Showing papers on "Liquid dielectric published in 2020"

Effect of surface modification of electrodes on charge injection and dielectric characteristics of propylene carbonate

Abstract: Space charge injected into a liquid dielectric by an electrode under an impulse voltage causes the electric field to be distorted, which affects the dielectric's insulation performance. In this study, the authors study the surface modification of three-electrode materials, namely aluminium, copper, and stainless steel, by sputtering of titanium dioxide (TiO 2 ) based on magnetron sputtering. The breakdown voltage of propylene carbonate and the surface morphology of the electrodes before and after modification were examined. Furthermore, the distribution of the space charge injected into the propylene carbonate from the different electrode materials was also measured based on the Kerr electro-optic effect. These results indicate that the breakdown voltage of propylene carbonate increased by 7, 4, and 9% after surface modification of aluminium, copper, and stainless steel, respectively. After sputtering of TiO 2 , micro-grooves on the surface of the three-electrode materials became smooth, and the surface roughness decreased, which changed the barrier height of the solid–liquid interface. The space charge injection mode of the three-electrode materials before and after surface modification was bipolar injection; however, the space charge injection amounts were markedly lower for the modified electrodes.

Powder-mixed electro-discharge machining performance of Inconel 718: effect of concentration of multi-walled carbon nanotube added to the dielectric media

Abstract: The present work reports an experimental investigation on Powder-Mixed Electro-Discharge Machining (PMEDM) of Inconel 718 superalloy using Multi-Walled Carbon Nanotubes (MWCNTs) dispersed in kerosene, as dielectric media. Effects of variation of peak discharge current along with concentration of carbon nanotubes in the dielectric fluid are studied in purview of machining performance indicators including material removal efficiency, tool wear rate, and surface integrity of the machined part. The obtained results are compared to that of conventional EDM which utilizes kerosene as dielectric media. Morphology and topography, these two aspects of machined surface integrity are deliberated. The following surface morphological features: uneven fusion structure, globules of debris, molten metal deposition, surface cracks, pockmarks, and recast layer are identified. Topographical study includes surface roughness, severity of surface cracking, recast layer thickness, transfer of foreign elements, surface metallurgical characteristics, residual stress, and micro-indentation hardness. It is observed that application of MWCNT mixed dielectric media substantially improves EDM performance of Inconel 718 over conventional EDM. This is due to excellent thermo-physical properties of carbon nanotubes.

Cooling of a lithium ion battery using phase change material with air/dielectric fluid media: A numerical study:

Abstract: This paper proposes a method of cooling lithium ion (Li-ion) batteries using a phase change material RT35 in combination with air or a dielectric fluid media (STO 50). Three-dimensional numerical i...

Design of a High Sensitivity Microwave Sensor for Liquid Dielectric Constant Measurement.

Abstract: In order to improve the sensitivity of liquid dielectric constant measurements, a liquid dielectric constant sensor based on a cubic container structure is proposed for the first time. The cubic container, which consists of a dielectric substrate with a split resonant ring (SRR) and microstrip lines, can enhance the electric field intensity in the measuring area. High sensitivity can be obtained from measuring the dielectric constant with the characteristics of the structure resonate. The research results show that the resonant frequency of the sensor is shifted from 7.69 GHz to 5.70 GHz, with about a 2 GHz frequency offset, when the dielectric constant of the sample varied from 1 to 10. A resonance frequency offset of 200 MHz for the per unit dielectric constant is achieved, which is excellent regarding performance. The permittivity of oil with a different metal content is measured by using the relation between the fitted permittivity and the resonant frequency. The relative error is less than 1.5% and the sensitivity of measuring is up to 3.45%.

Compact organic liquid dielectric resonator antenna for air pressure sensing using soft material.

Abstract: For the first time, a flexible and deformable liquid dielectric resonator antenna (LDRA) is proposed for air pressure sensing. The proposed LDRA can be made very compact as it has employed liquidized organic dielectric with high dielectric constant (~ 33) with low loss tangent (~ 0.05). Here, a soft elastomer container has been fabricated using soft-lithography method for holding the liquid, and an air cavity is tactfully embedded into the central part of a cylindrical DRA to form an annular structure that can be used for sensing air pressure. It will be shown that the inclusion of the air cavity is essential for making the antenna structure sensitive to pressure changes. Simulations and experiments have been conducted to verify the functionalities of the proposed organic LDRA as microwave radiator and as air pressure sensor. It has been proven to have higher antenna gain than the water LDRA in the frequency range of 1.8–2.8 GHz, while achieving a good air pressure sensitivity of 270 MHz/bar.

