Fabrication method

Numerous fabrication techniques have been used for making micromixers. In the past few years, there have been two obvious developments in micromixer fabrication the substrate was changed from silicon to polymers and the structures were changed from simple planar structure to complex 3D structure. Most of the early micromixers were fabricated in silicon. The mixing channels were either wet etched with KOH [16, 35, 42, 52, 53, 66] or dry etched using DRIB (deep reactive ion etching) [23, 26, 31, 37, 45, 54, 59, 96]. Anodically bonded glass on top of the channel 

The characteristic dimensionless numbers such as Reynolds number (Re) and Peclet number (Pe) are very suitable for discussing the operating conditions for micromixers. From the definitions in Equations (7.1) and (7.2), the relation 

In micromixers, the hydraulic diameter and the mixing path I are usually on 

P viscosity ratio of sheath flow to sample flow 

Nguyen, S. T. Wereley, Fundamentals and Applications of Microfluidics, 2nd edn, Artech House, Boston. 2006. 

Before we discuss various liquid-based microlenses, it is beneficial to provide a chapter that gives a brief introduction of the methods involved in their fabrication. The fabrication of these lenses takes advantage of advancements in modem microelectronic fabrication for integrated circuits (ICs), microelec-tromechanical systems (MEMS), and microfluidics that enable large scale, low cost, precise manufacturing. 

We also need to put fabrication into the perspectives of complete systems. Microlenses alone are of little use until they are combined with other optical components and electronics into systems. Hence, in this chapter, our descriptions of fabrication cover the basic techniques of microfabrication. However, this chapter is by no means a comprehensive discussion and readers wishing to learn more about microfabrication are encouraged to peruse other books that provide more complete reviews [1-11]. 

Conventional microfabrication has been based on planar technologies and thus limited to flat, rigid, solid surfaces. However, recent advances in technologies now allow fabrication on flexible surfaces and we will also provide examples of these devices. 

Silicon wafer on which devices have been fabricated. 

The activity of acid catalysts is increased using promoters. Various homogeneous catalysts and catalytic systems were tested in this reaction (e.g., BF3 and its adducts with (C H-l O, 

The best known polymerization of norbomene is the ring opening metathesis polymerization. The reaction is technically applied in the Norsorex process (7). 

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Processing. SAN copolymers may be processed using the conventional fabrication methods of extmsion, blow mol ding, injection molding, thermoforming, and casting. Small amounts of additives, such as antioxidants, lubricants, and colorants, may also be used. Typical temperature profiles for injection mol ding and extmsion of predried SAN resins are as follows (101). 

Ceramics. The properties of ferroelectrics, basically deterrnined by composition, are also affected by the microstmcture of the densifted body which depends on the fabrication method and condition. The ferroelectric ceramic process is comprised of the following steps (10,24,25) (/) selection of raw oxide materials, (2) preparation of a powder composition, (J) shaping, (4) densification, and (5) finishing. 

Although the LED is one of the most basic optoelectronic devices, there exists a variety of complex and interacting material and stmctural considerations in designing these devices. These include the choice of materials for emission wavelength of the LED as well as the geometry and fabrication methods of the device. The principal stmctural properties of commercially available LEDs are summarized in Table 1. 

HoUow-fiber fabrication methods can be divided into two classes (61). The most common is solution spinning, in which a 20—30% polymer solution is extmded and precipitated into a bath of a nonsolvent, generally water. Solution spinning allows fibers with the asymmetric Loeb-Soufirajan stmcture to be made. An alternative technique is melt spinning, in which a hot polymer melt is extmded from an appropriate die and is then cooled and sohdified in air or a quench tank. Melt-spun fibers are usually relatively dense and have lower fluxes than solution-spun fibers, but because the fiber can be stretched after it leaves the die, very fine fibers can be made. Melt spinning can also be used with polymers such as poly(trimethylpentene), which are not soluble in convenient solvents and are difficult to form by wet spinning. 

Fabrication methods that are generaby used to make these junctions are diffusion, ion implantation, chemical vapor deposition (CVD), vacuum deposition, and bquid-phase deposition for homojunctions CVD, vacuum deposition, and bquid-phase deposition for heterojunctions and vacuum deposition for Schottky and MIS junctions. 

Although all of the preceding processes can produce a p—n homojunction, the properties required from the junction and the fabrication costs determine the fabrication method. Methods of making other kinds of junctions are similar to those used to make homojunctions, but the materials on either side of such junctions are dissimilar. 

Melt Viscosity. The study of the viscosity of polymer melts (43—55) is important for the manufacturer who must supply suitable materials and for the fabrication engineer who must select polymers and fabrication methods. Thus melt viscosity as a function of temperature, pressure, rate of flow, and polymer molecular weight and stmcture is of considerable practical importance. Polymer melts exhibit elastic as well as viscous properties. This is evident in the swell of the polymer melt upon emergence from an extmsion die, a behavior that results from the recovery of stored elastic energy plus normal stress effects. 

