Plasma Technologies in Preservation of Cultural Heritage
Plasma cleaning systems can be used for a variety of surface cleaning purposes before processing. In industrial practice, it is already efficiently used for removing surface oxidation and clearing mineral residue from surfaces. It is also used for preparing surfaces of plastics and elastomers, as well as for cleaning ceramics. It is also used for cleaning the surface of glass as well as metal surfaces. Using efficient plasma cleaning systems eliminates the need for using chemical solvents. One major benefit of using plasma cleaning is that the entire process is, or can be, operator friendly and environmentally friendly.
Plasma can be characterized as an at least partially ionized gas and is a complex mixture of different components, such as charged particles (electrons and ions) and neutral species (atoms and molecules), in addition to radicals, UV photons and irradiated heat. In general, plasma can be classified according to its temperature in thermal and nonthermal plasmas. A thermal plasma is an almost completely ionized gas, whereby the temperatures of the charges and neutral species are approximately equal, with temperatures typically reaching at least 15,000 К (Eliasson and Kogelschatz, 1991). In comparison to thermal plasma, nonthermal ones are only partially ionized, indicating that the number of neutral species is much higher than the number of charged species, whereby the temperature of the different particles is not equal. The temperature of electrons is still in the range of several thousand Kelvin, but the temperature of the neutral species and ions can be close to ambient temperature. Thus, nonthermal plasmas are also termed cold plasmas.
For the generation of cold plasma, energy needs to be supplied to a gas, where electric energy sources have been shown to be the most convenient. The lifetime of the particles inside the plasma is quite small due to energy loss by collision processes, and therefore energy must be supplied continuously for plasma applications. The generation of cold plasma can be achieved under atmospheric pressure and/or lower pressure conditions (Hertwig et al., 2018).
A plasma treatment is usually, but not always, performed in a chamber or enclosure that’s evacuated (Vacuum plasma). The air within the chamber or enclosure is pumped out prior to letting the gas in. The gas then flows in the enclosure at low- pressure. This is done before any energy (electrical power) is applied. It is imperative to know that plasma treatment performed at a low temperature can easily process materials that are heat-sensitive. There are also multiple plasma systems operating in ambient air at atmospheric pressure (atmospheric plasma).
These types of plasma are sometimes referred to as ‘cold plasma’.
In general, there are a number of plasma definitions, according to various disciplines with which it is connected. The simplest ones tell us that sufficient additional energy to gas creates a plasma. In the case of cold plasma, suitable for the modification of natural polymers, the basis of the process remains on the excitation of gas at reduced pressure or atmospheric pressure by radiofrequency (RF) energy. A review of multiple cold-plasma reactors that are not covered by this chapter describes the technical aspects of most low-temperature plasma systems (Hnatiuc et al., 2012).
By producing high-frequency electric discharges, plasma generates ionized gas that can modify the surface properties of the material it is in contact with. Plasma treatment is a versatile and powerful technique commonly used in many industries for materials such as plastics, textiles, glass, and metals.
Low-Pressure Operating Plasma Systems for Metallic Objects Surface Treatment
The low-pressure hydrogen-based plasmas operating in the flowing regime where the reaction products are continuously pumped out of the system were studied broadly (Daniels 1979; Daniels 1981; Patscheider and Veprek, 1986; Veprek et ah, 1985, 1988; RaSkova et ah, 2002). The diffusion length of active particles is not too short at low pressure (typically, a pressure of hundred Pascals is used). Therefore, active particles are able to penetrate into the pores or holes in the surface and, thus, nearly the whole surface can be treated.
Vacuum and Gas Handling System
As it was described above, special systems are needed to complete the low-pressure plasma treatment of metallic objects. The general scheme of such systems is shown in Figure 3.1. The core of the experimental device is a vacuum chamber with a pumping system and a proper gas handling system.
FIGURE 3.1 General concept of the low-pressure plasma system for the surface treatment of archaeological objects.
