ASTM D748-14
(Specification)Standard Specification for Natural Block Mica and Mica Films Suitable for Use in Fixed Mica-Dielectric Capacitors
Standard Specification for Natural Block Mica and Mica Films Suitable for Use in Fixed Mica-Dielectric Capacitors
ABSTRACT
This specification covers the electrical, visual, and physical properties of natural blocks and films made from mica that are suitable for use in fixed mica-dielectric capacitors. The materials under this specification are of three forms and four classes. Samples should be taken and tested according to the test methods prescribed here. All samples should adhere to the specified values of conducting path, Q value, dielectric constant, weight loss on heating, thickness uniformity, temperature coefficient of capacitance and retrace, and amount of air inclusions, waves, cracks, tears, pin holes, stones, buckles, and ridges.
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 requirements will be necessary to identify particular characteristics. 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 specification 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 of spots 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 of block ...
SCOPE
1.1 This specification covers natural block mica2 and mica films (cut and uncut) suitable for use in the manufacture of fixed mica-dielectric capacitors, based on electrical, visual, and physical properties as determined by tests specified herein.
1.2 The values stated in inch-pound units are to be regarded as standard.
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NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation:D748 −14
Standard Specification for
Natural Block Mica and Mica Films Suitable for Use in Fixed
Mica-Dielectric Capacitors
This standard is issued under the fixed designation D748; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope* istics.Thesewillbeaddedtothespecificationastheirinclusion
2 becomes generally desirable, and the necessary test data and
1.1 This specification covers natural block mica and mica
methods become available. It is possible that natural block
films (cut and uncut) suitable for use in the manufacture of
mica and mica films that do not conform to the requirements of
fixedmica-dielectriccapacitors,basedonelectrical,visual,and
this specification for capacitor use are capable of meeting the
physical properties as determined by tests specified herein.
requirements for other critical electrical insulation purposes.
1.2 The values stated in inch-pound units are to be regarded
4.2 The system of classifying electrical quality of natural
as standard.
block and mica films (cut and uncut) covered by this specifi-
2. Referenced Documents
cation is based on a combination of electrical and physical
properties,andvisualqualitiesspecifiedherein,whichthemica
2.1 ASTM Standards:
must possess. This system differs radically from past practices
D149 Test Method for Dielectric Breakdown Voltage and
and previous concepts of mica quality for capacitor use. The
Dielectric Strength of Solid Electrical Insulating Materials
electrical classification system does not discriminate against
at Commercial Power Frequencies
the presence of spots and stains in even first quality electrically
D351 Classification for Natural Muscovite Block Mica and
selected mica, provided the mica conforms to specific and
Thins Based on Visual Quality
physical requirements. Appreciable amounts of air inclusions
D1082 Test Method for Dissipation Factor and Permittivity
and waviness also are permitted in all electrical quality classes,
(Dielectric Constant) of Mica
provided the mica meets specific electrical and physical
2.2 ASTM Adjuncts:
requirements. Mica meeting these requirements is acceptable
Test electrodes
without regard to color or origin. However, mica meeting these
Grid test electrodes
electrical and physical requirements but having lower visual
3. Terminology qualitythanthatmeetingtherequirementsforthevisualquality
classification is not considered generally as desirable.
3.1 For definitions pertaining to this specification see Clas-
sification D351.
4.3 In capacitor fabrication, one or more pieces of cut film
or block mica having lower than required electrical and
4. Significance and Use
physical properties will possibly prevent meeting the end
4.1 The properties included in this specification are those
requirements of the capacitor. It is therefore required that each
required to identify the types of natural block mica and mica
piece of block (cut) or film (cut or uncut), or both, be tested for
films (cut and uncut) suitable for use in the manufacture of
the electrical requirements and inspected for the visual require-
fixed mica-dielectric capacitors. It is possible that other re-
ments listed in this specification.
quirements will be necessary to identify particular character-
5. Forms
This specification is under the jurisdiction of ASTM Committee D09 on
5.1 This specification covers the following three forms of
Electrical and Electronic Insulating Materials and is the direct responsibility of
natural mica, suitable for use in the manufacture of mica-
Subcommittee D09.01 on Electrical Insulating Products.
dielectric capacitors:
Current edition approved Nov. 1, 2014. Published December 2014. Originally
approved in 1943. Last previous edition approved in 2011 as D748 – 00 (2011).
