Magnetic, Dielectric And Microstructural Properties Of Nickel−Zinc Ferrite Ni0.8zn0.2fe2o4 Prepared Via Conventional And Co-Precipitation Techniques

Nickel-zinc ferrite is a soft magnetic material having low magnetic coercivity and high resistivity values. The high electrical resistivity and excellent magnetic properties make this ferrite an automatic choice as a core material for power transformers in electronic and telecommunication applications in megahertz frequency regions .The properties of the Ni–Zn ferrite are very sensitive to the method of preparation, sintering temperature, sintering time and chemical composition . Most ferrites prepared via the conventional ceramic processing method have some drawbacks. Wet chemical methods are being pursued to overcome these drawbacks and to produce ultra-fine, homogeneous and reproducible ferrite powders using aqueous solutions of salts of constituent ions. In this study, Ni0.8Zn0.2 Fe2O4 was prepared via both conventional ceramic and the co-precipitation methods. The microstructure, magnetic and dielectric properties The permittivity in both the co-precipitation synthesis and the conventional synthesis increases with increasing sintering temperature. The dielectric constant and loss tangent of sample prepared via co- precipitation is much lower than the conventional due to the smaller grain size and higher porosity. The resistivities of samples prepared via co-precipittion were from 7.2x108 to 3.7x108 (Ωcm) and 4.3x106 to 8.2x105 (Ωcm) for samples prepared via conventional synthesis. In small grain microstructure the grain boundaries are more thus increase the resistivity. This resulted higher resistivity in co-precipitation than conventional methods. The highest saturation magnetisation, Ms, values in samples synthesized by co- precipitation was 63 Ms(Am2kg-1) and 60 Ms(Am2kg-1) for the conventional counterpart which were sintered at 1300°C. The higher Ms in co-precipitation is probably due to better crystalline structure. The coercivity and remanence of samples prepared via co-precipitation are lower than those of the conventionally synthesized samples. The initial permeability value lies between 4 and 17 for the co-precipitation and 14 to 24 for conventional synthesis. The widest frequency range is obtained at 700°C in co-precipitation method due to the small grin size. The relative loss factor (RLF) also studied and it was revealed that the RLF increased with sintering temperature. But It is thus concluded that it is the better of the two methods of preparation employed in this work for Nickel-Zinc Ferrite, Ni0.8 Zn0.2 Fe2 O4

The Effect of the Social Media Tools on Virtual Team Performance: The Mediating Role of Transactive Memory System Mapping with the Feature Richness

The communication tool is an important component of a virtual team, and virtual teams are highly dependent upon the communication tools for accomplishing their tasks and fulfilling their needs effectively. This research in progress builds upon the existing literature and employs the concept of feature richness of social media tools and a Transactive Memory System (TMS) approach to develop a conceptual framework for understanding the impact of social media tools on virtual team performance. Thus, a conceptual research model which postulates that TMS mediates the relationship between social media tools and virtual team performance, is developed. This research tries to establish an appropriate component-level mapping between the components of TMS construct and the feature richness factors to provide a deeper understanding about the effect of social media tools on TMS and consequently, the impact on virtual team performance

Relation between mean fluid temperature and outlet temperature for single U-tube boreholes

Ground-coupled heat pump (GCHP) systems usually utilize buried vertical heat exchangers, named borehole heat exchangers (BHEs). The accurate design or simulation of a GCHP system requires the calculation of the time-dependent outlet temperature from the BHEs, T out. However, the most widely employed BHE simulation models yield the time evolution either of the mean temperature of the BHE-ground surface, T sm, or of that of the fluid, T fm. In transient regime, it is not easy to relate T out to either T sm or T fm. In this paper we determine, through 3D finite element simulations, simple expressions of a dimensionless coefficient \u3c6 allowing the calculation of T out by means of a simulation model that yields T fm. These expressions hold for single U-tube BHEs, both in quasi-steady and in unsteady working conditions. We validate our 3D simulation code by comparison with an analytical BHE model. Then, we present applications of our expressions of \u3c6 to calculate the time-dependent values of T out through a BHE model that yields those of T fm. Finally, we show that the values of \u3c6 in quasi-steady working conditions can be used for a simple calculation of the effective borehole thermal resistance

Nanofluid suspensions as heat carrier fluids in single U-tube borehole heat exchangers

