# Development and Testing of Copper Filters for Efficient Application in Half-Face Masks
## 2 Experimental Materials and Methods  
In this research, the elemental Cu powder produced by Kovohuty Dolny Kubin, s.r.o., Slovakia with a purity of 99.5% was used. The induction furnace is designed for melting and holding various copper alloys in a 600 kg crucible, and then pouring the liquid metal into further equipment or ladles. The exact chemical composition of the delivered material was: min. 99.5% of Cu; <0.001% of P; <0.001% of Sn; <0.001% of Pb; <0.003% of Fe; <0.001% of Zn; and <0.14% of O\({}_{2}\). The powders were made by atomization with air and water. Water atomization is used primarily for ferrous compositions but can also be used to produce a range of nonferrous alloys. Commercial water-atomized powders are normally irregular in shape. Particle size distributions are relatively broad, i.e., 10-350 um (micrometer). A cooling rate is a function of a particle size. The representative operating conditions in water atomization are summarized in paper [15]. Production of gas- or air-atomized powders is lower than water-atomized ones due to its higher cost. The atomization ensures a low oxygen content of 100 ppm (parts per million) of the spherical powders (ferrous and non-ferrous alloys). Commercial air- or gas-atomized powders are normally regular in shape [16; 17]. Four types of copper powders were used in this study. The basic powder parameters are shown in Table 1.  
Sintering of the free-flowing powders was carried out in the Carbolite electric tube furnace. The sintering temperature was 865 \({}^{\circ}\)C; the heating rate was 10 \({}^{\circ}\)C\(\cdot\)min\({}^{-1}\); and the holding time was 1 h after gradual cooling. The sintering environments of nitrogen and hydrogen were employed. Morphological characterization was performed using the Olympus optical microscope, model GX71, and the TESCAN scanning electron microscope. The measurements of pressure losses in filters were carried out using a newly created experimental device which allows determining the pressure losses at a known airflow. The prototype is intended for testing various filtration materials while its universal applicability consists of the possibility of adjusting the components that are used to attach the filtration material and manufactured by 3D printing. The 3D printing facilitates the fast production and adjustment of a holder for the filtration material to meet current requirements and the determination of a pressure drop in the respective filter that is embedded, for instance, in a filter case. Negative pressure in the system is achieved using a continuously controllable radial fan while the negative pressure generated on the fan side may amount to 900 Pa. According to EN 149 +A, the standard of filtration half masks, the maximum allowable pressure in inhalation or exhalation is 300 Pa. This prototype device is equipped with the measuring technology consisting of a differential pressure sensor (BD Sensors 816-CS4M-1-0-8-0-1-Y00-M-000) which detects a difference in pressure before and after the filtration material, a sensor of atmospheric pressure (BD Sensors 111-JS38-1-8-100-100-1-000), a thermometer, and an anemometer that identifies a velocity profile of the flowing air on the plane located on the mouth of the measuring stand. Replacing the fan with artificial lungs facilitated the measurement of a pressure drop in the filter in the mode simulating spontaneous breathing. Automatic evaluation of measurement results and graphical representation of the measured values were carried out using the basic software programmed at the Department of Power Engineering at the Faculty of Mechanical Engineering of the Technical University of Kosice. Measurements and recording of the required physical parameters were carried out using the measuring technology and Raspberry Pi 3 microcomputer. The measured values were visualized in a Qt Creator graphical environment programmed in the C++ language.