The vacuum chamber can be made of stainless steel in case of an internal electrode system (see Section 3.1.2 and Figure 3.3) or it can be made of Pyrex or Quartz glass if external electrodes are used. The chamber size can be from 20 cm in its diameter with the height from 50 cm up to a couple of meters in case of a stainless steel reactor. For the glass reactor, a diameter starting at 10 cm and its length from 40 cm up to meters is applicable. Both kinds of reactors must be easily open for the manipulation of the treated samples and be equipped with non- conductive sample holders (ideal material is glass). An exception is if samples are simply placed on the bottom electrode in case of the stainless steel reactor chamber. The fused silica optical window is necessary for the plasma process monitoring in case of the stainless steel reactor. High vacuum pressure is not needed for the process; thus, silicon rubber gaskets are sufficient for the reactor sealing. The glass reactor should have flanges isolated from the rest of the device (i.e. mounted at the floating potential) to avoid their role as grounded electrodes. The detailed scheme of the vacuum and gas handling system is given in Figure 3.2.
The pumping system needs a simple mechanic pump like a two-stage rotary oil pump or at least a two-stage membrane pump; the use of a scroll pump is possible but it is more expensive. The base pressure of the pump should be in the order of about 1 Pa to ensure proper pumping before the process. The pumping speed depends on the volume of the plasma reactor, so it is better to consult this parameter directly with vacuum technologies suppliers. In general, the pump with a speed of about 4 m'Vhour is sufficient for small systems. A mechanical filter at the pump input should be installed to avoid the intake of small grains falling off from the treated samples during manipulation with them because they can mechanically damage the pump. The strongly corroding hydrochloric acid and other less corrosive species are generated by the plasma process (Chapter 5), thus it is necessary to avoid their introduction to the pump, although the membrane pump can be constructed in a chemical version which would be less sensitive to their effects.
FIGURE 3.2 Scheme of the vacuum and gas handling system. 1 - High-pressure cylinder; 2 - on/off valve; 3 - mass flow controller; 4 - airing valve; 5 - on/off valve; 6 - cold trap with a reactive absorber of HC1; 7 - regulating valve; 8 - vacuum pump; 9 - operating pressure gauge; 10 - atmospheric pressure indicator/gauge.
Exhaust gas must be led into the outer open space, not to the laboratory because of hydrogen presence and to avoid potential higher hydrogen concentration in the closed volume (hydrogen is explosive in the air in concentrations between 3% and 97%). A simple cooled trap (liquid nitrogen cooling is not necessary, simple water cooling is sufficient) filled with material easily corroded by hydrochloric acid with a large surface (like aluminum shavings) is a good solution. The operational pressure is in the order of hundreds of Pascal. To be able to set the pressure and also for a good pump start, it is useful (but not necessary) to add some regulating valve at the pump input. The gauge operating in the range of about plus-minus 2 orders around the operating pressure must be installed close to the reactor chamber. The additional simple gauge showing pressure up to atmospheric values is useful but not necessary.
Plasma is generated in the gaseous mixture containing mainly hydrogen because this element is the most important in the plasma chemical reactions with the corrosion products (Chapter 5). The initial studies (Veprek et al., 1988) also used much more complicated gaseous mixtures containing methane, argon and nitrogen. Later studies (Fojtfkova et ah, 2015b, 2015c; Sazavska et ah, 2012) used hydrogen-argon mixtures only. If the hydrogen-argon gaseous mixture is used, the surface bombardment by atomic argon (both neutral and ionic) species contributes to the surface processes by ablation because of 40 times higher mass of argon ions than hydrogen atoms. The negative effect is in the significantly higher heating of the treated object, especially in the atomic scale (Note that the surface bombardment is by inert gas atoms. They lose not only their kinetic energy, but also some part or even all excitation energy. All this energy becomes heat because argon is non-reactive). This very local (only a few surrounding atoms at the surface are affected) overheating can lead to the selective melting of the surface material. This was observed when tin atoms from the copper matrix were removed from the bronze surface after the plasma treatment although the bulk material temperature was about 100 °C below the tin melting point (Fojtfkova et al., 2015a, for example, see Figure 3.10).
To ensure a proper gaseous mixture flow through the system, the mass flow controllers are the best solution. To be able to vary the gas mixture composition, two separate lines are recommended, although pure hydrogen or a pre-prepared hydrogen-argon mixture is used. The gases should be supplied from high-pressure gas cylinders. Hydrogen also can be prepared directly at the device from water by electrolysis. This solution is better for safety reasons but in this case, hydrogen contains some humidity that can complicate the monitoring of the plasma treatment process (see later).
Finally, the system must be equipped with an ambient air valve to be able to open the system after the plasma process completing.