The borehole heat exchanger (BHE) is a critical component to improve energy efficiency and decreasing environmental impact of ground-source heat pump systems. The lower thermal resistance of the BHE results in the better thermal performance and/or in the lower required borehole length. In the present study, effects of employing a nanofluid suspension as a heat carrier fluid on the borehole thermal resistance are examined. A 3D transient finite element code is adopted to evaluate thermal comportment of nanofluids with various concentrations in single U-tube borehole heat exchangers and to compare their performance with the conventional circuit fluid. The results show, in presence of nanoparticles, the borehole thermal resistance is reduced to some extent and the BHE renders a better thermal performance. It is also revealed that employing nanoparticle fractions between 0.5% and 2 % are advantageous in order to have an optimal decrement percentage of the thermal resistance

Prophylactic Effects of Ephedrine, Ondansetron and Ringer on Hemodynamic Changes during Cesarean Section under Spinal Anesthesia — a randomized clinical trial

Background: Hemodynamic change during spinal anaesthesia for cesarean section is prevalent. Objective: Comparing the prophylactic effects of ephedrine, ondansetron and ringer on hemodynamic changes in patients undergoing cesarean section with spinal anaesthesia. Material and methods: This randomized clinical trial was carried out on pregnant women undergoing elective cesarean sec­tion referred to teaching hospitals of Mashhad, Iran. Patients allocated to three groups of intravenous ondansetron (O) (4 mg, 5 min),intramuscular ephedrine (E) (25 mg, 25 min) and ringer (R) (500 ml, 20 min) prior to spinal anaesthesia. Anaesthesia inducted with 10–15 mg of bupivacaine. Vital signs were recorded every 3 minutes for 18 minutes including systolic blood pressure (SBP), diastolic blood pressure (DBP), mean arterial pressure (MAP), pulse rate (PR), pulse oximetry (SpO2). Results: Ninety patients with a mean age of 29.4 ± 5.4 years were studied in three groups of O (n = 30), E (n = 30), R (n = 30). Results showed a statistically significant difference in the incidence rate of hypotension 12 minutes after spinal anaesthesia in the three groups, but no statistically significant difference was found in the rest of minutes among the three groups. Dur­ing follow-up minutes, bradycardia was observed in only one patient (1.1%) of Group O and no cases of this sign were observed in other minutes and other groups. Conclusion: Intramuscular administration of ephedrine 25 minutes prior to the spinal anaesthesia leads to better prevention of systolic blood pressure changes compared with intravenous ondansetron and ringer, while administration of ondansetron and ringer had the same effects on reducing hemodynamic changes

A new estimate of sand and grout thermal properties in the sandbox experiment for accurate validations of borehole simulation codes

Ground-coupled heat pumps usually employ fields of borehole heat exchangers (BHEs), which must be designed by suitable models. In order to validate a BHE model, it is advisable to compare the computation results with experimental data. A well-known data set was provided by Beier et al. (Geothermics 2011, 40) through a laboratory model usually called “sandbox”. Several authors proposed estimates of the thermal properties of the sandbox grout and sand. In this paper, we present a new estimate of those properties, obtained by means of 2D finite-element simulations that consider all the details of the experimental setup, including the thin aluminum pipe at the BHE boundary. Our results show that the measured temperatures in the fluid and in the sand can be reproduced very accurately by considering thermal conductivities 0.863 W/(mK) for the grout and 3.22 W/(mK) for the sand, volumetric heat capacities 4.6 MJ/(m3K) for the grout and 3.07 MJ/(m3K) for the sand, and a slightly enhanced heat capacity of the water contained in the BHE. The 2D simulations are validated by comparison with an analytical solution and by 3D simulations

Conception et développement in vitro d'agents de contraste hautement efficace en IRM. Apport de la dynamique moléculaire sur le signal RMN

