High-Vacuum Technology

A Practical Guide, Second Edition

Marsbed H. Hablanian

• Publisher: Routledge
• Year: 2017
• ISBN:
  9781351440684
• Edition: 2
• Topic: Physical Sciences
• Subtopic: Mechanics

Offering a basic understanding of each important topic in vacuum science and technology, this book concentrates on pumping issues, emphasizes the behavior of vacuum pumps and vacuum systems, and explains the relationships between pumps, instrumentation and high-vacuum system performance. The book delineates the technical and theoretical aspects of the subject without getting in too deep. It leads readers through the subtleties of vacuum technology without using a dissertation on mathematics to get them there. An interesting blend of easy-to-understand technician-level information combined with engineering data and formulae, the book provides a non-analytical introduction to high vacuum technology.

1

Introduction

1.1 NATURE OF VACUUM

The word “vacuum” is used to describe a very wide range of conditions. At one extreme, it refers to nearly complete emptiness, a space without matter, or more specifically, space in which air and other gases are absent.

At the other extreme, vacuum is any air or gas pressure less than a prevailing pressure in an environment or, specifically, any pressure lower than the atmospheric pressure. An example of conditions approaching the first meaning is intergalactic space. Examples of the second meaning are pressures existing at the inlet of an ordinary vacuum cleaner and in a straw used for drinking.

The basic property involved here is the density of the gas. The degree of vacuum can easily be described in terms of gas particle density instead of pressure. Scientific and engineering interest in vacuum spans an enormous range of gas densities—15 orders of magnitude—in other words, it involves density changes of a million billion times. Corresponding to this wide variation in density, there are industrial and laboratory processes in which certain ranges in this wide variety of conditions are used. Production of a high degree of vacuum and an understanding of high-vacuum technology were closely associated with the development of physics as a science beginning approximately 350 years ago.

At atmospheric pressure, there are about 2 × 10¹⁹ molecules in a cubic centimeter of air. At the altitude of earth-orbiting satellites, the corresponding number is 10⁹. Resistance to motion is reduced accordingly, letting satellites stay in orbit for many years. At atmospheric conditions, molecules can move a distance of only a few millionths of a centimeter without colliding with one another, while at the altitude of orbiting satellites, this distance between collisions can be several miles. In a high-vacuum chamber, collisions between molecules are much less frequent than with walls of the chamber.

The existence of great travel distances without collision is used, for example, in a television tube, where a focused beam of electrons must travel from the electron gun on one end to the picture screen in the front without dispersion or scattering. Inside the television picture tube, the degree of vacuum is about one billionth of an atmosphere.

1.2 SCOPE OF VACUUM SCIENCE AND TECHNIQUE

The commonest example of partial vacuum involves respiration. When we breathe we expand our lungs and increase their volume. This produces a pressure inside which is slightly lower than in the outside air and permits additional air to enter the lung. Similarly, the suction in an ordinary drinking straw is created by inhaling the air from the straw, thereby reducing the pressure on the liquid surface inside the straw relative to the pressure above the liquid in the drinking cup. The excess pressure pushes the liquid into the straw. If anyone doubts this process, let the person try to drink through a straw after taking a very deep breath.

Partial vacuum or rarefied gas conditions are used in many common devices. For example, vacuum exists in automobile engine pistons when they are withdrawn to let in atmospheric air. A much higher degree of vacuum is used in a thermos bottle but for a very different purpose. The air is removed between the double walls of the bottle to reduce the amount of heat conduction from the hot to the cold wall. In an electric light bulb, vacuum is used during a stage of its manufacture to remove atmospheric oxygen and prevent a chemical reaction with very hot tungsten filaments.

A television picture tube is a vacuum tube. In addition to the increased travel distance of a coherent stream of electrons, vacuum environment also prevents rapid oxidation of the electron emitters (cathodes). The same two basic requirements are used for thin-film coating applications. For example, metallic films deposited on various substrates (ranging from plastic toys to microelectronic circuits) are produced in vacuum chambers instead of electroplating. This assures that the evaporated metal does not become oxidized and that it can reach the substrate without colliding with a large number of air molecules.

1.3 USES AND APPLICATIONS OF VACUUM TECHNOLOGY

These few examples just described illustrate a variety of possible applications and the importance of high-vacuum technology. Many commonly observed and utilized properties of gases are changed substantially when the gas is highly rarefied. Life, with the possible exception of some very hardy viruses, cannot be supported.

Animals do not survive exposure to high vacuum longer than a few seconds. Volatile liquids, including water, would evaporate at a very high rate without an atmosphere surrounding our planet. Lakes and rivers would evaporate within a few days if the resulting “steam” were to be removed and condensed elsewhere. Ordinarily, water and other fluids evaporate slowly because the evaporating molecules can return into the fluid after colliding with air molecules present above the liquid surface.

