Engineers Clean JWST Secondary Reflector with Carbon Dioxide Snow

Carbon dioxide cleaning (CO₂ cleaning) comprises a family of methods for

parts cleaning Parts cleaning is a step in various industrial processes, either as preparation for surface finishing or to safeguard delicate components. One such process, electroplating, is particularly sensitive to part cleanliness, as even thin layers of oil ...

and sterilization, using

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

in its various phases. Due to being non-destructive, non-abrasive, and residue-free, it is often preferred for use on delicate surfaces. CO₂ cleaning has found application in the

aerospace Aerospace is a term used to collectively refer to the atmosphere and outer space. Aerospace activity is very diverse, with a multitude of commercial, industrial, and military applications. Aerospace engineering consists of aeronautics and astron ...

, automotive,

electronics Electronics is a scientific and engineering discipline that studies and applies the principles of physics to design, create, and operate devices that manipulate electrons and other Electric charge, electrically charged particles. It is a subfield ...

medical Medicine is the science and Praxis (process), practice of caring for patients, managing the Medical diagnosis, diagnosis, prognosis, Preventive medicine, prevention, therapy, treatment, Palliative care, palliation of their injury or disease, ...

, and other industries. Carbon dioxide snow cleaning has been used to remove particles and organic residues from metals,

polymer A polymer () is a chemical substance, substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeat unit, repeating subunits derived from one or more species of monomers. Due to their br ...

ceramic A ceramic is any of the various hard, brittle, heat-resistant, and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcela ...

glass Glass is an amorphous (non-crystalline solid, non-crystalline) solid. Because it is often transparency and translucency, transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window pane ...

es, and other materials, and from surfaces including

hard drive A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating hard disk drive platter, pla ...

s and

optical surfaces Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultravio ...

Applications

cleaning has found application in many industries and technical areas, including aerospace, automotive, electronics, medical, manufacturing, basic and applied research, and

optics Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of optical instruments, instruments that use or Photodetector, detect it. Optics usually describes t ...

. The different carbon dioxide cleaning methods can remove gross contamination,

paint Paint is a material or mixture that, when applied to a solid material and allowed to dry, adds a film-like layer. As art, this is used to create an image or images known as a painting. Paint can be made in many colors and types. Most paints are ...

overlayer An overlayer is a layer of adatoms adsorption, adsorbed onto a surface, for instance onto the surface of a single crystal. On single crystals Adsorbed species on single crystal surfaces are frequently found to exhibit long-range ordering; that ...

s, grease,

fingerprint A fingerprint is an impression left by the friction ridges of a human finger. The recovery of partial fingerprints from a crime scene is an important method of forensic science. Moisture and grease on a finger result in fingerprints on surfa ...

s, particles down to nanometers in size,

hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and Hydrophobe, hydrophobic; their odor is usually fain ...

and organic residues, and

radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is conside ...

residues. Materials cleaned include metals, polymers, ceramics, and glasses. The key limitation is that the contamination must be on the surface, not buried within the material. Porous materials are not good candidates for pellets or snow, but can be cleaned using liquid or supercritical CO₂.

Methods

Carbon dioxide cleaning refers to several different methods for parts cleaning, making use of all phases of : basic methods include solid

dry ice Dry ice is the solid form of carbon dioxide. It is commonly used for temporary refrigeration as CO2 does not have a liquid state at normal atmospheric pressure and Sublimation (phase transition), sublimes directly from the solid state to the gas ...

pellets, liquid , snow (a hybrid method), and supercritical . The different forms of cleaning can clean many types of objects, from large generators to small and delicate parts, including hard drives and optics.

Pellets

In pellet cleaning ("

dry-ice blasting Dry-ice blasting is a form of carbon dioxide cleaning, where dry ice, the solid form of carbon dioxide, is accelerated in a pressurized air stream and directed at a surface in order to clean it. The method is similar to other forms of media bla ...

"), relatively large pellets of solid CO₂ are fired at the surface to be cleaned. These pellets impinge on the surface, mechanically dislodging contaminant particles. Pellet cleaning is only appropriate for surfaces sturdy enough to withstand significant impacts.

