Moisture stress

Thermomechanlcal Behavior. Requirements for optical performance Impose unprecedented requirements for dimensional stability of polymers used In hlgh-concentratlon reflectors. Requirements for mechanical compatibility are also strict for photovoltaic systems subjected to moisture and thermal stresses. Moisture, temperature, and UV, separately and In combination, can change the volume and thus the stress state of polymers. For example, temperature and humidity cycles alone do not cause surface micro-cracks In polycarbonate. However, In the presence of UV radiation, such cycles cause microcracks, while UV alone does not [32]. An understanding of these relationships Is essential to permit reliable design of equipment that uses polymers. 

Subjected to stress, moisture and temperature uses peel joint As above, but uses single-lap shear joint loaded in tension As above but uses a wedge test Exposure to moisture and temperature... 

Another approach employing concepts of continuum damage mechanics (e.g., Lemaitre and Chaboche 1994) was presented by Perreux and Suri (1997). Studying the effects of cyclic fatigue on [ 55°]3 glass fiber/poly epoxy composite pipes they noticed that in the absence of stress moisture uptake data could be modeled as a process of a two-phase diffusion (see Sect. 3.6). Damage in the axial direction was related to the decrease in the axial stiffness, namely and its effect on... 

Alloying with both chromium and nickel increases scaling resistance. Stainless steels or nickel alloys, except alloy 400 (N04400), are required to provide satisfactory oxidation resistance at temperatures above 705 °C. Thermal cycling, applied stresses, moisture and sulfur-bearing gases will decrease scaling resistance. 

Corrosion fatigue is a type of failure (cracking) which occurs when a metal component is subjected to cyclic stress in a corrosive medium. In many cases, relatively mild environments (e.g., atmospheric moisture) can greatly enhance fatigue cracking without producing visible corrosion. 

A combination of excellent chemical and mechanical properties at elevated temperatures results in rehable, high performance service to the chemical processing and related industries. Chemical inertness, heat resistance, toughness and flexibiUty, stress-crack resistance, excellent flex life, antistick characteristics, Htfle moisture absorption, nonflammability, and exceptional dielectric properties are among the characteristics of these resins. 

Dielectric Film Deposition. Dielectric films are found in all VLSI circuits to provide insulation between conducting layers, as diffusion and ion implantation (qv) masks, for diffusion from doped oxides, to cap doped films to prevent outdiffusion, and for passivating devices as a measure of protection against external contamination, moisture, and scratches. Properties that define the nature and function of dielectric films are the dielectric constant, the process temperature, and specific fabrication characteristics such as step coverage, gap-filling capabihties, density stress, contamination, thickness uniformity, deposition rate, and moisture resistance (2). Several processes are used to deposit dielectric films including atmospheric pressure CVD (APCVD), low pressure CVD (LPCVD), or plasma-enhanced CVD (PECVD) (see Plasma technology). 

A case provides mechanical support and protection for the devices, interconnects, and substrate mounted in it it also helps to dissipate heat during component operation and offers protection to the contents of the package from environmental stresses, contaminants, and, in the case of hermetic packages, moisture. 

Cases can be classified as either hermetic or nonhermetic, based on their permeabiUty to moisture. Ceramics and metals are usually used for hermetic cases, whereas plastic materials are used for nonhermetic appHcations. Cases should have good electrical insulation properties. The coefficient of thermal expansion of a particular case should closely match those of the substrate, die, and sealing materials to avoid excessive residual stresses and fatigue damage under thermal cycling loads. Moreover, since cases must provide a path for heat dissipation, high thermal conductivity is also desirable. 

Dehydration. Residual liquid and physisorbed moisture on particle surfaces can be eliminated on beating to - 200° C. Temperatures ia excess of 1000°C may be requited to eliminate cbemisorbed water (29). Kaolin must be beated to 700°C to Hberate tbe water of crystallisation and produce tbe desired dehydrated aluminosiUcate. As with biader burnout, rapid gas evolution from rapid dehydration can result ia catastrophic stress development within a body. 

Plasticity. Plasticity may be defined as the property of a material that permits it to be deformed under stress without mpturing and to retain the shape produced after the stress is removed. When water is added to dry clay in successive increments, the clay becomes workable, that is, readily shaped without mpturing. The workabiUty and retention of shape develop within a very narrow moisture range. 

FIG. 20-70 The influence of moisture as a percentage of sample saturation S on granule deformabihty. Here, deformation strain (AL/L) is measured as a function of applied stress, with the peak stress and strain denoted by tensile strength and critical strain (AL/L) of the material. Dicalcium phosphate with a 15 wt % binding solution of PVP/PVA Kolhdon VAG4. [Holm et al., Powder Tech., 43, 213 (1.9S.5J,] With land permission from Elsevier Science SA, Lausanne, Switzerland. 

The confinement of the cracks to a specific area of the cooler suggests that condensate from atmospheric moisture initially formed in this area and dissolved a corrodent from the atmosphere such as ammonia, sulfur dioxide, or oxides of nitrogen. Since the previous cooler had been in service for 20 years, it is conjectured that the rapid failure of this exchanger was due principally to very high bending stresses, which may have been induced during construction of the cooler. 

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