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

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

Influence of sintering temperature on the structural, magnetic and dielectric properties of Ni0.8Zn0.2Fe2O4 synthesized by co-precipitation route.

The polycrystalline Ni–Zn ferrite powder with the chemical formula Ni0.8Zn0.2Fe2O4 has been synthesized using co-precipitation route. The toroidal and pellet form samples were sintered at various temperatures from 700 to 1300 °C/5 h in steps of 200 °C. The structures of samples were studied by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and the energy dispersive X-ray spectroscopy (EDXS). The magnetic and dielectric measurements were carried out using a vibrating sample magnetometer (VSM) and the impedance analyzer, respectively. The highest density of 4.48 g cm−3 was obtained for the sample sintered at 1300 °C. It was found that the initial permeability increased from 4 to 17 and the RLF was in the order of 10−3 to 10−4 in the frequency range of 1.0 MHz to 1.0 GHz. The dielectric constant and dielectric loss were lower compared to the reported values for conventional solid state technique. The electrical resistivity is in the order of 108 Ω cm. Therefore, low relative loss factor and high resistivity make these ferrites particularly useful as inductor and transformer materials for high frequency applications

Finite element analysis of heat transfer in double U-tube borehole heat exchangers

La presente Tesi di Dottorato tratta dell’analisi agli elementi finite dei processi di trasmissione del calore in scambiatori verticali con il terreno detti “Borehole Heat Exchangers”, BHEs, con riferimento a BHEs a doppio tubo a U. Nella valutazione di Test di Risposta Termica (TRTs), per il progetto di campi di BHEs, e nella simulazione dinamica di pompe di calore accoppiate al terreno (Ground-Coupled Heat Pumps, GCHPs), la temperatura media Tm del fluido operatore in un BHE è abitualmente approssimata dalla media aritmetica della temperatura in ingresso e di quella in uscita del fluido. Nei TRT, questa approssimazione causa una sovrastima della resistenza termica dello scambiatore. Nella simulazione dinamica delle GCHPs, questa approssimazione introduce un errore nel calcolo della temperatura in uscita dallo scambiatore di calore. In questa Tesi, la distribuzione di temperatura del fluido nello scambiatore viene studiata mediante accurate simulazioni 3D agli elementi finiti, quindi vengono proposte semplici correlazioni adimensionali che consentono di determinare la temperatura media Tm del fluido in qualsiasi condizione operativa, date le temperature in entrata e in uscita e la portata in volume del fluido e la conducibilità termica della malta sigillante. Vengono inoltre analizzati gli effetti della distribuzione di temperatura superficiale dello scambiatore sulla resistenza termica dello stesso. Si mostra che la resistenza termica 2D di una sezione trasversale non è influenzata dalla differenza della temperatura di bulk fra coppie di tubi, ma è influenzata dalla conducibilità termica del terreno circostante se l’interasse fra i tubi è elevato. Si mostra anche che, se vengono considerati i veri valori della temperatura media di bulk del fluido e della temperatura alla superficie esterna dello scambiatore, la resistenza termica 3D dello scambiatore coincide con quella 2D di una sezione trasversale, a condizione che questa possa essere considerata invariante lungo lo scambiatore

Electrical and Magnetic Properties of NiZn Ferrite Prepared by Conventional and Solar Sintering