Snow cleaning

In CO₂ snow cleaning, compressed liquid or gaseous

is expelled from a nozzle, condensing into a mixture of solid particles and gas, which impact the surface to be cleaned. Jet velocities are frequently supersonic. Snow cleaning works by a combination of momentum transfer (mechanically dislodging contaminant particles) and

solvent A solvent (from the Latin language, Latin ''wikt:solvo#Latin, solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a Solution (chemistry), solution. A solvent is usually a liquid but can also be a solid, a gas ...

action. The CO₂ sublimates on contact, increasing in volume up to 800 times, thereby generating pressure to sweep particles away. The CO₂ also dissolves hydrocarbon contaminants, and its low temperature embrittles residues such as fingerprints, making them easier to blow away. Snow cleaning has found application in the

aeronautical Aeronautics is the science or art involved with the study, design, and manufacturing of air flight-capable machines, and the techniques of operating aircraft and rockets within the atmosphere. While the term originally referred solely to ''ope ...

, automotive,

optical Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultravio ...

semiconductor A semiconductor is a material with electrical conductivity between that of a conductor and an insulator. Its conductivity can be modified by adding impurities (" doping") to its crystal structure. When two regions with different doping level ...

, and

space Space is a three-dimensional continuum containing positions and directions. In classical physics, physical space is often conceived in three linear dimensions. Modern physicists usually consider it, with time, to be part of a boundless ...

industries. It can provide a gentle cleaning, appropriate for delicate surfaces. The effectiveness of carbon dioxide snow cleaning has been demonstrated via light

microscopy Microscopy is the technical field of using microscopes to view subjects too small to be seen with the naked eye (objects that are not within the resolution range of the normal eye). There are three well-known branches of microscopy: optical mic ...

, particle counting,

scanning electron microscopy A scanning electron microscope (SEM) is a type of electron microscope that produces images of a sample by scanning the surface with a focused beam of electrons. The electrons interact with atoms in the sample, producing various signals that ...

, microprobing,

X-ray photoelectron spectroscopy X-ray photoelectron spectroscopy (XPS) is a surface-sensitive quantitative spectroscopic technique that measures the very topmost 50-60 atoms, 5-10 nm of any surface. It belongs to the family of photoemission spectroscopies in which electro ...

atomic-force microscopy Atomic force microscopy (AFM) or scanning force microscopy (SFM) is a very-high-resolution type of scanning probe microscopy (SPM), with demonstrated resolution on the order of fractions of a nanometer, more than 1000 times better than the opti ...

, and

mass spectroscopy Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a ''mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is used ...

. Equipment costs for a carbon dioxide snow cleaning system can range from US$1500 for a basic system to $50,000 for a high-end automated unit. Material costs are comparatively low, although ultra-pure CO₂ must often be used to avoid the introduction of new contaminants.

Supercritical fluid

At temperatures and pressures above its critical point, CO₂ can be maintained as a

supercritical fluid A supercritical fluid (SCF) is a substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist, but below the pressure required to compress it into a solid. It can effuse through porous sol ...

, exhibiting extremely low

viscosity Viscosity is a measure of a fluid's rate-dependent drag (physics), resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for e ...

and high

solvency Solvency, in finance or business, is the degree to which the current assets of an individual or entity exceed the current liabilities of that individual or entity. Solvency can also be described as the ability of a corporation to meet its long- ...

. To apply this method, parts to be cleaned are enclosed in a pressure vessel that is then filled with supercritical CO₂. This method is appropriate for small and delicate parts such as microelectronics, and is not ideal for particulate removal. Aside from cleaning, applications of

supercritical carbon dioxide Supercritical carbon dioxide (s) is a fluid state of carbon dioxide where it is held at or above its critical temperature and critical pressure. Carbon dioxide usually behaves as a gas in air at standard temperature and pressure (STP), or a ...

include targeted chemical supercritical fluid extraction and materials processing.