The boiling temperature of a liquid is reduced if the liquid is placed in a vacuum. Water will boil at room temperature if it is subjected to a pressure of about 1/40 of an atmosphere. An astronaut working outside of a space station cannot hear the “noise” of the hammer or wrench. Sound cannot be transmitted through high vacuum because sound is created by pressure perturbations in the air.

The transmission of electromagnetic radiation, however, light, radio waves, and so on, is not affected and is even improved by the absence of air. This is the reason, for example, for placing telescopes and other instruments in space.

These examples indicate the basic uses of vacuum: removal of gas to impede heat transfer, removal of gas to permit travel of particles through a required distance, evaporation or drying, particlarly at a lower-than-usual temperature, and removal of chemically active gases. In vacuum metallurgy, removal of oxygen and reduction of the amount of dissolved gases in the melt are of primary importance. Many metals, such as tungsten, molybedenum, tantalum, and titanium, cannot be molten in atmospheric air. They react rapidly and sometimes violently when heated to high temperatures in the presence of oxygen.

Large accelerators built for investigations in the field of high-energy physics are essentially gigantic vacuum tubes. Even though vacuum tubes have almost disappeared from radio sets and television receivers, they are employed in devices used for the generation and transmission of radio, radar, and television signals.

With the advent of space exploration, large vacuum chambers have been built to simulate conditions existing in interplanetary space. Such chambers may be as large as 10 m in diameter and 30 m high. Entire smaller rockets, subassemblies of large rockets and satellites, can be placed in such chambers for long periods of time. The performance of mechanisms and instruments can be tested without the presence of air or the presence of absorbed layers of water vapor on sliding surfaces. Even the presence of unshielded radiation of the sun can be simulated.

In the microelectronic industry, high-vacuum chambers are used for precisely controlled deposition of thin films of desired materials on various substrates. The rapid development of sophisticated electronic computers in recent years would not be possible without the existence of refined techniques in high-vacuum technology.

1.4 PRODUCTION OF VACUUM

Vacuum can be produced by a few basic methods. It can be produced by mechanical displacement of gases from an enclosed space by means of pistons, rotating vanes, lobes, and so on. Any compressor, in principle, can be converted into a vacuum pump simply by connecting the chamber to the inlet of the device instead of the discharge. This includes air and steam ejectors, vapor jet pumps, and turbocompressors. To be effective in high-vacuum work, such compressors are usually redesigned to enhance certain required performance characteristics. A vacuum can also be produced simply by condensing and freezing the gas in the enclosed space. Normally, this requires very low temperatures (cryogenic pumping).

A vacuum can also be created by chemical reactions with the gas, which produce solid residues, thereby removing the gas from space (chemical pumping or gettering). Physical absorption can also be used as a means of removing gases from space. Highly porous substances are used for this purpose (e.g., activated charcoal, activated alumina, or minerals called zeolites), often in conjunction with cryogenic temperatures. Such substances have extremely large internal surface areas which can act as gas “sponges.”

The final method of producing a vacuum, developed approximately 30 years ago (primarily at Varian Associates), involves ionization of the gas, accelerating the ions in a high-voltage field, and driving the gas directly into a wall or target electrode.

To produce a high vacuum in a chamber, the evacuation process must start at atmospheric pressure. Due to the enormous range of gas densities involved, a single pumping device cannot be used efficiently throughout the evacuation process. Usually, a sequence of at least two different devices is used. A coarse vacuum is produced either by mechanical means or by cryosorption, the higher degree of vacuum is developed by vapor jet pumps (called diffusion pumps), turbomolecular pumps, cryosorption pumps, and finally, by ion-gettering pumps.

The instruments used to measure the degree of vacuum must also span a very wide range of gas densities (or pressures). At least two different vacuum gauges are used in most vacuum systems. At the higher pressures, gauges based on force measurement can be used, such as liquid-filled manometers and various diaphragm gauges.

At a high degree of vacuum, the pressure force and force difference between degrees of vacuum become so small that direct force measurements become impractical and indirect ways must be used. For example, one of the most common methods involves ionizing the gas and measuring the intensity of the ion current, which depends on the amount of gas present. Such gauges, called ionization gauges, are used at gas densities as low as a million billion times less than 1 atm.