The production of vitreous siUca from chemical precursors was first described in patents filed in 1934, including a fabrication method in which fine, high purity powders were produced by decomposing silanes (39). Forms were then cast from aqueous sHps. More importantiy, a dame hydrolysis process which used SiCl as the chemical precursor was described (40). This latter approach led to a marked improvement in glass purity and served as the basis for the processes used in the 1990s to make synthetic vitreous siUca. 

Cost and Quality. Many factors affect catalyst support cost including which raw materials are used, the purity of the raw materials, the chemical processing steps required, the fabrication method used, the severity of calcination conditions, and the extent of the quaHty assurance procedure. In... 

Ease of cure, easy removal of parts from mold surfaces, and wide availabiHty have made polyesters the first choice for many fiber-reinforced composite molders. Sheet mol ding compound, filament winding, hand lay-up, spray up, and pultmsion are all weU adapted to the use of polyesters. Choosing the best polyester resin and processing technique is often a challenge. The polyester must be a type that is weU adapted to the processing method and must have the final mechanical properties requked by the part appHcation. Table 1 Hsts the deskable properties for a number of fiber-reinforced composite fabrication methods. 

Table 1. Resin Properties Required for Various Fabrication Methods...

Some of these devices have a respectable quantum efficiency of charge generation and collection, approaching 0.4 (20). The nature of the polymeric binder has a large effect on the device performance (21), and so does the quaUty and source of the dye (22). Sensitivity to the environment and fabrication methods results in some irreproducibiUties and batch-to-batch variances. However, the main advantage of the ZnO-based photoreceptor paper is its very low cost. 

All these methods require careful design and control of fabrication methods to assure success. 

Materials selection for low-temperature service is a specialized area. In general, it is necessaiy to select materials and fabrication methods which will provide adequate toughness at all operating conditions. It is frequently necessaiy to specify Charpy V-notch (or other appropriate) qu fication tests to demonstrate adequate toughness of carbon and low-aUoy steels at minimum operating temperatures. 

The design is viable only if it can be produced economically. The choice of production and fabrication method is largely determined by the choice of material. But the production route will also be influenced by the size of the production run, and how the component will be finished and joined to other components each class of material has its own special problems here they were discussed in Chapters 14, 19, 24 and 25. The choice of material and production route will, ultimately, determine the price of the product, so a second major iteration may be required if the costing shows the price to be too high. Then a new choice of material or component design, allowing an alternative production path, may have to be considered. 

If, after fabrication, heat treatment is not possible, materials and fabrication methods must have optimum corrosion resistance in their as-fabricated form. Materials that are susceptible to stress corrosion cracking should not be employed in environments conducive to failure. Stress relieving alone does not always provide a reliable solution. 

Since these assumptions are not always justified for plastics, the classical equations cannot be used indiscriminately. Each case must be considered on its merits and account taken of such factors as mode of deformation, service temperature, fabrication method, environment and so on. In particular it should be noted that the classical equations are derived using the relation. 

Another important aspect of the fatigue of all materials is the statistical nature of the failure process and the scatter which this can cause in the results. In a particular sample of plastic there is a random distribution of microcracks, internal flaws and localised residual stresses. These defects may arise due to structural imperfections (for example, molecular weight variations) or as a result of the fabrication method used for the material. There is no doubt that failure... 

Conceptually elegant, the SOFC nonetheless contains inherently expensive materials, such as an electrolyte made from zirconium dioxide stabilized with yttrium oxide, a strontium-doped lanthanum man-gaiiite cathode, and a nickel-doped stabilized zirconia anode. Moreover, no low-cost fabrication methods have yet been devised. 

Since these assumptions are not always justifiable when applied to plastics, the classic equations cannot be used indiscriminately. Each case must be considered on its merits, with account being taken of such factors as the time under load, the mode of deformation, the service conditions, the fabrication method, the environment, and others. In particular, it should be noted that the traditional equations are derived using the relationship that stress equals modulus times strain, where the modulus is a constant. From the review in Chapter 2 it should be clear that the modulus of a plastic is generally not a constant. Several approaches have been used to allow for this condition. The drawback is that these methods can be quite complex, involving numerical techniques that are not attractive to designers. However, one method has been widely accepted, the so-called pseudo-elastic design method. 

Generally the lower the process pressure, the larger the product that can be produced. With most labor-intensive fabricating methods, such as RP hand lay-up with TS plastic, relatively slow process curing reaction time of the plastic can be used so that there is virtually no limit on size (Fig. 8-63). 

An important criteria is to understand and properly apply the interrelations of design requirements with materials of construction and fabricating methods. RPs has some mechanical, formability, and other characteristics that differ from other materials (steel, aluminum, wood, etc.). So what is new all materials have certain characteristics that... 

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