Experimental investigation of saturation boiling of HFE-7000 dielectric liquid on rough copper surfaces

Abstract: Experiments are performed to investigate saturation boiling of the Novec HFE-7000 dielectric liquid on uniformly heated, 10 × 10 mm rough Cu surfaces in the horizontal upward facing orientation. The six surfaces employed have average roughness, Ra = 0.039, 0.134, 0.21, 0.28, 0.33 and 0.58 μm. The critical heat flux (CHF) and nucleate boiling heat transfer coefficient, hNB, and the corresponding surface superheats (ΔTsat), and the surface temperature excursion prior to boiling incipience are correlated in terms of Ra. Recorded still photos and video images of boiling in the different regions helped interpretation of experimental data and observations. For the HFE-7000 liquid, CHF increased ~39%, from 21.6 to 30 W/cm2, and the maximum nucleate boiling heat transfer coefficient (hMNB) increased ~2.32 times, from 0.82 to 1.9 W/cm2 K, and the corresponding surface superheats, ΔTexc, decreased ~38% and 41%, with increased Ra from 0.039 to 0.58 μm, respectively. The value of ΔTexc decreased from ~36.4 K to ~10 K with increased Ra from 0.039 to 0.58 μm. To quantify the effect of liquid properties, the values and correlations of CHF, hMNB, and hNB,sat are compared to those of PF-5060 dielectric liquid, which has different properties and ~64% higher boiling point (Table 1). The values of hNB,sat, hMNB, and CHF are ~11%, ~16–28%, and ~41% higher than for the PF-5060 dielectric liquid, respectively. The heat flux for liquid natural convection prior to boiling incipience is practically independent of Ra, but ~10.5% higher than for PF-5060 liquid. The present correlation of hNB,sat is in general agreement with widely referenced correlations by others.

Electrical Discharge Machining Non-Conductive Ceramics: Combination of Materials

Abstract: One of the promising processing methods for non-conductive structural and functional ceramics based on ZrO2, Al2O3, and Si3N4 systems is electrical discharge machining with the assistance of an auxiliary electrode that can be presented in the form of conductive films with a thickness up to 4–10 µm or nanoparticles - granules, tubes, platelets, multidimensional particles added in the working zone as a free poured powder the proper concentration of which can be provided by ultrasound emission or by dielectric flows or as conductive additives in the structure of nanocomposites. However, the described experimental approaches did not reach the production market and industry. It is related mostly to the chaotic development of the knowledge and non-systematized data in the field when researchers often cannot ground their choice of the material for auxiliary electrodes, assisting powders, or nano additives or they cannot explain the nature of processes that were observed in the working tank during experiments when their results are not correlated to the measured specific electrical conductivity of the electrodes, particles, ceramic workpieces or nanocomposites but depends on something else. The proposed review includes data on the main electrophysical and chemical properties of the components in the presence of heat when the temperature in the interelectrode gap reaches 10,000 °C, and the systematization of data on ceramic pressing methods, including spark plasma sintering, the chemical reactions that occur in the interelectrode gap during sublimation of primary (brass and copper) and auxiliary electrodes made of transition metals Ti, Cr, Co, and carbon, auxiliary electrodes made of metals with low melting point Zn, Ag, Au, Al, assisting powder of oxide ceramics TiO2, CeO2, SnO2, ITO, conductive additives Cu, W, TiC, WC, and components of Al2O3 and Zr2O workpieces in interaction with the dielectric fluid - water and oil/kerosene medium.

