UDC 621.762
MANUFACTURE OF THE CARBON-CARBON ELECTRODES FOR THE CDI UNITS D.V. Kudin, I.V. Gurin, G.V. Taran, V.I. Golota, A.N. Bukolov, L.M. Zavada National Science Center “Kharkov Institute of Physics and Technology”, Kharkov, Ukraine E-mail:
[email protected]; tel./fax +38(057)349-10-81 A new method for manufacturing the carbon-carbon electrodes with the frame from the thermally-expanded graphite and titanium electrical lamellas is developed. It is shown that the manufactured electrodes have high mechanical and electrical properties. It is experimentally proved that the interlayer interface resistance through the “titanium contact-carbon frame-carbon cloth” system was 0.2…0.3 Ω, which is approximately the resistance of the carbon material.
INTRODUCTION The nuclear power industry, in comparison with the heat power industry, has always been considered to be an ecologically safe power source having minimal impact on the environment. However, in the process of operation of the nuclear-industrial complex, a significant amount of radioactive waste requiring expensive technology for recycling or disposal is produced. A large proportion of this waste is low-active liquid effluents formed as a result of production and disposal of nuclear fuel as well as during the operation of the nuclear-industrial complex as a result of water contamination by a small amount of radioactive isotopes. Traditional methods for deactivation of such contamination are expensive, inefficient and result in the accumulation of solid radioactive waste. For example, when the ion exchange technology is used, the significant amount of used ion exchange resins containing radioactive ions is formed. For evaporation of the low-level liquid wastes, a considerable amount of energy is required. An alternative for evaporation is the ion-exchange technologies. The proposed capacitive deionization method for the treatment of the low-level (> 105 Bq/l) liquid radioactive wastes using mesoporous carbon materials is referred to such technologies. Taking into consideration, that low-level liquid wastes are mainly aqueous solutions with low content of radioactive ions, the CDI method is the most attractive for the treatment of low-level (> 105 Bq/l) liquid wastes. The CDI method is based on the adsorption/desorption of ions dissolved in water on the high surface area of mesoporous carbon materials under the applied electric field. Due to the manfacture of carbon cloth materials with the high surface area (800…2000 m2/g) in the middle of 90s, a lot of experiments on capacitive deionization of water are carried out in many leading scientific countries (USA, Netherlands, South Africa, Australia, China, Italy, etc.). The perspectiveness of this technology can be explained by the fact that no chemical reagents are used in the process of decontamination and there is no water decomposition in comparison with other technologies. The first systems using the CDI principle for salt water desalination and ion removal from solutions have already been created [14].
The carbon graphite materials are widely used as the electrodes, especially in aggressive environment. Carbon cloth, fabric and felt are also widely used, especially those which are exposed to the activation processes which result in a significant increase of the carbon material specific area. The effectiveness of using nanostructures (fullerenes, nanotubes and nanofibers), graphite powders, including activated powders, as well as exfoliated graphite is being studied [57]. At the same time, the issue related to the creation of a reliable electrical contact with the carbon electrode inevitably occurs. Among the main methods of connecting carbon electrodes, mechanical contacts are more popular. The methods of contact soldering through the previously applied layer of copper (using electrolytic deposition method) and gluing by conductive adhesive are also widely used. The methods of direct soldering and welding are less popular. The most common issues which occur at the electrical connection with carbon are the issuesrelated to low chemical and/or temperature resistance of metals, mechanical strength, high electrical transition resistance, corrosion resistance. The goal of the work was the development of the methods for manufacturing the electrodes with reliable and efficient electrical contacts based on carbon materials.
THE METHOD FOR MANUFACTURING THE CARBON-CARBON MATERIALS To solve the abovementioned issues, the experiments on the development of the combined methods for the manufacture of carbon-carbon materials with simultaneous formation of electric contact groups were carried out. The method is based on the manufacture of the electrode using the activated carbon cloth with the formation of carbon-carbon frame around its perimeter. The mechanical strength of the electrode, reliable and uniform electrical contact on the perimeter and corrosion resistance for salt water solutions should be provided by the frame. However, when creating such a frame, physical and chemical properties of the activated carbon cloth on the working surface of the electrode should not degrade. To form the frame, the ultrafast method for obtaining the carbon-carbon composite material in methane using resistive heating of the working area by the direct transmission of the electric current was used. ISSN 1562-6016. PASТ. 2016. №4(102), p. 113-117.
