Evaluation of the acoustic performances of metal powder based materials

Transcription

1 PROCEEDINGS of the 22 nd International Congress on Acoustics Materiasl for Noise Control: Paper ICA Evaluation of the acoustic performances of metal powder based materials Federico Rossi (a), Beatrice Castellani (b), Massimo Palombo (c), Elena Morini (d), Andrea Nicolini (e) (a) University of Perugia, CIRIAF,Via G. Duranti, Perugia, ITALY, (b) University of Perugia, CIRIAF,Via G. Duranti, Perugia, ITALY, (c) Consorzio IPASS Scarl, Via G. Guerra, Perugia, ITALY, (d) University of Perugia, CIRIAF,Via G. Duranti, Perugia, ITALY, (e) University of Perugia, CIRIAF,Via G. Duranti, Perugia, ITALY, Abstract Electrodes in Molten Carbonate Fuel Cells (MCFC) are constituted by porous metal mixtures. Metals employed are nickel for the cathodic electrode and nickel-chrome for the anodic one. Plates are built from the metal powders treated by tape casting and sintering processes. Material characteristics such as porosity and density may be properly designed by varying powder particle size, organic binder composition and sintering conditions. These materials may be potentially valuable as acoustic absorbers and their acoustical properties may be adjusted by changing the material s characteristics. Generally materials for MCFC require no impurities and their cost is a critical issue; in acoustical applications, purity requirements are not necessary and scrap samples may be used, contributing strongly to cut costs. This paper deals with an investigation on a typical MCFC material to verify their suitability for noise insulation and absorption. For this purpose, a measurement campaign by Kundt tube on MCFC electrodes was carried out by varying the plate porosity, width, mixture content and purity; optimal configuration has been found in terms of maximum absorption frequency. Results suggested that, for acoustical application, some metal powder maybe substituted by a polymer powder in order to further reduce costs and further improve absorption. Manufacturing procedures are actually under study. Keywords: metal powder; acoustic performance; porous materials; acoustic absorption.

2 1 Introduction Noise pollution is a major concern which propels engineers to evolve new methods and new materials to meet comfort and regulatory requirements. When a sound wave strikes an acoustical material the sound wave causes the fibers of the absorbing material to vibrate. This vibration causes tiny amounts of heat due to the friction and thus sound absorption is accomplished by way of energy to heat conversion. The more fibrous a material is the better the absorption; conversely denser materials are less absorptive. The sound absorbing characteristics of acoustical materials vary significantly with frequency. In general low frequency sounds are very difficult to absorb because of their long wavelength. For the vast majority of conventional acoustical materials, the material thickness has the greatest impact on the material s sound absorbing qualities. While the inherent composition of the acoustical material determines the material s acoustical performance [1]. Therefore, porous sound absorption materials must keep appropriate porosity features. According to the substance, porous sound absorption materials are divided into three categories: organic polymer porous materials, porous metallic materials and inorganic porous materials. Most organic porous materials cannot endure high temperature and the porous metallic materials have a higher cost, while typical inorganic materials are fragile and hazardous to health [2]. In order to reduce costs, a new type of metallic materials are proposed, deriving from fuel cell component production. Fuel cells (FC) are electrochemical generators in which a fuel (typically hydrogen but also fuels such as methane and methanol from which hydrogen is extracted) and an oxidant (oxygen or air) to produce DC electricity, water and heat. Among FC, molten-carbonate fuel cells (MCFCs) are high-temperature fuel cells that operate at temperatures of 600 C and above. MCFC are widely investigated and are of scientific interest because high temperatures allow to obtain low internal losses with large benefits in terms of generated electric power. Several research activities have been undertaken at University of Perugia (Italy). A new geometry for small sized MCFCs have been proposed in [3]. The global efficiency and the lifetime of the small sized MCFCs have been investigated in [4-7] with experimental tests. MCFCs are resulted also suitable for ethanol reforming [8]. The technology has been also proposed as a methodology for separation and capture of CO 2 in [9-10]. The proposed configuration is composed of a nickelchrome porous anode and a nickel oxide porous cathode. In general, materials for MCFCs require very low impurity and air bubbles content which make their cost a critical issue. Nickel-chrome samples have been investigated as potentially valuable materials for noise absorption. Their acoustic properties may be tuned by changing the porosity fitting the peculiar application. Since in acoustical application the above mentioned requirements are not so important, refuse samples may be used contributing strongly to cut costs. In this paper the acoustic properties, i.e. acoustic insulation and transmission loss, of single layer and multilayer sintered and not-sintered panels obtained by metal mixtures of nickel and nickel-chrome have been measured. 2