High On-Off Ratio Graphene Switch via Electrical Double Layer Gating

Abstract: This paper discusses the production and investigation of novel graphene switches by gating through an electrical double layer (EDL). Controlled voltage biases across a liquid dielectric and graphene induce electrochemical reactions within the dielectric and produce high electric fields in an EDL at the surface of graphene. As the electrochemical reactions occur within the dielectric, the EDL strength separates the electrochemically-produced ions based on their polarity, and provides the necessary molecular activation and deactivation energies to form weak, reversible molecular bonds between the produced ions and graphene. The reversible bonds between the ions and graphene are used to dynamically alter the electronic transport through graphene, which introduces an exciting assortment of device possibilities. Whereas traditional graphene devices are unuseful for electronic switches or digital logic due to an insufficient bandgap of graphene, the presented graphene electrochemical field-effect transistors (GEC-FETs) exhibit ON-OFF ratios larger than 104 with OFF-resistances as high as 10 $\text{M}\Omega $ . Channel current, gate voltage, and dielectric medium are varied and compared to show their effect on device performance. The presented device and associated techniques show potential for integration in graphene digital-logic architectures.

Powder mixed electric discharge machining – A review

Abstract: In current world scenario, there has been a steep rise in advancement of technology for progress of mankind, which has induced an urgent need to develop and adopt latest efficient methods for machining and manufacturing. Spark machining or Electro-Discharge-Machining (EDM) is known as the recent and upcoming advanced process for machining kinds of material in which electrical current can be conductive. In this technique, the machining operation is carried out due to the heat energy generated by the electric arc which is induced between the electrodes, generally tool termed as cathode and work material as anode. The machining zone is submerged in the circulating pool of dielectric fluid contained in machining tank. Powder-mixed EDM (PMEDM) is a variant of EDM process which involves suitable particles in form of powder, doped in dielectric fluid while machining. The use of powder added dielectric has resulted in enhanced machining of materials, including superalloys and composites. The process has been remarked to have improved the finish as well as the state of the machined surface and also decrease the solidified recast layer and white layer thickness. The modified state of the surface characterizations like increased micro-hardness and toughness, high corrosion and wear resistance were also observed. However, it has also been reported that very high concentration and size of powder had adverse effect on MRR and surface roughness (Ra). Hydrocarbon oils are generally used as dielectric in EDM, but led to pollution due to production of harmful hydrocarbon gases. Hence, bio dielectric like vegetable ester oils is effectively used as an alternate option to hydrocarbon oils. Also, rotating the tool was found to increase the machining rate and influence in lowering the surface roughness. MRR and surface characteristics can also be improved with use of ultrasonic assisted EDM where ultrasonic vibration is induced either to the tool or the workpiece or the machining tank. Further study, investigation and optimization of parameters is required on powder characteristics to render PMEDM commercially feasible.

Performance Analysis of Vegetable Oil as Dielectric Fluid in Electric Discharge Machining Process of Inconel 800

Abstract: Electric discharge machining (EDM) is a spark erosion process widely used to machine difficult-to-cut material by conventional machining method. The major elements of EDM process are work piece, electrode and dielectric fluid. In this work, an attempt is made to investigate vegetable oil as dielectric fluid and their process performance for machining of Inconel 800 in EDM process. The effects of dielectric fluids are investigated with vegetable oils and conventional dielectric fluid namely Sunflower oil and Kerosene respectively. The important input variables considered in this study are pulse on/off time, current and voltage. Three levels of energy setting are employed for machining with selected dielectric fluids. The output parameters are considered tool wear rate (TWR), material removal rate (MRR) and surface roughness (SR). In the present study effect of vegetable oil as dielectric fluid and the results are compared with conventional dielectric fluid. The result revealed that vegetable oils are successfully employed as dielectric fluids and they are having similar dielectric properties and erosion mechanism compared to conventional dielectric fluid. This proposed vegetable oil based dielectric fluids showed higher MRR than conventional dielectric. It shows vegetable oils have similar dielectric properties compared with conventional dielectric and it is possible to replace as dielectric fluid in EDM process.

Experimental Models of the Variation of HFE-7100 and HFE-7000 Electric Properties With Temperature

Abstract: HFE-7000 and HFE-7100 have shown a promising importance in the electrohydrodynamic (EHD) domain. Their dielectric properties are investigated in a temperature range between −20 and 60 °C for relative permittivity and electric conductivity and between −20 and 80 °C for breakdown voltage. The aim is to electrically characterize these liquids and to develop experimental models to be used in numerical simulations. Mathematical models of electric properties are essential for understanding the physical phenomena that can affect the performance of EHD systems. The development of these models can then help design more reliable EHD devices and optimize their performance.

