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Support Material) Content VACUUM TECHNOLOGY PRACTICE COURSE (SUPPORT MATERIAL) CONTENT INTRODUCTION…………………………………………………………………………… 3 1. CONCEPT OF VACUUM………………………………………………………………. 4 1.1. Gas laws…………………………………………………………………………….. 4 1.2. Kinetic theory of gases……………………………………………………………… 5 1.3. Gas flow…………………………………………………………………………….. 10 1.4. Pressure of gas, levels of vacuum, vacuum units…………………………………… 12 2. VACUUM SYSTEMS CALCULATION……………………………………………….. 15 3. VACUUM SYSTEMS PROJECTION………………………………………………….. 16 3.1. Materials for vacuum systems……………………………………………………… 16 3.2. Gas load sources……………………………………………………………………. 22 3.3. Absorption, diffusion, and outgassing……………………………………………… 23 3.4. Elements of vacuum systems……………………………………………………….. 26 3.5. Throughput capacity of vacuum systems elements………………………………… 26 4. CLEANING……………………………………………………………………………… 35 5. VACUUM PUMPS……………………………………………………………………… 43 5.1. Mechanical pumps………………………………………………………………….. 43 5.2. Condensers………………………………………………………………………….. 47 5.3. Jet and diffusion pumps…………………………………………………………….. 47 5.4. Molecular and turbomolecular pumps……………………………………………… 52 5.5. Sorption pumps……………………………………………………………………... 57 5.6. Cryotechnology and cryopumps……………………………………………………. 67 6. VACUUM MEASUREMENT………………………………………………………….. 78 6.1. Mechanical vacuum gauges………………………………………………………… 78 6.2. Spinning rotor gauges………………………………………………………………. 84 6.3. Direct electric pressure measuring transducers……………………………………... 85 6.4. Thermal conductivity vacuum gauges……………………………………………… 85 6.5. Thermal mass flowmeters…………………………………………………………... 89 6.6. Ionization gauges…………………………………………………………………… 91 6.7. Combined vacuum gauges………………………………………………………….. 100 7. VACUUM COMPONENTS, SEALS, AND JOINTS…………………………………... 104 7.1. Vacuum hygiene…………………………………………………………………….. 104 7.2. Joining technologies in vacuum technology………………………………………... 106 7.3. Components………………………………………………………………………… 114 7.4. Vacuum sealing technology………………………………………………………… 119 8. LEAK DETECTION…………………………………………………………………….. 129 8.1. Basic principals……………………………………………………………………... 129 8.2. Mass spectrometers and residual gas analysis……………………………………… 131 8.3. Leak detection with tracer gases…………………………………………………… 136 8.4. Application notes…………………………………………………………………… 140 9. VACUUM SYSTEM OPERATING…………………………………………………….. 143 9.1. Electronic integration of vacuum system…………………………………………… 143 9.2. Pressure control……………………………………………………………………... 145 9.3. Technic for operating low-vacuum system…………………………………………. 146 9.4. Technic for operating fine-vacuum system………………………………………… 147 9.5. Technic for operating high-vacuum system………………………………………… 148 9.6. Technic for operating ultrahigh-vacuum system…………………………………… 149 REFERENCES ……………………………………………………………………………… 154 INTRODUCTION Historically, vacuum technology has been of major importance in the evolution of electronics ever since the first vacuum tube was constructed. Means for obtaining and measuring vacuums have proven indispensable in the development of nearly all electronic devices. Likewise, such means are basic to the understanding of surface physics and surface chemistry. For many experiments and purposes, pressures of the order of 10-6 Torr (10-6 mm Hg or about 10-9 atmosphere) are low enough, even though surface phenomena studies show that at 10-6 torr a surface becomes covered with adsorbed gas in only a few seconds. Other experiments, however, require the use of ultrahigh vacuum. At lower pressures the number of collisions of gas phase atoms and molecules with a surface is so small that a clean surface remains free of contamination long enough to do experiments on the surface itself. Recent surface studies have required pressures of 10- 9torr or lower. At this pressure no significant surface contamination occurs for a several-hour period. Pressures of 10-9 torr and below are generally called the ultrahigh vacuum range. Ultrahigh vacuum is a requisite for many experiments that involve either the reaction between a surface and a gas or the properties of the surface itself. Currently, the understanding and application of techniques of ultrahigh vacuum are somewhat restricted to specialists who of necessity are familiar with meeting reports, subject literature that appears in scientific journals, and parts of textbooks. From this material these men have gained a working knowledge of the inherently simple techniques needed in ultrahigh vacuum experiments. It has been clear for some time that a book devoted to the study of ultrahigh vacuum was needed. The main idea of the book is to cover all aspects of vacuum science and technology in order to enable engineers, technicians, and scientists to develop and work successfully with the equipment and “environment” of vacuum. Beginners in the field of vacuum shall be able to start and