3 2 Material characteristics Materials that are employed as electrodes on MCFCs are soft ribbon like plates constituted by a porous metal mixture. Metals employed in the mixture are nickel for cathode and nickel-chrome for anode. MCFCs plates are made from the metal powders which are treated by tape casting and sintering processes. Figure 1 shows the tape casting machine that has been used to make the materials to test in the present paper. It is available in the facilities of University of Perugia at Pentima Bassa (Terni), Italy. Figure 1. Tape casting machine. In the tape casting process the metal powders are held together by an organic binder (slurry); the metal slurry is spread continuously over a conveyor belt with the help of a doctor blade to obtain a flat layer. This slurry layer is subsequently dried for further sintering process. Figure 2 shows the difference a sintered and a non-sintered material. 3

4 Sintered Panel Non-Sintered Panel Figure 2. Sintered and non-sintered panels. Material characteristics as well porosity and density have been properly designed by varying powder particle size, organic binder composition and sintering conditions. Some cilindrical samples have been tested; their dimensions are shown in Figure 3. Figure 3. Cylindrical Shape samples. More in detail, the measurements were reported about the following optima samples, determined by a preliminary investigation: 1) Sintered Panel (thickness 1,2 mm); 2) Multilayer panel constituted by Sintered Panel (thickness 1,2 mm) + not-sintered panel (thickness 0,5 mm); 4

5 3) Multilayer panel constituted by Sintered Panel (thickness 1,2 mm) + not-sintered panel (thickness 0,5 mm) + Sintered Panel (thickness 1,2 mm). 3 Acoustic measurement methodology Acoustic absorption and insulation properties of the proposed material were tested by impedance tube techniques. Measurements were carried out on two diameters cylindrical shape samples: 100 mm and 29 mm as in Figure 4, which are respectively related for [100, 1600] Hz and [1600, 6400] Hz frequency ranges. Sample 3 Panel Layer aggregation Sample 3 29 mm diameter Figure 4. Sample tested by impedance tube technique. The test equipment is constituted by: Brüel & Kjær PULSE data acquisition system (model 3560-B-030); Brüel & Kjær Impedance tube model 4206; Brüel & Kjær amplifier model 7206; Brüel & Kjær ¼ microphones model 4187; Brüel & Kjær preamplifiers model 2670; Brüel & Kjær pistonophone model Acoustic absorption measurements were carried out according to ISO [11]. Acoustic insulation measurements were led by a methodology based on the same assumptions of ISO , a four microphone transfer function method which is able to determine the Transmission Loss (TL) [12]. TL is determined by a two-load method: two successive acquisitions are made for each sample by modifying the characteristics of a tube extremity. A 5

6 reflective and an absorbing material are installed on a tube terminal for the two acquisitions. Channels phase displacement errors are avoided by a calibration procedure. Signal to noise ratio is kept over 10 db for each measurement session. Figure 5 shows some measurement phases. Large Tube Absorption measurement Small Tube Transmission Loss measurement Figure 5. Some measurement phases 4 Acoustic measurement results The absorption measurement average results are reported in Figure 6 for the [100, 6400] Hz frequency range. Figure 6. Absorption measurement The absorption shows an increasing trend for the three Samples. They have comparable and low absorption values for low frequencies (below 600 Hz). For the average frequencies, Sample 1 has the lowest values and Sample 3 have the highest values; Sample 2 has an halfway trend. 6