The electrical properties of polycrystalline NiZn ferrite, Zn0.44Ni0.38Fe2.18O4, were investigated by impedance spectroscopy over the frequency and temperature ranges, 5 Hz to 2 MHz and 10-600 K and by magnetic permeability measurements at room temperature. Samples were sintered in either conventional or solar furnaces followed by quenching or slow cooling to ambient temperature. Depending on processing conditions, the room-temperature electrical resistivity of conventionally sintered samples varied by seven orders of magnitude, from 5 ohm cm for a sample quenched from 1250°C to 10 Mohm cm for a sample quenched from 400°C. These differences were attributed to variations in oxygen content of the ferrite which decreased with increasing quench temperature. Oxygen deficiency led to mixed valence of Fe in the octahedral B sites of the spinel structure and was responsible for high electronic conductivity with low activation energy at low temperatures in oxygen-deficient samples. By contrast, in oxygen-stoichiometric samples, Fe on the tetrahedral A sites was believed to be divalent and Fe on the octahedral B sites to be entirely trivalent. Electron hopping between A and B sites had much higher activation energy and dominated the conductivity at high temperature for all samples. Samples sintered in the solar furnace were much more conductive than ones that were slow-cooled after conventional sintering and this is attributed to the relatively rapid cooling rate after exposure in the solar furnace, which preserved some of the oxygen deficiency present at high temperature. For the same reason, samples that were slow cooled in N2 were also much more conductive. Solar-sintered samples with higher density (96%) had higher real permeability than slow-cooled, conventionally sintered ones (86%) mainly due to a combination of their lower resistivity and higher density. Resistivity seems to have a greater correlation with the imaginary permeability than density has

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

On the optimisation of age of the air in the breathing zone of floor heating systems: The role of ventilation design

For a mechanically ventilated space, the nominal age of the air can be obtained by the reciprocal of the air change rate. However, values of the local mean age of the air in practice may differ to some extent from the nominal one since the nominal time constant employs as reference the theoretical optimum model. This discrepancy could become more prominent in spaces conditioning with both ventilation and heating system where the indoor air pattern is affected by turbulent mixed convection flow. Given importance of knowledge on the distribution of age of the air in these spaces, the present study provides insights on how ventilation design in floor heating systems can optimise the delivery of fresh air into the breathing zone. In this context, by establishing a computational fluid dynamic (CFD) model validated against experimental data, the local mean age of the air as well as the indoor air quality (IAQ) indices in the breathing zone of the floor heating system are examined under different ventilation modes. Six different ventilation scenarios are regarded in order to find the optimal ventilation design in terms of the delivery of the fresh air and ventilation effectiveness in occupied space. Furthermore, the integrated effects of the floor temperature and inlet supply temperature along with the ventilation design on the distribution of local age of the air are addressed. The obtained results indicate that the up-supply ventilation strategy is more efficient than down-supply one in the ventilation effectiveness and delivery of fresh air into the breathing zone. Moreover, it is shown that the mean age of the air in the breathing zone of the up-supply ventilation increases with increment of the Ri (Richardson number), whereas an increase in Ri improves the delivery of fresh air in down-supply mode. For a given floor temperature, the similar trend is also observed in the variation of age of the air with the characteristic temperature of supply inlet, namely the temperature difference between inlet supply and surrounding walls

Techno-economic analysis of a novel retrofit solution for the domestic hot water system: A comparative study

The retrofit solution for domestic hot water (DHW) system in existing buildings requires to ensure the long-term energy security and efficiency as well as to minimise occupants’ disturbance, construction works and installation costs. In this regard, the present study performs a techno-economic evaluation on a novel retrofit solution for DHW production in a pilot building. The proposed solution appoints a substantial role to the thermal energy storage through a 2-pipe hot water network utilisable for both DHW and heating purposes. The first storage level is provided by a centralised buffer storage supplied by a PV-BESS-driven heat pump while the second level consists of decentralised modular tanks installed in each dwelling for the production and storage of hot water. Firstly, experimental thermal performance of the proposed decentralised storages is investigated. By developing a dynamic simulation code, the energy efficiency of the proposed solution is compared to that of the existing system in the pilot building as well to that of a typical centralised system as a benchmark solution. Finally, economic analysis of the retrofit solution is performed to address capital expenditures of the system, including purchasing and installation costs, as well as its life cycle cost (LCC). The obtained results indicate that the proposed system reduces the annual energy consumption for DHW production more than 7,200 kWh, with respect to the existing DHW system. Furthermore, it is shown that, in the proposed system, the fraction of thermal loss from piping network decreases by 31.5%, compared to a typical DHW centralised system. Economic assessment of the proposed solution implies that this system, in terms of both mechanical and electrical components, requires 13.7% lower initial investment than a typical centralised system. However, the cost of control systems in this system is higher since it is inherently a control-based system