Effect of bubble flow on partial discharge under non-uniform AC electric fields in FC-72

Abstract: The focus of this study is on thermal bubbles which limits the application of phase-change cooling technology in high electric fields. In order to investigate the impact of the boil-induced bubble flow on the partial discharge (PD), a PD test chamber is set up which is modified to adjust the intensity of the bubble flow. PD signals and characteristics of the bubble flow are recorded under different heating conditions and electric fields. It is found that thermal bubbles have negligible impact on the PD characteristics under a non-uniform field, but the discharge frequency decreases as the boiling intensity increases. A deviation occurs in the trajectory of bubbles, which makes discharge in bubbles impossible. It is also found that the heat induced fluid affects the density of dielectric liquid and ionization coefficient of the discharge environment, which reduces PD frequency.

Stability of micro dry wire EDM: OFAT and DOE method

Abstract: Micro dry wire electrical discharge machining (μDWEDM) is an environmental-friendly machining process where gas is used as the dielectric fluid instead of liquid. In this process, certain modifications of wire electrical discharge machining (WEDM) are required during the machining operation for stable machining. In μDWEDM, the process is considered stable if the machining is continuous without any interruption due to wire breakage or wire lag. However, in the present state of the arts, stable and smooth machining process using μDWEDM remains a critical issue. Hence, the objectives of this research are to establish a stable μDWEDM process using two different experimental approaches: one-factor-at-a-time (OFAT) and design of experiment (DOE) method. The investigation was performed on a stainless steel (SS304) with a tungsten wire as the electrode using integrated multi-process machine tool, DT 110 (Mikrotools Inc., Singapore). Types of dielectric fluid, dielectric fluid pressure, polarity, threshold voltage, wire tension, wire feed rate, wire speed, gap voltage, and capacitance were the controlled parameters. The machining length of the microchannels was measured using scanning electron microscope (SEM) (JEOL JSM-5600, Japan). Analysis based on these two experimental approaches shows that stable μDWEDM process is achievable when the types of dielectric fluid, dielectric fluid pressure, polarity, threshold voltage, wire tension, wire feed rate, and wire speed remain as the fixed parameters while the capacitance and gap voltage remain as the controlled parameters.

Effect of an electric field on the stability of binary dielectric fluid mixtures.

Abstract: We consider the phase stability of binary fluid mixtures with constituents of contrasting dielectric properties in the presence of a static applied electric field, E0. The dielectric fluid is modeled using a recently developed field-theoretic representation for the equilibrium behavior of a system of polarizable molecular species [J. M. Martin et al., J. Chem. Phys. 145, 154104 (2016)]. The dielectric displacement of the fluid, D, is obtained from a direct E0 derivative of the fluid’s free energy, illuminating coupled structural and electrostatic fluctuations that manifest in the dielectric properties of the fluid. Linearizing D with respect to E0 yields an explicit, molecularly based expression for the dielectric constant of the fluid mixture, ϵ, through the relation D = ϵE0. In the linear response regime, the composition dependence of ϵ completely specifies the applied field-dependent contribution to the fluid’s miscibility, which we enumerate as a contribution χE to a Flory interaction parameter. Using a Gaussian approximation to the field theory, we obtain an expression for χE that relates structural and electrostatic contrast between dissimilar molecules to miscibility in the presence of an applied field. Specifically, contrast between wavevector-dependent, single-molecule correlation functions, Λ^A/B(k), emerges as a necessary ingredient for electric field-induced mixing, corresponding to χE < 0. The character of χE is considered in three classes of binary systems: a binary simple fluid, a homopolymer blend, and a homopolymer solution. Within each system, the form for Λ^A/B accounts for molecular architecture effects, such as chain connectivity. Our findings elucidate the conditions for which one should expect electric field induced mixing or demixing for each class of mixture.