1.5 BRIEF HISTORY OF DEVELOPMENT

Vacuum technology is a relatively new technology although speculations about the possibility of creating a vacuum go back perhaps 2000 years. Ancient Greek philosophers speculated about the nature of vacuum. For a variety of reasons, the existence of vacuum was doubted and even rejected. Aristotle (384–322 B. C.) thought, for example, that vacuum was not possible because he assumed that the concept of empty space would invite the concept of motion without resistance (i.e., motion at infinite velocity). Such opinions persisted for many years and were held by many notable writers, including Roger Bacon (1214–1299) and René Descartes (1596–1650).

Galileo (1564–1642) was among the first to conduct experiments attempting to measure forces required to produce vacuum with a piston in a cylinder. Many of these early experiments were associated with the pumps used to remove water from mines. Torricelli (1608–1647), an associate of Galileo’s, was first to use mercury instead of water for such experiments, thereby reducing to convenient dimensions the size of the apparatus required. He produced a vacuum by filling a glass tube (closed at one end) with mercury and submerging the open end in a pool of mercury. He demonstrated that the mercury column “suspended” in the tube was always 76 cm above the level in the pool, regardless of the size, length, shape, or degree of tilt of the tube. He correctly explained that the pressure of atmospheric air acting on the surface of the mercury pool supports the mercury column in the tube. In honor or Torricelli, the unit of measure of degree of vacuum, the unit “millimeters of mercury” (mmHg) was called torr. Recently, however, the International Standards Organization established a new unit, called the pascal in honor of Blaise Pascal (1623–1662), who contributed to understanding of vacuum physics, carried out many early experiments, demonstrated his findings to large audiences, and was among the first to devise a barometer.

Otto von Guericke (1602–1686) was first to modify water pumps for direct pumping of air. He produced sufficient vacuum inside two gasketed copper hemispheres (essentially large suction cups, about 50-cm-diameter) to conduct his famous demonstration in Magdeburg (1654), where horses hitched to each side of the sphere could not pull the hemispheres apart until air was readmitted. (In retrospect, it may be observed that this experiment was not conducted correctly because the horses did not apply a maximum force simultaneously [Ref. 33].) Many such early experiments were also made by Robert Boyle (1627–1691), who made an improved vacuum pump.

The understanding of vacuum phenomena stimulated many branches of science and engineering, such as hydraulics, hydrostatics, pneumatics, and physics of rarefied gases. Vacuum technique was associated with many important discoveries in physics: for example, electrical discharges in gases, x-rays (1895), and the existence of electrons (1897).

In the twentieth century, the development of vacuum science and technology has been associated with radio tubes, high-energy particle physics, atomic energy, isotope separation, optical and microelectronic coating, processing of heat-sensitive fluids such as photographic emulsions, and vacuum metallurgy. Important recent advances have been associated with the requirements in processing microwave tubes that led to the development of ion-getter pumps and ultrahigh-vacuum techniques.

Many modern instruments and tools would be unthinkable without high vacuum. Examples are electron and ion microscopes, mass spectrometers, ion implanters, and extremely sensitive leak detectors. Even the landing of human beings on the moon could not have been accomplished without the existence of space simulation technology and an understanding of vacuum science.

1.6 HISTORY OF VACUUM PUMPS

Most writers on the history of vacuum pumps attribute the first invention to Otto von Guericke, in about 1650, following Torricelli’s experiments with vacuum in mercury-filled tubes in 1644 (or 1643). However, others mention Galileo and his attempts to measure the force of vacuum formation using cylinders, pistons, and weights (1640).

The initial attempts to create vacuum were made by pumping water out of filled containers using a variety of well-known water pumps. Water pumps go back at least to ancient Alexandria and were used in ancient Roman mines. Many sixteenth-century mining engineers probably produced 0.3-atm vacuum without knowing it when they tried to pump water from above. They staged their pumps at about 20-ft levels. Galileo reported in his book (Leyden, 1638) that he first heard about the 33- or 34-ft height limit from a mine worker.

This discovery must have been before 1630 because in that year G. B. Baliani reported to Galileo the failure of a water p...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface to the Second Edition
• Preface to the First Edition
• Chapter 1 Introduction
• Chapter 2 Properties of Gases
• Chapter 3 Fluid Flow and Pumping Concepts
• Chapter 4 Vacuum Systems
• Chapter 5 Coarse Vacuum Pumps
• Chapter 6 Vapor Jet (Diffusion) Pumps
• Chapter 7 Turbomolecular Pumps
• Chapter 8 Cryogenic Pumps
• Chapter 9 Gettering and Ion Pumping
• Chapter 10 Overloading of Vacuum Pumps
• Chapter 11 Ultrahigh Vacuum
• Chapter 12 Vacuum Gauges and Gas Analyzers
• Chapter 13 Leak Detection
• Appendix
• Index