The electrodes were made from the activated carbon viscose cloth “Busofit”. The cloth was unrolled on the cutting table. After this, the special electrode blanks with the diameter of 100 mm were used. Then, the layer of phenolic varnish was applied to the perimeter of the electrode blanks. The lacquer was prepared on the basis of phenol-formaldehyde resin SF-11. To manufacture the electrodes, 4 layers of the cloth were used. To ensure the reliable electrical contact and mechanical strength, as well as to separate the electrodes from the tool set after the process, the two rings made of the thermally-expanded graphite foil TMG-L/B2-1x1000x1000 with the thickness of 1 mm (TEG) were used. The rings were cut so that their inner and outer diameter corresponded to the inner and outer diameter of the electrode frame, 100 and 120 mm respectively. To sinter the electrodes, the special tool set was developed and manufactured (Fig. 1).
The process of sintering was performed in pyrolysis furnace AGAT-1.6. The set number of layers for the prepared carbon cloth with frame rings made of TEG and lamellas were mounted between the two graphite bowls and bound between the electrodes of the furnace as shown in Fig. 2.
Fig. 2. The chamber of the furnace AGAT-1.6 with the mounted tool set Fig. 1. The view of the tool set for electrode sintering As shown in Fig. 1, the tool set for sintering consists of two graphite bowls with cylindrical walls. Between these bowls the blank electrodes were bound. The first tool set was manufactured from the industrial graphite GE-0 and MPG-7. The thickness of the tool set walls was 10 mm. However, while using this tool set for electrode sintering, it was found that the power of the existing furnaces (40 V, 6000 A) was not enough for warming up the electrode blanks. Because of this, the tool sets were manufactured from the graphite of our own production (pyrocarbon-bound graphite), the resistivity of which at the room temperature is several times higher than that of the industrial graphite and is 8…20 mΩ∙m. The side walls were made as a T-shaped profile. This approach allows significant reduction of the current and power consumption for the furnace at keeping the set temperature of sintering. In the process of the electrode manufacture, the metal (titanium) lamellas were used. The width of the lamellas was 10 mm and the thickness was 0.5…1 mm. To provide a good contact throughout the area of the frames where such lamellas were used, one of the graphite bowls was made with the corresponding grooves (see Fig. 1, on the top, on the left). The depth of the grooves corresponded to the thickness of lamellas and was 0.5…1 mm.
The assembly was installed so that the process of sintering could be observed through the upper viewing window. After installing the assembly, the chamber was sealed and filled with natural gas. The voltage was applied to the electrodes. The voltage was gradually increased until the arc burning in the gap occurred. The arc burning could be visually observed – the glowing dot appeared in the place where the cloth was bound. The temperature of the dot glowing was at the level of ~ 1200…1500 °C. After the arc burning occurred, the heating in the place of contact was exponentially propagated around the whole diameter, due to which the light ring appeared. After the ring was closed, the heating was switched off. Such a behavior of the assembly can be explained by the following factors: Impregnation by a phenolic resin significantly increases the electrical resistance of the carbon cloth in the place of contact. However, when heated, the phenolic resin carbonizes and locally graphitizes producing the electrically conducting glass-carbon layers. Taking into account that the carbonization process is exothermal [8] it results in progressive process realization. Carbonization of phenol-formaldehyde resin is additionally improved as a result of pyrolytic graphite deposition from natural gas (methane) due its thermal destruction (pyrolysis). Thus, at the temperature of 1200…1500 C0, that was observed on the surface of the
layers during the process, graphite deposition occurs at a high rate. The deposition of pyrolytic graphite results in the formation of the pyrolytic graphite matrix, which in its turn improves the electrical conductivity of the material. Both described processes provide sintering of carbon fabric layers with each other and with TEG due to the formation of carbon-carbon composite in the sintering area. This composite provides not only a good conductivity and reliable electrical contact around the whole perimeter of the electrode, but also has sufficient mechanical strength. The titanium lamellas located between the layers of the carbon cloth were also heated to the mentioned temperature. As a result, the titanium was melted and welded to the graphite cloth. Also, the carbides were formed. Due to this, the strength of the connection was additionally increased and better electrical contact was provided. The typical regimes for sintering the frames are presented in Table. Time and power parameters for the process of frame sintering using the furnace AGAT-1.6 Samp- Time since le the process No start, s 40 80 1 131 162 15 2 60 85 15 3 60 85
Voltage, V 15 20 25 27 15 22 23 15 23 23
Current, Power А consumption, kW 670 10 1650 33 2800 70 3350 90.5 2000 30 3000 66 4400 101.2 2000 30 4000 92 4400 101.2
As can be seen from the table, the time of frame sintering is a few minutes. The blank electrode is heated only in the area of frame formation. The working surface of the electrode (carbon cloth) is protected and is not heated. This approach preserves the necessary properties of the activated carbon cloth on the working surface of the electrode. The formed electrode with TEG frame and titanium lamellas is shown in Fig. 3.
