Vacuum pumps can be broadly categorized according to three techniques:
Positive displacement pumps use a mechanism to repeatedly expand a cavity, allow gases to flow in from the chamber, seal off the cavity, and exhaust it to the atmosphere. Examples include: piston pump, diaphragm pump, liquid-ring pump, sliding vane rotary pump, miltiple-vane rotary pump, rotary piston pump, rotary plunger pump and roots pump.
Momentum transfer pumps, also called molecular pumps, use high speed jets of dense fluid or high speed rotating blades to knock gaseous molecules out of the chamber. Examples include: turbine pump, turbomolecular pump, liquid jet pump, vapor jet pump, diffusion pump and diffusion ejector pump.
Entrapment pumps capture gases in a solid or absorbed state. These include: absorption pump, sublimation pump, sputter-ion pump amd cryopump.
Positive displacement pumps are the most effective for low vacuums, but their high backstream flows through mechanical seals generally limit their usefulness in high vacuums. Momentum transfer pumps in series with positive displacement pumps are the most common configuration used to achieve high vacuums, but they stall at low vacuums. (Hence the need for a positive displacement pump in series.) Entrapment pumps can be added to reach ultrahigh vacuums, but they have a maximum operational time since they do not exhaust materials. They periodically saturate and require regeneration, which usually means bringing the system back up to higher pressures and temperatures. The available operational time is usually unacceptably short in low and high vacuums, thus limiting their use to ultrahigh vacuums. Pumps also differ in details like manufacturing tolerances, sealing material, pressure, flow, admission or no admission of oil vapor, service intervals, reliability, tolerance to dust, tolerance to chemicals, tolerance to liquids and vibration.
Thursday, November 29, 2007
Vacuum pumps can be broadly categorized according to three techniques:
In most mechanical systems, friction or vibration are often considered to be negative attributes because they results in wear and unnecessary energy dissipation. Tribological issues such as friction, wear, and vibration have always plagued developers of small-scale mechanical devices. As devices get smaller and even reach the nanoscale, this problem becomes more acute due to the extreme surface-to-volume ratios these devices have. In nanomachines damage to even a single atom layer could mean disaster. Nanotechnology researchers basically have two ways to address this problem: they either could apply traditional tribological methods by trying to integrate dampers and low-friction materials with their nano- and microdevices - which becomes increasingly complex and costly at the nanoscale - or they could try and develop intrinsic damping materials that have hard, low-fiction surfaces to lower wear yet still maintain high compressibility and elastic properties to provide resistance to vibrations and shocks. Finding materials that address these problems individually is not difficult but fabricating a structure that combines all of them is nearly impossible because of the conflicting nature of these attributes. However, in what researchers have dubbed a 'nanomattress', a unique structure containing aligned carbon nanotubes (CNTs) covered with a hard layer of diamond-like carbon (DLC) results in a carbon-based composite material with outstanding mechanical properties.
The hybrid film was prepared by first growing a dense, vertically aligned, multi-walled CNT network (CNTs are ca. 7 µm long with 200 nm diameters) by thermal chemical vapor deposition (CVD) on a conductive silicon substrate. The prepared sample was then placed in a patented, off-plane, double-bend, filtered cathodic vacuum arc PVD system with substrate pulse biasing to deposit a 5 µm a-D film on the surface of the CNTs. In order to confine the coating to the top of the CNT film and prevent it from seeping through, a negative substrate pulse bias was used. Because of the negative voltage pulse on the CNT substrate, the high-energy positive carbon ions were attracted primarily to the tips of the CNTs, forming a-D nanospheres. As these spheres increase in size, they coagulate into a thick, solid a-D film on top of the CNTs. The top layer that formed in this manner adhered very well to the tips of the CNTs, as the carbon ions formed strong covalent C–C bonds to them. With a hard, solid top, the nanomattress is now capable of distributing the forces applied to it uniformly to the underlying CNTs, which act like the springs of a mattress.
This information was derived from: http://www.nanowerk.com/spotlight/spotid=3453.php
Monday, November 26, 2007
Tribology, the study of the friction and wear of materials, comes from the Greek word, tríbein, meaning to rub. The first tribological coating for controlling friction and wear was titanium carbide (TiC), introduced in 1969 on cemented carbide cutting tool inserts using chemical vapor deposition (CVD). The problem with the CVD process was that the substrate temperature during deposition was about 1000ºC so that CVD could not be used to coat high speed steel tooling, which is softened at those temperatures. To overcome this obstacle, workers began using physical vapor deposition (PVD) techniques that provide ion bombardment of the growing film, resulting in good film adhesion and densification of the film.
The first commercially successful PVD hard coating was titanium nitride (TiN). Balzers deposited it with their low voltage electron beam process, Ulvac with their hot hollow cathode process, and Multi-Arc with their cathodic arc process. Since the cost of the arc coating equipment was less than that of competing deposition processes, the cost of the arc coatings was lower, and the use of cathodic arc deposited hard coatings became widespread.
Initially, sputtering was not used for the commercial deposition of the tribological films because the quality of the films did not equal that of films produced by low voltage electron beam or cathodic arc methods. This situation was significantly improved with the introduction of closed-field unbalanced magnetron sputtering that provided for a higher degree of substrate ion bombardment during deposition.
