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ASTM D748-2018 pdf free download

ASTM D748-2018 pdf free download.Standard Specification for Natural Block Mica and Mica Films Suitable for Use in Fixed Mica-Dielectric Capacitors
4. Significance and Use
4.1 The properties included in this specification are those required to identify the types of natural block mica and mica films (cut and uncut) suitable for use in the manufacture of fixed mica-dielectric capacitors. It is possible that other re- quirements will be necessary to identify particular character- istics. These will be added to the specification as their inclusion becomes generally desirable, and the necessary test data and methods become available. It is possible that natural block mica and mica films that do not conform to the requirements of this specification for capacitor use are capable of meeting the requirements for other critical electrical insulation purposes.4.2 The system of classifying electrical quality of natural block and mica films (cut and uncut) covered by this specifi- cation is based on a combination of electrical and physical properties, and visual qualities specified herein, which the mica must possess. This system differs radically from past practices and previous concepts of mica quality for capacitor use. The electrical classification system does not discriminate against the presence ofspots and stains in even first quality electrically selected mica, provided the mica conforms to specific and physical requirements. Appreciable amounts of air inclusions and waviness also are permitted in all electrical quality classes, provided the mica meets specific electrical and physical requirements. Mica meeting these requirements is acceptable without regard to color or origin. However, mica meeting these electrical and physical requirements but having lower visual quality than that meeting the requirements for the visual quality classification is not considered generally as desirable. 4.3 In capacitor fabrication, one or more pieces of cut film or block mica having lower than required electrical and physical properties will possibly prevent meeting the end requirements of the capacitor. It is therefore required that each piece ofblock (cut) or film (cut or uncut), or both, be tested for the electrical requirements and inspected for the visual require- ments listed in this specification.
7. Classes
7.1 This specification covers the following four classes and subdivisions of natural block mica or mica films. The class of mica desired shall be specified by the purchaser. Block mica or mica films shall conform to all of the requirements of its respective class, unless otherwise specified by the purchaser. 7.1.1 Class C-1 Special—Class C-1 special mica films, in addition to having the highest Q obtainable for mica in the megacycle range, also have the highest Q generally obtainable for mica in the audio frequency range (1000 min at 1 kHz). Such mica therefore is particularly suitable for capacitors which must meet highest Q requirements at both high and low frequencies. 7.1.2 Class C-1—Class C-1 block mica or mica films have the highest Q value obtainable for mica (2500 min at 1 MHz in capacitors) and are suitable for use in all sizes and types of silver and foil electrode, molded and clamped unit capacitors, including the most critical types, for use in high stability tuned circuits, as well as high current radio frequency capacitors used in radio transmitter circuits. N OTE 1—Based 4 on commercial experience Class C-1 block mica or mica films are satisfactory for the manufacture ofall ofthese types ofmica capacitors. However, it has been found that some “medium to heavy” air-stained mica will produce a slightly lower yield ofhighest stability and high current radio frequency types of capacitors, as well as a somewhat lower yield of satisfactory silver electrode mica laminations. Likewise, it is also possible that some “medium to heavy” wavy mica will also adversely influence the application of silver electrodes. In addition, there is some possibility that excessive waviness will cause cracked laminations in molded capacitors due to the high molding pressures employed, thus making it less suitable from a stacking standpoint. Similarly, excessive amounts of either air inclusions or waviness could cause some reduction in unit volume capacitance of foil electrode capacitors.

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