Liquid CO₂ washing

Liquid CO₂ washing, like supercritical fluid CO₂ washing, relies on the high solvent power of CO₂, but at lower temperatures and pressures, the latter making it simpler to implement. Because liquid CO₂ does not have the solvent power of the supercritical fluid, agitation and

surfactant Surfactants are chemical compounds that decrease the surface tension or interfacial tension between two liquids, a liquid and a gas, or a liquid and a solid. The word ''surfactant'' is a Blend word, blend of "surface-active agent", coined in ...

s may be added to improve the effectiveness of the method. Liquid CO₂ has been used in

dry cleaning Dry cleaning is any cleaning process for clothing and textiles using a solvent other than water. Clothes are instead soaked in a water-free liquid solvent (usually non-polar, as opposed to water which is a Solvent#Solvent classifications, polar ...

and machined parts degreasing.

History

Carbon dioxide cleaning was contemplated in the 1930s, and the "pellet" approach was developed in the 1970s by E.E. Rice, C.H. Franklin, and C.C. Wong. The introduction of CO₂ snow cleaning, with its ability to remove sub-micron-scale particles, is credited to Stuart Hoenig of the

University of Arizona The University of Arizona (Arizona, U of A, UArizona, or UA) is a Public university, public Land-grant university, land-grant research university in Tucson, Arizona, United States. Founded in 1885 by the 13th Arizona Territorial Legislature, it ...

, who first published on the topic in 1985–1986. Hoenig traveled the US to demonstrate the technology, eventually attracting the interest of

The BOC Group BOC Limited is a British based multinational, industrial gas company. Formerly listed on the London Stock Exchange, since 2006 it has been a subsidiary of Linde plc. History Brin's Oxygen Company was formed in 1886, by two French brothers, ...

, which developed Venturi nozzles for the process, and

Hughes Aircraft The Hughes Aircraft Company was a major American aerospace company, aerospace and defense contractor founded on February 14, 1934 by Howard Hughes in Glendale, California, as a division of the Hughes Tool Company. The company produced the Hughes ...

, which developed straight nozzles. CO₂ snow cleaning was further developed by the Fraunhofer Institute for Manufacturing Engineering and Automation IPA, for the purpose of removing

from aircraft

fuselage The fuselage (; from the French language, French ''fuselé'' "spindle-shaped") is an aircraft's main body section. It holds Aircrew, crew, passengers, or cargo. In single-engine aircraft, it will usually contain an Aircraft engine, engine as wel ...

s. Nozzle design is the most significant factor in carbon dioxide snow cleaning performance, affecting the size and velocity of the dry ice particles. Variations in nozzle design have been developed by W.H. Whitlock, L.L. Layden, Applied Surface Technologies, and Sierra Systems Group.

Issues

Safety

CO₂ cleaning may present certain safety risks. If the process is used to remove hazardous materials, precautions must be taken to avoid exposure to these materials in the vent stream. Because the CO₂ stream is

cryogenic In physics, cryogenics is the production and behaviour of materials at very low temperatures. The 13th International Institute of Refrigeration's (IIR) International Congress of Refrigeration (held in Washington, DC in 1971) endorsed a univers ...

, it may cause injury with direct skin contact. In addition, care must be taken to prevent the concentration of carbon dioxide in the work area from exceeding safe levels.

Contamination

Some commercial grades of carbon dioxide may contain traces of heavy

s, which can be left behind on the surface being cleaned. Abrasive particles originating in the cleaning equipment itself may need to be filtered out as well. The low temperature of the carbon dioxide stream can also induce moisture

condensation Condensation is the change of the state of matter from the gas phase into the liquid phase, and is the reverse of vaporization. The word most often refers to the water cycle. It can also be defined as the change in the state of water vapor ...

on the part, which may be mitigated with hot plates,

heat gun A heat gun is a device used to emit a stream of hot air, usually at temperatures between , with some hotter models running around , which can be held by hand. Heat guns usually have the form of an elongated body pointing at what is to be hea ...

heat lamp An infrared heater or heat lamp is a heating appliance containing a high-temperature emitter that transfers energy to a cooler object through electromagnetic radiation. Depending on the temperature of the emitter, the wavelength of the peak of th ...

s, or dry boxes.

