyst was increased by enhancing the MoO₃/V₂O₅ content. Thus, the catalyst with high MoO₃/V₂O₅ loading (20%MoO₃/20%V₂O₅/60%MCM-41) indicated the highest catalytic activity and could convert 99.06% of dibenzothiophene under the optimum conditions. Mass and FT-IR analysis demonstrated that the major product of dibenzothiophene oxidation was its corresponding sulfone. The catalyst could be recycled five times without any considerable reduction in its catalytic activity. The kinetics of the reaction fitted the pseudo-first-order equation pretty well. Eventually, a reaction mechanism for the catalytic oxidation of DBT in the presence of MoO₃/V₂O₅/MCM-41 was proposed.     

Optimization of Hydroforming Process for Deep Drawing of AA7075 Using Finite Element Simulation and Response Surface Methodology

Aluminum 7075 is a functional aerospace alloy that has high strength and low formability during room temperature deep drawing. Therefore, to avoid warm forming process and relative difficulties, careful control in the process design is needed to enhance quality of drawn part and improve die life. In the present work, an attempt was made to experimentally and numerically study the effect of hydrodynamic deep drawing process such as die geometry and fluid pressure on thinning ratio and punch force. Statistical analysis based on response surface methodology was carried out to correlate empirical relationship between die corner radius, punch corner radius and die fluid pressure as process factors and maximum thinning ratio and punch force as responses. Sensitivity analysis was also performed to find out the factor having greatest impact on aforementioned responses. Further, optimization was carried out by using desirability approach to find out optimal parameter setting to attain minimum thinning and forming force simultaneously. Results indicated that punch and die corner radius followed by fluid had significant effect on the process quality characteristics. Moreover, setting of 9.3 mm punch corner radius, 2 mm die corner radius and 26 MPa fluid pressure resulted in 71% desirability incorporating 31% thinning and 175 MPa punch force. The obtained optimal results were then verified through both FE simulation and confirmatory experiment.     

Effects of alfalfa particle size and specific gravity on chewing activity, digestibility, and performance of Holstein dairy cows

Two experiments were carried out to test the effects of alfalfa particle size and functional specific gravity (FSG) on chewing activity, digestibility, rumen kinetics, and production of lactating dairy cows fed corn silage based rations. In experiment 1, water-holding capacity (WHC), insoluble dry matter, hydration rate, and FSG changes were determined in alfalfa hay (varying in particle size) and corn silage. Reduction of particle size increased bulk density, FSG, and the rate of hydration, and decreased WHC of alfalfa. In experiment 2, 9 midlactation Holstein dairy cows fed total mixed rations containing 3 sizes of alfalfa hay (with geometric mean 7.83, 4.04, and 1.14 mm) were used in a replicated 3 x 3 Latin square design. The diets contained 20, 20, 35, 7, 7.5, 10, 0.3, 0.1, and 0.1% of DM alfalfa, corn silage, barley, soybean meal, beet pulp, wheat bran, dicalcium phosphate, vitamin premix, and salt, respectively. The geometric means (GM) of rations were 3.34, 2.47, and 1.66 mm in long, medium, and fine alfalfa treatments, respectively. Reduction of particle size increased daily NDF intake (kg), but decreased the proportion of physically effective factor (pef) and physically effective NDF (peNDF) in the ingested rations. Reduction of particle size increased the FSG of rations and intake of DM but reduced digestibility of NDF and ash. Reduction of particle size decreased ruminal mean retention time (RMRT), but increased the ruminal particulate passage rate. Milk and FCM yield were not affected by treatments. The rumen pH, total chewing activity, rumination, eating time, and milk fat were reduced as particle size decreased, but milk protein increased. This study showed that reduction of forage particle size increased bulk density, FSG, and hydration rate of alfalfa and was the most influential factor affecting DMI, milk composition, and chewing behavior. Reduction of forage particle size had minimal impact on digestibility and milk production.