7 From about 4100 Hz Sample 2 and Sample 3 have almost the same values of absorption. For high frequencies the maximum values of absorption are reached: about 0.3 for Sample 1, about 0,4 for Samples 2 and 3. TL measurement results are reported in Figure 7. Figure 7. Transmission loss measurement Samples 1, 2, and 3 have a growing trend with the frequency. Sample 1 TL ranges from 3 db at 100 Hz to the highest TL of 10 db at 6400 Hz. Sample 2 has TL from 3 db near 100 Hz that grows to 15 db at 6400 Hz. Sample 3 has TL from 6 to 10 db around 100 Hz and grows to 20 for 6400 Hz. 5 Conclusions In this paper the acoustic properties of materials that are employed as electrodes on MCFCs are investigated. Metals employed are nickel for cathode and nickel-chrome for anode and the plates are made from the metal powders which are treated by tape casting and sintering processes. In general, materials to be used in MCFCs require very low impurity and air bubbles content. This issue makes their cost a critical aspect. However, in acoustic application such a requirement it is thought not to be important and refuse samples may be used contributing strongly to cut costs. Acoustic absorption and insulation properties of the proposed material are tested by impedance tube techniques. Two diameters cylindrical shape samples are analyzed: 100 mm and 29 mm as in Figure 5, which are respectively related for [100, 1600] Hz and [1600, 6400] Hz frequency ranges. Absorption (A) and Transmission Loss (TL) measurement results are presented. Results suggested that, for acoustical applications, the analyzed materials don t have good absorption properties (except for high frequencies) but they may be used in multilayer panels to improve their insulation properties. Furthermore, some metal powder may be substituted by a polymer powder in order to further reduce costs and further improve absorption. Manufacturing procedures are actually under investigation and future works foresee further investigations to obtain a design of the Samples that optimize their performance. 7

8 References [1] M.E. Delany, E.N. Bazley. Acoustic properties of fibrous absorbent material. Appl Acoust, 3, 1970, [2] Duan C., Cui G., Xu X., Liu P. Sound absorption characteristics of a high-temperature sintering porous ceramic materials. Applied Acoustics, 2012, 73: [3] Cotana, F., Rossi, F., Nicolini, A. A new geometry high performance small power MCFC. Journal of Fuel Cell Science and Technology, 1 (1), 2004, [4] Rossi, F., Nicolini, A.Experimental investigation on a novel electrolyte configuration for cylindrical molten carbonate fuel cells. Journal of Fuel Cell Science and Technology, 8 (5), 2011, art. no [5] Rossi, F., Nicolini, A. A cylindrical small size molten carbonate fuel cell: Experimental investigation on materials and improving performance solutions. Fuel Cells, 9 (2), 2009, [6] Rossi, F., Nicolini, A., Di Profio, P. Small size cylindrical molten carbonate fuel cells and future approaches for decreasing working temperature. ECS Transactions, 2008, 12 (1), [7] Cotana, F., Filipponi, M., Castellani, B. A cylindrical molten carbonate fuel cell supplied with landfill biogas. Applied Mechanics and Materials, 2013, 392, [8] Rossi, F., Nicolini, A. Ethanol reforming for supplying molten carbonate fuel cells. International Journal of Low-Carbon Technologies, 8 (2), 2013, [9] Rossi, F., Nicolini, A., Palombo, M., Castellani, B., Morini, E., Filipponi, M. An innovative configuration for CO2 capture by high temperature fuel cells. Sustainability, 2014, 6 (10), [10] Roohani Isfahani, SN., Sedaghat, A. A hybrid micro gas turbine and solid state fuel cell power plant with hydrogen production and CO2 capture. International Journal of Hydrogen Energy, 2016, 41, [11] ISO , Acoustics - Determination of sound absorption coefficient and impedance in impedance tubes - Part 2: Transfer-function method, [12] Lung, T.Y., Doige, A.G. A Time-averaging Transient Testing Method for Acoustic Properties of Piping Systems and Mufflers. J. Acoust. Soc. Am, 73, 1983,