Static charge

Ionization Ionization or ionisation is the process by which an atom or a molecule acquires a negative or positive Electric charge, charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged at ...

caused by the flowing gas can result in potentially damaging

static charge Static electricity is an imbalance of electric charges within or on the surface of a material. The charge remains until it can move away by an electric current or electrical discharge. The word "static" is used to differentiate it from curren ...

buildup on non-

conductive In physics and electrical engineering, a conductor is an object or type of material that allows the flow of Electric charge, charge (electric current) in one or more directions. Materials made of metal are common electrical conductors. The flow ...

parts. This can be mitigated by grounding or positive ionization sources.

References

{{reflist, 30em, refs= {{cite patent , inventor-last = Swain , inventor-first = Eugene A. , inventor2-last = Carter , inventor2-first = Stephen R. , inventor3-last = Hoenig , inventor3-first = Stuart A. , publication-date = Jun 30, 1992 , title = Carbon dioxide snow agglomeration and acceleration , country-code = US , patent-number = 5125979 A {{cite journal, last1=Sherman, first1=Robert, last2=Adams, first2=Paul, title=Carbon Dioxide Snow Cleaning – The Next Generation of Clean, journal=Precision Cleaning, date=1995, pages=271–300, url=http://infohouse.p2ric.org/ref/02/01017.pdf, access-date=24 September 2015 {{cite web, title=Applications, website=Carbon Dioxide Snow Cleaning, url=http://www.co2clean.com/applications, publisher=Applied Surface Technologies, access-date=23 September 2015 {{cite web, title=FAQ, website=Carbon Dioxide Snow Cleaning, url=http://www.co2clean.com/faq, publisher=Applied Surface Technologies, access-date=23 September 2015 {{cite web, title=AFM, website=Carbon Dioxide Snow Cleaning, url=http://www.co2clean.com/afm, publisher=Applied Surface Technologies, access-date=24 May 2016 {{cite journal , last1 = Chernoff , last2 = Sherman , year = 2010 , title = Resurrecting dirty atomic force microscopy calibration standards , journal = Journal of Vacuum Science & Technology B, Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phenomena , volume = 28 , issue = 3, page = 643 , doi = 10.1116/1.3388847 , bibcode = 2010JVSTB..28..643C {{cite web, title=What is Dry Ice Blasting (Cleaning)?, url=http://www.coldjet.com/en/information/what-is-dry-ice-blasting.php, publisher=Cold Jet, access-date=23 September 2015 {{cite web, title=About Us, website=Carbon Dioxide Snow Cleaning, url=http://www.co2clean.com/, publisher=Applied Surface Technologies, access-date=4 August 2015 {{cite web, title=Cleaning Methods, url=http://www.co2clean.com/cleaners, website=Carbon Dioxide Snow Cleaning, publisher=Applied Surface Technologies, access-date=13 August 2015 {{cite web, title=Safety Issues, website=Carbon Dioxide Snow Cleaning, url=http://www.co2clean.com/safety, publisher=Applied Surface Technologies, access-date=23 September 2015 {{cite web, title=How does CO2 Blasting Work?, url=http://www.coldjet.com/en/information/how-does-it-work.php, publisher=Cold Jet, access-date=23 September 2015 {{cite web, title=Industries and applications, url=http://www.coldjet.com/en/industries/index.php, publisher=Cold Jet, access-date=23 September 2015 {{cite news, title=Space probes: sterile launch into outer space, url=http://phys.org/news/2015-08-space-probes-sterile-outer.html, access-date=4 August 2015, work=Phys Org, agency=Fraunhofer-Gesellschaft, date=August 3, 2015 {{cite journal, last1=Weibel, first1=Gina, last2=Ober, first2=Christopher, title=An overview of supercritical CO ₂ applications in microelectronics processing, journal=Microelectronic Engineering, volume=65, issue=1–2, date=2003, pages=145–152, url=https://www.researchgate.net/publication/222729244, doi=10.1016/S0167-9317(02)00747-5 Cleaning tools Industrial processes Broad-concept articles Cleaning methods

cleaning Cleaning is the process of removing unwanted substances, such as dirt, infectious agents, and other impurities, from an object or environment. Cleaning is often performed for beauty, aesthetic, hygiene, hygienic, Function (engineering), function ...