experts shall be able to deepen their knowledge and find the necessary information and data to continue their work. 3 1. CONCEPT OF VACUUM 1.1. Gas laws For a proper understanding of phenomena in gases, more especially at low pressures, it is essential to consider those phenomena from the point of view of the kinetic theory of gases. This theory rests essentially upon two fundamental assumptions. The first of these postulates is that matter is made up of extremely small particles or molecules, and that the molecules of the same chemical suhstan1·csubstance are exactly alike as regards size, shape, mass, and so forth. The second postulate is that the molecules of a gas are in constant motion, and this motion is intimately related to the temperature. In fact, the temperature of a gas is a manifestation of the amount or intensity of molecular motion. In the case of monatomic molecules (such as those of the rare gases and vapors of most metals) the effect of increased temperature is evidenced by increased translational (kinetic) energy of the molecules. In the case of diatomic and polyatomic molecules increase in temperature also increases rotational energy of the molecule about one or more axes, as well as, vibrational energy of the constituent atoms with respect to mean positions of equilibrium. However, in the following discussion only the effect on translational energy will he considered. According to the kinetic theory a gas exerts pressure on the enclosing walls because of the impact of molecules on these walls. Since the gas suffers no loss of energy through exerting pressure on the solid wall of its enclosure, it follows that each molecule is thrown back from the wall with the same speed as that with which it impinges, but in the reverse direction; that is, the impacts are perfectly elastic. Suppose a molecule of mass m to approach the wall with velocity v. Since the molecule rebounds with the same speed, the change of momentum per impact is 2mv. If υ molecules strike unit area in unit time with an average velocity v, the total impulse exerted on the unit area per unit time is 2mvυ. But the pressure is defined as the rate at which momentum is imparted to a unit area of surface. Hence, 2 = (1.1) It now remains to calculate υ. Let n denote the number of molecules per unit volume. It is evident that at any instant we can consider the molecules as moving; in six directions corresponding to the six faces of a cube. Since the velocity of the molecules is v, it follows that, on the average, ( /6) molecules will cross unit area in unit time. Equation (1) therefore becomes 1 (1.2) = 3 Since 2 = (1.3) where ρ denotes the density, equation (2) can be expressed in the form 1 (1.4) = 3 which shows that, at constant temperature,2 the pressure varies directly as the density, or inversely as the volume. This is known as Boyle’s, law. Also, from equation (2) it follows that the total kinetic energy of the molecules in volume V is 1 3 (1.5) = 2 2 Now it is a fact that no change in temperature2 occurs if two different gases, originally at the same temperature, are mixed. This result is valid independently of the relative volumes. Consequently, the average kinetic energy of the molecules must be the same for all gases at any given temperature and the rate of increase with temperature must be the same for all gases. We may therefore define temperature in terms of the average kinetic energy of the molecules, and this 4 suggestion leads to the relation 1 3 (1.6) = 2 2 where T is the absolute temperature (degrees2 Kelvin), defined by the relation = 273.16 + (t = degrees Centigrade), and k is a universal constant, known as the Boltzmann constant ( = 1.380649 × 10 ). Combining equation 6 with 5, it follows that −23 = (1.7) This is an extremely� useful relation, as will be shown subsequently, for the determination of n for given values of P and T. Also, from equation 3 and the last equation, it follows that (1.8) = that is, (1.9) = which is known as Charles' law. Lastly, let us consider equal volumes of any two different gases at the same values of P and T. Since P and V arc respectively the same for each gas, and is constant at constant value of T, it follows from equations 5 and 7 that n must be the same for1 both2 gases. That is, equal volumes of all gases at any given values of temperature and pressure c2ontain an equal number of molecules. This was enunciated as a fundamental principle by Avogadro in 1811, but it required about 50 years for chemists to understand its full significance. On the basis of Avogadro's law, the molecular mass, M, of any gas or vapor is defined as that mass in grams, calculated for an ideal gas which occupies, at 0° C and one atmosphere, a volume = 22,414.6 = 22.4146 This is therefore designated the molar
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