Powder additives influence on dielectric strength of EDM fluid and material removal

Abstract: Electrical discharge machining (EDM) is used to machine difficult-to-machine materials having high hardness and toughness. One of the recent advancements in the EDM process is the powder mixed electrical discharge machining (PMEDM) process, in which the metallic or abrasive additives in the form of fine powders are added to the dielectric fluid. PMEDM was found to improve machinability in terms of higher material removal rate (MRR) and enhanced tool wear index (TWI), by reducing the breakdown strength of the dielectric. In PMEDM process, machining happens with relatively larger spark gap with enhanced machining characteristics. This paper investigates the influence of powder mixing on the breakdown strength of the liquid dielectric and the gap voltage. Determination of dielectric strength was carried out with a specially designed experimental set-up adhering to ASTM standard D1816 - 97. Tests were conducted using silicon carbide, alumina, copper and aluminium powders. The effect of varying the grain size was also studied. An experimental set-up was also designed and realised to measure the influence of powder mixing on the gap voltage and MRR in machining of titanium alloy. The results have shown significant improvement in MRR and TWI.

Modelling of material removal rate in the magnetic field and air-assisted electrical discharge machining:

Abstract: In the present research work, an attempt has been made to develop the mathematical model to predict the material removal rate in the electrical discharge machining process when the assistance of ai...

CFD Simulation of Two-Phase Immersion Cooling Using FC-72 Dielectric Fluid

Abstract: With more development in electronics system capable of having larger functional densities, power density is increasing. Immersion cooling demonstrates the highest power usage efficiency (PUE) among all cooling techniques for data centers and there is still interest in optimizing immersion cooling to use it to its full potential. The aim of this paper is to present the effect of inclination and thermal shadowing on two-phase immersion cooling using FC-72. For simulation of boiling, the RPI (Rensselaer Polytechnic Institute) wall boiling model has been used. Also, two empirical models were used for calculation of bubble departure diameter and nucleate site density. The boundary condition was assumed to be constant heat flux and the bath temperature was kept at boiling temperature of FC-72 and the container pressure is assumed to be atmospheric. this study showed that due to the thermal shadowing, boiling boundary layer can lay over the top chipset and increases vapor volume fraction over top chipsets. This ultimately causes increase in maximum temperature of second chip. The other main observation is with higher inclination angle of chip, maximum temperature on the chip decreases up to 3°C.

Theoretical analysis of the transition from field emission to space-charge-limited emission in liquids and gases

Abstract: Discharge formation and breakdown in liquids have critical implications for water purification, medicine, and combustion, thereby motivating characterization of liquid behavior under high voltages. While several experiments with dielectric liquids have demonstrated that current transitions from Fowler–Nordheim (FN) scaling for field emission to Mott–Gurney (MG) scaling for space-charge-limited emission (SCLE) with increasing voltage, there is no common model linking electron emission from the liquid to gas phase. We show that applying a theory unifying FN, MG, and the Child–Langmuir law (CL) for SCLE at vacuum as a function of electron mobility and gap distance [Darr et al., Appl. Phys. Lett. 114, 014103 (2019)] yields excellent agreement with dielectric liquid emission experiments [Dotoku et al., J. Chem. Phys. 69, 1121 (1978)]. Specifically, current follows FN scaling at lower voltages before space charge contributions dominate, although none of the dielectric liquids considered achieve MG scaling in the experimental regime of interest. Considering a higher mobility representative of a vapor in the theory demonstrates the feasibility of achieving CL scaling for the gaps of the size considered experimentally at reasonably achievable applied voltages. Increasing the gap distance by an order of magnitude eliminates the contribution of space charge; decreasing gap distance by an order of magnitude causes a transition to MG. The implications of these results on electron emission in liquids and during a phase change to vapor and gas will be discussed.

Capillary-gravity waves on the interface of two dielectric fluid layers under normal electric fields

Abstract: In this paper, we consider capillary-gravity waves propagating on the interface of two dielectric fluids under the influence of normal electric fields. The density of the upper fluid is assumed to be much smaller than the lower one. Linear and weakly nonlinear theories are studied. The connection to the results in other limit configurations is discussed. Fully nonlinear computations for travelling wave solutions are achieved via a boundary integral equation method. Periodic waves, solitary waves and generalised solitary waves are presented. The bifurcation of generalised solitary waves is discussed in detail.