Rapid Discharge Sintering Research Articles

Surface properties of nanostructured NiO undergoing electrochemical oxidation in 3-methoxy-propionitrile

Nanostructured nickel oxide (NiO) was deposited in the configuration of thin film (thickness, l=2–6μm) onto fluorine-doped tin oxide (FTO) substrates via plasma-assisted rapid discharge sintering (RDS). Electrochemical cycling of RDS NiO in 3-methoxy-propionitrile (3-MPN) revealed two characteristic peaks of NiO oxidation which were associated to the surface-confined redox processes Ni(II)→Ni(III) and Ni(III)→Ni(IV). Grazing angle X-ray photoelectron spectroscopy (XPS) was conducted ex-situ on NiO electrodes in both pristine and oxidized states. Oxidized NiO samples for XPS experiments were obtained in the potentiostatic mode through the polarization of NiO at its two characteristic potentials of oxidation. The XPS analysis allowed to ascertain the electronic structure of the nanoporous NiO framework, and verify the adsorption of perchlorate and chloride anions onto NiO surface due to the compensation of the charge stored in oxidized NiO.XPS also revealed that the spectrum within the region characteristic of Ni 2p ionization does not vary considerably with the state of charge of the nickel centres. This finding is in evident contrast to what has been observed for the same system when it undergoes electrochemical oxidation in aqueous electrolyte.

Cobalt Sulfide as Counter Electrode in p-Type Dye-Sensitized Solar Cells

We proposed a novel application of cobalt sulfide (CoS) in the configuration of transparent thin film as anode in p-type dye-sensitized solar cell (p-DSC). The anodes here considered have been prepared using a water-based method that is suitable for the large scale production of large-area electrodes. The photoactive cathodes of the p-DSC were mesoporous nickel oxide (NiO) thin films deposited via rapid discharge sintering. The NiO electrodes were sensitized with the benchmark dye erythrosine B (ERY), while the couple I−/I3− was the redox mediator. The CoS anodes showed higher electrocatalytic efficiency in comparison with the commonly used platinized Fluorine-doped Tin Oxide (Pt-FTO). This was determined by means of electrochemical impedance spectroscopy of CoS based dummy cells, with CoS showing a lower charge-transfer resistance with respect to Pt-FTO. The overall conversion efficiency of the p-DSC employing ERY-sensitized NiO as photoactive cathode and CoS anode was 0.026 %, a value very close to that obtained with Pt-FTO anodes (0.030 %). The external quantum efficiency spectra of the p-DSCs with CoS anodes were similar to those obtained with Pt-FTO anodes under illumination with AM 1.5 solar simulator.

Electrochemical Characterization of Rapid Discharge Sintering (RDS) NiO Cathodes for Dye-Sensitized Solar Cells of <i>p</i>-Type

Nickel oxide (NiO) thin films with thickness ranging in the interval 0.2 - 3.5 μm have been deposited onto conductive transparent substrate via the method of plasma-assisted rapid discharge sintering (RDS) with microwave heating starting from NiO nanoparticles with diameter 50 nm. The optical and electrochemical properties of the RDS NiO films in the pristine state were characterized in non aqueous electrolyte with the solvent 3-methoxy-propionitrile (3-MPN). Upon electrochemical cycling of NiO in 3-MPN we observed two characteristic oxidation peaks referring to the two nickel centred processes Ni(II)→Ni(III) and Ni(III)→Ni(IV), which are both localized prevalently on the surface of the metallic oxide. The oxide films prepared with the RDS method were also sensitized with different types of commercial dyes, either organometallic (N719, black dye) or organic (squaraine 2, erythrosine B), to compare the corresponding p-type dye-sensitized solar cells (p-DSCs). All dyes here employed matched the energies of their frontier orbitals with the upper edge of NiO valence band and the redox level of the triiodide/iodide couple. The comparison of the performances of the p-DSCs based on RDS NiO which differed exclusively for the nature of the sensitizer showed that the extent of electronic conjugation in the structure of the dye is crucial for the control of the photovoltaic performance of the corresponding p-DSC.