One of the early themes in PVD tribological coatings was that high hardness was the most important property. It is true that a coating used for a machining application must be hard, but it is now understood that a coating should be both hard and ductile if it is going to perform well in a tribological application. Superlattice, multilayer, nanostructured, MAX phase, and carbon nitride (a form of diamond-like-carbon (DLC) film) coatings have succeeded in achieving a measure of success in providing both hardness and ductility.
Many DLC films are produced by PVD techniques including cathodic arc, filtered cathodic arc, sputtering, reactive sputtering, and low pressure CVD, and plasma assisted or plasma enhanced CVD processes. The hardness of DLC films covers the range from hard to superhard with hardness of 20-80 GPa. Whereas hard coatings such as titanium nitride, titanium aluminum nitride, and multilayer films have been used very successfully for tooling applications, the DLC films have been very successful where low friction and low wear are needed such as on gears and bearings.
There are two areas that will be very important for the future of tribological coatings. The first of these is nanolayered and nanocomposite coatings, which have already had a major impact on tribological coatings. Another area that should have a major impact on tribological coatings is the use of ionized PVD. Here the recent introduction of high power pulse magnetron sputtering (HPPMS) is used to provide a high degree of ionization of the sputtered material, improving the quality of the coatings and allowing the deposition of films that previously could not be done with conventional sputtering.
Most of this information was obtained from an article by William Sproul on page 46: http://online.qmags.com/SVC1007/
Thursday, November 22, 2007
Thin film photovoltaics, using inorganic or organic compounds as active layers, represent the most promising technology for significantly beating the cost of conventional solar amorphous or crystalline silicon electrical systems. These technologies have the potential to provide low cost, ac mains solar power by using non-silicon solar cells and low cost plastic substrates. Traditional silicon cell manufacturers have been constrained by the shortage of silicon, high prices of silicon, its weight and fragility and the difficulty of processing it.
See the table at right for more detail on the different types of thin film photovoltaic technologies including CIGS, DSSC and CdTe. These companies are doing extraordinary things with various thin films techniques, usually roll-to-roll (thin film vacuum coating or thin film ink-jet printing).
IDTechEx finds that the market for thin film photovoltaics beyond silicon will reach at least $1 billion in 2012 after a slow ramp up and grow rapidly after that to $6 billion in 2014. The global solar energy market is expected to reach $34 billion in 2010 and $100 billion in 2050 and most of that latter figure will be achieved by non-silicon photovoltaics. The market for printed batteries will reach $170 million in 2012 and $560 million in 2014. www.idtechex.com/power
Wednesday, November 21, 2007
The commercial acceptance of hard coatings for cutting tools is driven by demands on machining productivity, environmental mandates, and increased usage of new difficult-to-cut materials. Improved cutting performance is derived from synergies of machine tool system and cutting tool development. The latter strives for an optimized combination of tool material, hard coating and cutting edge geometry.
The integration of hard coatings in cutting tools has reached a mature stage after more than three decades of proven performance and productivity benefits in industrial metal cutting. Chemical vapor deposition (CVD) was the first technology used, which advanced from single layer to current multilayer types combining TiC, TiN, TiCN and Al2O3. Some 25 years later it is evident that novel physical vapor deposition (PVD) compositions have surpassed the limited set of available CVD coatings. The relative economics are debatable but it is accepted that total life cycle consideration and environmental friendliness favor PVD technology. Succeeding generations of PVD hard coatings have become commercially available, notably TiCN, TiAlN and AlCrN, with demonstrated success in expanding areas of application.
Intensive development of the superhard coatings diamond (diamond-like-carbon DLC) and cubic boron nitride (CBN) peaked in the 90’s. It seems that an insurmountable technical barrier – extremely high residual stress during deposition – has not been overcome and still prevents successful commercialization.
In mulitlayer coatings, particular properties suitable to the cutting wear conditions can be engineered by creative combinations of coating layers. For example, AlCrN-TiSiN performs well in high speed carbide drilling of 1045 steel. Although AlCrN has low thermal conductivity, it has been shown that the multilayer construction with TiSiN which has high thermal conductivity effectively raises the composite conductivity, minimizing the heat concentration at the critical drill outer corner. This apparently accounts for better performance of certain multilayer coatings with carbide drills. Another example is the TiAlN-WC/C coating designed for drills used for dry machining.
The figure: coating selection guide based on field performance and commercial acceptance.
An excellent historical overview of the development of PVD coatings leading up the latest innovations written by Dennis Quinto. Jump to page 17 for more... http://online.qmags.com/SVC1007/
Monday, November 19, 2007
Advances in thin films technologies are transforming traditional manufacturing processes and whetting the appetite of industry.Multiple new techniques in thin films technology are vying for supremacy in two rapidly growing sectors—microelectromechanical systems (MEMS) and highly-efficient photovoltaic systems.
Silicon wafer-based cells may soon give way to a new generation of thin film photovoltaics. According to The Information Network, a New Tripoli, Pa.,-based market research company for the semiconductor industry, manufacturing methods using cadmium telluride, CdTe, and copper indium gallium selenide, CIGS, will see a tremendous boost in research dollars for future equipment manufacture. A 275% increase in investment by 2010 is expected, which is a testament to their anticipated impact.CIGS and CdTe rely on the deposition of nanoparticles of the precursor materials on the substrate, followed by in-place sintering. The U.S. Dept. of Energy's National Renewable Energy Laboratory, Golden, Colo., says the best CIGS cell now matches the efficiency of the best polycrystalline-silicon cell, about 19.5% efficiency. Potential applications include newspaper-like roll-to-roll printing and CIGS-based inks.
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