L'objectif du travail de thèse est de concevoir et développer 2 classes d'agent de contraste (AC) intrinsèquement efficace en IRM. La première classe comprend le Gd (III)-méso-tétra (4-pyridyl) porphyrine (Gd (TPyP)) macromoléculaire. Son efficacité a été comparé avec deux produits de référence : la Mn(III)-méso-tétra(4-sulfonatophenyl)porphyrine (Mn(III)TSPP), et Fe(II)-méso-tétra(N-methylpyridinium)porphyrine (Fe(II)TMPyP) à de deux champs magnétiques de 20 MHz et 60 MHz. En particulier, la relativité r1 la plus grande est obtenue pour le complexe Gd(TPyP) (24 mM-1s-1à 60 MHz) soit une efficacité au moins 6 fois plus importante que pour les complexes de Gd conventionnels. Afin d'améliorer sa biocompatibilité médicale, sa solubilité de l'eau, et augmenter son efficacité en IRM, la Gd-porphyrine a été conjuguée avec les nanoparticules de chitosan. Ainsi, une valeur de 38 mM-1s-1 à 60 MHz 9 fois plus élevée que celle de Dota-Gd a été obtenue dans l'eau. Dans la 2ème partie de la thèse, nous avons développé le concept de greffage de Mn dopé à la surface de nanoparticules ZnS afin d'améliorer l'accessibilité de l'eau au moment magnétique du Mn. Les nanoparticules ont une relaxivité r1 variant de 21.57 à 74.12 mM-1s-1 pour un taux de Mn passant de 0.1 à 0.3, par rapport au produit commercial Mn DPDP (r1=2.8 mM-1.s-1 à 42 MHz). Par la suite, l'influence de la granulométrie de Mn0.3Zn0.7S sur son efficacité a été étudiée. Leur r1 a diminué de 74.12 à 42.81 mM-1s-1 avec l'augmentation de la dimension particulaire. Pour expliquer le mécanisme, une simulation numérique de la dynamique moléculaire de l'eau au voisinage des nanoparticules de MnZnS a été développée. Les résultats de la simulation de la structure cristalline de MnZnS sont concordent avec les valeurs expérimentales par la cristallographie aux rayons X. Le modèle a été amélioré en incluant un effet de la concentration variable au Mn dans un environnement aqueux.The objective of this thesis is design and development of two types of MRI contrast agents (CA) with high efficiency, including macromolecule and nanoparticles. The first substance is Gd(III)-meso-tetra(4-pyridyl)porphyrin (Gd(TPyP)). Its efficiency has been compared with two metalloporphyrin compounds, Mn(III)-meso-tetra(4-sulfonatophenyl) porphyrin (Mn(TSPP)) and Fe(II)-meso-tetra(N-methylpyridiniumyl)porphyrin (Fe(TMPyP)) in presence of two magnetic field of 20 and 60 MHz. Among the metallated porphyrins, Gd(TPyP) exhibits the highest r1 of 24 mM-1s-1 (6-fold higher compared to r1 of Gd-DOTA). In the next step, Gd(TPyP) has been conjugated to chitosan nanoparticles in order to improve its biocompatibility and water solubility. The small water-soluble Gd(TPyP)-conjugated chitosan nanoparticles (~40 nm) show higher (56%) r1 of 38 mM-1.s-1 at 3T than the one of Gd(TPyP) in ethanol and 9-fold greater than r1 of Gd-DOTA. The second complex developed as CA is MnxZn1-xS ( 0.1 =x=0.3) nanoparticles while the majority of Mn atoms localized on/ or close to the surface of ZnS nanoparticles to enhance their efficiency as MRI CA. Mn:ZnS nanoparticles exhibits higher r1 compared with the one of commercial Mn-DPDP (r1=2.8 mM-1s-1 at 42 MHz), which significantly increases from 20.34 to 75.5 mM-1s-1 with Mn content in the range of 0.1-0.3. Thereafter, effect of particle size on relaxivity of Mn0.3Zn0.7S has been investigated. We observed that r1 decreases with increasing particle size due to decreasing the surface to volume ratio from 75.5 to 42.81 mM-1s-1. In order to obtain the insight through the relaxivity of Mn:ZnS nanoparticles, computational technique has been carried out to predict the interaction and dynamic of Mn:ZnS and solvent (water) via molecular dynamic simulations (MDs). Thereby, MnxZn1-xS with different dopnat contents ( 0.1 =x=0.3) has been modeled via MDs. We achieved to reproduce the crystal structure of MnZnS precisely, within a few percent of experimental values. The study has been completed successfully by adding MnZnS nanoparticles in aqueous solution

Comparative studies on the structure and electromagnetic properties of Ni-Zn ferrites prepared via co-precipitation and conventional ceramic processing routes

The magnetic and physical properties of ferrites are very sensitive to microstructure, which in turn critically depends on the manufacturing process. In this study, nickel zinc ferrite powder with composition Ni0.8Zn0.2Fe2O4 has been prepared via conventional ceramic processing and co-precipitation methods. The toroidal and pellet form samples were sintered at various temperatures such as 1100, 1200 and 1300 °C. The microstructure, magnetic and dielectric properties of both samples were studied. The X-ray diffraction patterns confirm the formation of single-phase cubic spinel structure for the co-precipitation technique after sintering. The microstructure studies of Ni0.8Zn0.2Fe2O4 show the grain size increases and the porosity decreases with temperature for both methods. The controlled permeability with small loss and wide operational frequency range are found in the co-precipitation samples. Dielectric constants decrease with increase of frequency and increase with sintering temperature in both methods. Consequently, the homogenous microstructure with the low-loss high-performance of nickel zinc ferrite has been discovered by means of co-precipitation method