Comparison of the photoelectrochemical properties of RDS NiO thin films for p-type DSCs with different organic and organometallic dye-sensitizers and evidence of a direct correlation between cell efficiency and charge recombination

Undyed mesoporous NiO in the configuration of thin film (thickness 2–3 μm) presents photoelectrochemical activity as cathode of a p-type dye-sensitized solar cell (p-DSC) towards the reduction of triiodide to iodide under irradiation with a solar simulator. The photoelectroactivity of the oxide prepared via microwave plasma sintering (or rapid discharge sintering, RDS) has been observed in the spectral range 300–500 nm with the incident photon-to-current conversion efficiency (IPCE) reaching a maximum of 8.7 % at 375 nm. Upon sensitization, the characteristic photoelectrochemical activity of NiO can be either enhanced or depressed depending on the nature of the dye-sensitizer. The comparative analysis of the JV and IPCE curves of the p-DSCs based on bare NiO and four differently sensitized NiO cathodes reveals that N719, black dye (BD), and commercial squaraine 2 (SQ2) decrease the efficiency of conversion of dyed NiO with respect to bare NiO in the range of photoelectroactivity of the latter (300–500 nm). The fourth dye P1 represents the sole exception since its employment brings about an enhancement of the quantum efficiency of P1-sensitized vs. unsensitized NiO up to a maximum of 21 % within the spectral interval of reference for NiO (300–500 nm). Outside the range of NiO photoelectrochemical activity, i.e., λ > 500 nm, only N719 does not introduce a gain of quantum efficiency with respect to bare NiO despite the observation of spectral sensitization up to 580 nm for N719-sensitized NiO. The impedance spectra recorded under illumination shows a direct proportionality between the overall efficiency (η) of the variously sensitized p-DSCs and the amplitude of the semicircle which is generally associated with the process of charge recombination at the electrode/electrolyte interface with η decreasing with the increase of the recombination resistance.

Fabrication of Efficient NiO Photocathodes Prepared via RDS with Novel Routes of Substrate Processing for p‐Type Dye‐Sensitized Solar Cells

Abstractp‐type dye sensitized solar cells (p‐DSCs) derived from nickel oxide (NiO) photocathodes have been obtained via rapid discharge sintering (RDS) of parent metal oxide nanoparticles deposited onto differently treated substrates utilizing a plasma atmosphere with microwave radiation as heat source. This method produces NiO thin films (0.6<l<6 μm) with mesoporous features and large surface areas as required for efficient dye‐loading and high photocurrents. Erythrosine B (ERY) was used to sensitize the oxide in the visible spectrum. We have analyzed and compared the photoelectrochemical performances of the p‐DSCs assembled with the various types of NiO samples prepared by RDS techniques with different treatments of the supporting substrate prior to, or during, spray deposition of the NiO nanoparticles. The best photovoltaic performances were obtained when the transparent conducting substrate (TCS) was heated during spraying. We believe that this is because the charge transfer through the NiO film and the charge collection at the TCS/NiO film interface were the most efficient with this sample. To our knowledge, the photovoltaic performances reported here are the best achieved with the commercial dye ERY as sensitizer.

Dye sensitised solar cells with nickel oxide photocathodes prepared via scalable microwave sintering

Photoactive NiO electrodes for cathodic dye-sensitised solar cells (p-DSCs) have been prepared with thicknesses ranging between 0.4 and 3.0 μm by spray-depositing pre-formed NiO nanoparticles on fluorine-doped tin oxide (FTO) coated glass substrates. The larger thicknesses were obtained in sequential sintering steps using a conventional furnace (CS) and a newly developed rapid discharge sintering (RDS) method. The latter procedure is employed for the first time for the preparation of p-DSCs. In particular, RDS represents a scalable procedure that is based on microwave-assisted plasma formation that allows the production in series of mesoporous NiO electrodes with large surface areas for p-type cell photocathodes. RDS possesses the unique feature of transmitting heat from the bulk of the system towards its outer interfaces with controlled confinement of the heating zone. The use of RDS results in a drastic reduction of processing times with respect to other deposition methods that involve heating/calcination steps with associated reduced costs in terms of energy. P1-dye sensitized NiO electrodes obtained via the RDS procedure have been tested in DSC devices and their performances have been analysed and compared with those of cathodic DSCs derived from CS-deposited samples. The largest conversion efficiencies (0.12%) and incident photon-to-current conversion efficiencies, IPCEs (50%), were obtained with sintered NiO electrodes having thicknesses of ~1.5-2.0 μm. In all the devices, the photogenerated holes in NiO live significantly longer (τ(h) ~ 1 s) than have previously been reported for P1-sensitized NiO photocathodes. In addition, P1-sensitised sintered electrodes give rise to relatively high photovoltages (up to 135 mV) when the triiodide-iodide redox couple is used.