TESTING The electrical characteristics of the formed electrodes were identified. The resistance of the external frame was 0.1…0.2 Ω. the transitional contact resistance of the titanium lamellas was 0.2…0.3 Ω, the transitional contact resistance of the “frame-carbon fabric” was 0.3…0.6 Ω, that is significantly (by several times) lower than in case of mechanical contact and is at the level of electrical resistance of the carbon fabric layer. To determine the effectiveness of the proposed method for the contact formation, the characteristics that determine the energy consumption for the various types of CDI modules were compared. For such a comparison, the four types of CDI modules were used. The first module used the electrodes manufactured by the method described above using the carbon material “Busofit”. For the second module, the welded titanium frame and the carbon material SAUT-1S (NSC KIPT, Ukraine) were used [24]. For the third module, the carbon aerogel glued to the titanium plates by the conductive adhesive under the technology of Livermore National Laboratory, USA (LLNL, USA) [9] was used. The fourth module was manufactured using the carbon felt, which was glued to the titanium plate by the conductive adhesive under the technology of the Oak Ridge National Laboratory, USA (ORNL, USA)). In the first two modules, the solution is pumped perpendicular to the surface of the electrodes. In the last two modules it is pumped along the electrode surface. A significant portion of power used in the process of capacitive deionization of water is consumed due to the Joule heating of water and the electrodes during the flow of electric current. The Joule heating of the electrodes is performed due to the carbon material resistance, current collector resistance and contact resistance between the current collector and the carbon material. The integral characteristic determining the amount of power consumption is the serial internal resistance which appears on the waveform for the DC charging of the CDI module as a “step” voltage at the start of the CDI module charging [14]. The dependence of the “step voltage” amplitude [24] on the charging current density when charging the abovementioned CDI units with DC is shown in Fig. 4.
Fig. 4. The dependence of the voltage (power consumption) on the current density for the CDI units Fig. 3. The view of the electrode after frame formation
As can be seen from Fig. 4, the power consumption at the use of airgel that is glued to the titanium plates by a conductive adhesive is the highest. It is related both to the high specific resistance of the carbon airgel and to the high contact resistance. The results for the various designs in which the carbon material is glued to the titanium plates (ORNL) and the designs developed by the NSC KIPT based on the titanium frame and the carbon material SAUT-1C are similar. However, it is necessary to take into account that the design developed by ORNL has a much larger contact area between the carbon material and the titanium plates of the current collectors. As in the design using the titanium frame and the material SAUT-1S, the contact between the titanium frame and the cloth is created only along the perimeter of the electrode, the electrodes become permeable for pumping the solution, unlike ORNL design, in which the solution is pumped along the electrode surface. Fig. 4 shows that the power consumption for the CDI module based on the carbon material “Busofit” with the carbon frame manufactured by the NSC KIPT method is the lowest (almost twice lower in comparison with the module based on the carbon cloth SAUT-1S with the titanium frame). At the same time, the angle of the curve is decreased. This additionally demonstrates the effectiveness of the design.
CONCLUSIONS The effectiveness of the method for manufacturing the electrodes for capacitive deionization of water using the carbon frame (frame) by carbonization and graphitization of the fillers under methane environment was proposed and experimentally proved. It is shown that the power consumption of the Joule heating for the CDI module using the carbon material Busofit with the carbon frame is much lower than that of the alternative variants.
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Article received 06.04.2016
УГЛЕРОД-УГЛЕРОДНЫЕ ЭЛЕКТРОДЫ ДЛЯ СИСТЕМ ЕМКОСТНОЙ ДЕИОНИЗАЦИИ ВОДЫ Д.В. Кудин, И.В. Гурин, Г.В. Таран, В.И. Голота, А.Н. Буколов, Л.М. Завада Разработана методика изготовления углерод-углеродных электродов с фреймом на основе термически расширенного графита и титановыми контактами. Показано, что изготовленные электроды имеют высокие электрические и механические свойства. Экспериментально установлено, что переходное контактное сопротивление в системе «титановый контакткарбоновый фреймуглеродная ткань» составляет 0,2…0,3 Ом, что примерно равно удельному сопротивлению углеродной ткани.
ВУГЛЕЦЬ-ВУГЛЕЦЕВІ ЕЛЕКТРОДИ ДЛЯ СИСТЕМ ЄМНІСНОЇ ДЕІОНІЗАЦІЇ ВОДИ Д.В. Кудін, І.В. Гурін, Г.В. Таран, В.І. Голота, О.М. Буколов, Л.М. Завада Розроблено методику виготовлення вуглець-вуглецевих електродів із фреймом на основі термічно розширеного графіту та титановими контактами. Показано, що виготовлені електроди мають високі електричні та механічні властивості. Експериментально встановлено, що перехідний контактний опір у системі «титановий контакткарбоновий фреймвуглецева тканина» становить 0,2…0,3 Ом, що приблизно дорівнює питомому опору вуглецевої тканини.