Influence of Gas Type on the Thermal Efficiency of Microwave Plasmas for the Sintering of Metal Powders

Microwave plasmas have enormous potential as a rapid and energy efficient sintering technology. This paper evaluates the influence of both plasma atmosphere and metal powder type on the sintering temperatures achieved and the properties of the sintered powder metal compacts. The sintering is carried out using a 2.45 GHz microwave-plasma process called rapid discharge sintering (RDS). The sintering of three types of metal powder are evaluated in this study: nickel (Ni), copper (Cu) and 316L stainless steel (SS). An in-depth study of the effects of the plasma processing parameters on the sintered powder compacts are investigated. These parameters are correlated with the mechanical performance of the sintered compacts to help understand the effect of the plasma heating process. The substrate materials are sintered in four different gas discharges, namely hydrogen, nitrogen oxygen and argon. Thermocouple, pyrometer and emission spectroscopy measurements were taken to determine the substrate and the discharge temperatures. The morphology and structure were examined using scanning electron microscopy and X-ray diffraction. The density and hardness of the sintered compacts were correlated with the plasma processing conditions. As expected higher densities were obtained with powders with lower sintering temperatures i.e. nickel and copper when compared with stainless steel. Under the power input and pressure conditions used, the highest substrate temperature attained was 1,100°C for Cu powder sintered in a nitrogen atmosphere. In contrast under the same processing conditions but in an argon plasma, the temperature achieved with SS was only 500°C. The effect of the plasma gas type on the sintered powder compact chemistry was also monitored, both hydrogen and nitrogen yielded a reducing effect for the metal in contrast with the oxidising effect observed in an oxygen plasma.

Microwave-assisted rapid discharge sintering of a bioactive glass–ceramic

Bioactive glass-ceramics have been developed as successful bone graft materials. Although conventional sintering in an electrically-heated furnace is most commonly used, an alternative microwave plasma batch processing technique, known as rapid discharge sintering (RDS), is examined to crystallise the metastable base glass to form one or more ceramic phases. Apatite-mullite glass-ceramics (AMGC) were examined to elucidate the effects of RDS on the crystallization of a bioactive glass-ceramic. By increasing the fluorine content of the glass, the fluorapatite (FAp) and mullite crystallization onset temperatures can be reduced. Samples were sintered in a hydrogen and hydrogen/nitrogen discharge at temperatures of ≈800 and 1000 °C respectively with the higher sintering temperature required to form mullite. Results show that the material can be densified and crystallised using RDS in a considerably shorter time than conventional sintering due to heating and cooling rates of ≈400 °C/min.

Novel rapid discharge sintering technique for damage-free diamond metal matrix composites

The performance of diamond/nickel metal matrix composites (MMCs) produced using a novel microwave plasma technique, rapid discharge sintering (RDS), and conventional tube furnace sintering in argon is compared. The MMCs were sintered at temperatures between 850 and 1050°C in both cases. The RDS treatments were carried out at 20 mbar in plasmas containing hydrogen or hydrogen–nitrogen gases. The addition of nitrogen gas to the hydrogen plasma facilitated a substantial increase in composite firing temperatures. A significant reduction in sintering times, to 10 min from several hours, was achieved using the RDS technique. A further advantage of the RDS treatments was the absence of any diamond graphitisation (as detected by X-ray diffraction), which was reflected in higher sintered densities and flexural strength for RDS than for furnace sintering.

Rapid discharge sintering of nickel–diamond metal matrix composites

In this study rapid discharge sintering (RDS) and furnace sintering of nickel–diamond metal matrix composites (MMCs) is compared. Nickel–diamond powder composites (80–20% by weight respectively) were uniaxially pressed into 20 mm discs at compaction pressures of 100, 200 and 300 MPa. Discharge sintering was carried out using a microwave plasma formed with hydrogen and hydrogen/nitrogen as the discharge gases and tube furnace sintering carried out in a argon or a hydrogen/nitrogen (3:1) atmosphere. Discs pressed to 300 MPa were treated at both 850 and 1000 °C. The properties of the sintered nickel–diamond composites were characterized using density, approximate flexural strength, hardness, wear resistance, scanning electron microscopy (SEM) and X-ray diffraction (XRD). The RDS samples sintered at 1000 °C achieved the maximum approximate disc flexural strength of 473 MPa within a 20 min treatment time compared with 6 h for furnace sintered samples. Samples sintered using the RDS technique exhibited increased hardness values and a finer nickel matrix over furnace sintered samples. Using the RDS technique it has been possible to process nickel–diamond MMCs without oxidation or graphitisation at temperatures above 900 °C. Minimal diamond destruction was observed during abrasive wear testing of the RDS samples compared with damage and pull-out observed for furnace sintering.

Plasma power can slash small run sintering times

A group of Irish researchers have demonstrated that rapid discharge sintering can dramatically reduce sintering times for processing small numbers of green compacts. But while mainstream sintering technologies have the upper hand for the time being, commercial prospects for plasma are being assessed…