5.1.1 Form 1—Full-knife trimmed natural block mica 0.007
DOI: 10.1520/D0748-14.
to 0.035 in. (0.18 to 0.89 mm) in thickness.
Coutlee, K. G., “Electrical Quality Classification of Raw Mica by a Rapid,
Direct-Reading Test Method,” Proceedings, ASTM, Vol 46, 1946, p. 1486.
5.1.1.1 It is acceptable for a certain percentage of block
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
mica thinner than 0.007 in. to be accepted under this
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
specification, upon agreement between the purchaser and the
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. manufacturer.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D748−14
5.1.2 Form2—Half-knifetrimmednaturalblockmica0.007 and coupling purposes where high Q value, high stability, and
to 0.035 in. in thickness. low temperature coefficient are not required.
5.1.3 Form 3—Films (cut or uncut) split or manufactured
NOTE 2—Experience has shown that silver electrode molded capacitors
from natural block mica in any range from 0.0008 up to and
made with Class C-3 mica, which contained slightly conducting spots and
including 0.006 in. (0.020 to 0.15 mm).
stains but contained “very slight” air inclusions and “nearly flat” waves,
had temperature coefficient and capacitance stability characteristics just as
6. Grades (Sizes)
good as that obtained with capacitors made with the best Class C-1 mica.
6.1 For grades see Classification D351.
8. Electrical and Physical Properties, and Visual
7. Classes Qualities
7.1 This specification covers the following four classes and
8.1 Natural block mica and mica films shall conform to the
subdivisions of natural block mica or mica films. The class of requirements as to electrical and physical properties and visual
mica desired shall be specified by the purchaser. Block mica or
qualities as prescribed in Table 1. Visual qualities not covered
mica films shall conform to all of the requirements of its in this specification are permitted provided mica meets the
respective class, unless otherwise specified by the purchaser.
electrical and physical requirements.
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
9. Test Methods
megacycle range, also have the highest Q generally obtainable
9.1 The properties enumerated in this specification shall be
for mica in the audio frequency range (1000 min at 1 kHz).
determined in accordance with the following:
Such mica therefore is particularly suitable for capacitors
9.2 Grading According to Size—Classification D351.
which must meet highest Q requirements at both high and low
frequencies.
9.3 Electrical Conductivity—See Annex A1.
7.1.2 Class C-1—Class C-1 block mica or mica films have
NOTE 3—For the purpose of this specification, electrical conductivity in
the highestQ value obtainable for mica (2500 min at 1 MHz in
spotted and stained areas of block mica is revealed when visible sparking
capacitors) and are suitable for use in all sizes and types of
or glowing takes place inside or on the surface of the mica in the vicinity
silver and foil electrode, molded and clamped unit capacitors,
of the test probe, and not by actual puncture of the mica by the
including the most critical types, for use in high stability tuned
high-potential current. If actual puncture of the test specimen does take
place this indicates the presence of mechanical faults, such as pinholes,
circuits,aswellashighcurrentradiofrequencycapacitorsused
tears, or cracks which extend completely through the mica.While this test
in radio transmitter circuits.
method had been found suitable for controlling conductivity in spots and
NOTE 1—Based on commercial experience Class C-1 block mica or stains and dielectric weakness due to mechanical faults in block mica, an
even greater factor of safety will be realized if this flash test is applied
micafilmsaresatisfactoryforthemanufactureofallofthesetypesofmica
directly to capacitor films. In this instance the purpose of the test is to
capacitors. However, it has been found that some “medium to heavy”
detect dielectric weakness due to any cause.
air-stained mica will produce a slightly lower yield of highest stability and
high current radio frequency types of capacitors, as well as a somewhat
9.4 Q Value or Dissipation Factor—Test Method D1082 at
lower yield of satisfactory silver electrode mica laminations. Likewise, it
1 MHz, or by the rapid, direct-reading method described in
is also possible that some “medium to heavy” wavy mica will also
adversely influence the application of silver electrodes. In addition, there Appendix X1.
is some possibility that excessive waviness will cause cracked laminations
NOTE 4—In cases of dispute arising from borderline cases ofQ value or
in molded capacitors due to the high molding pressures employed, thus
dissipationfactoranddielectricstrength,thetestspecimensshallbebaked
making it less suitable from a stacking standpoint. Similarly, excessive
foraminimumperiodof2hatatemperatureof121°C(250°F),andtested
amounts of either air inclusions or waviness could cause some reduction
immediately upon cooling to room temperature.
in unit volume capacitance of foil electrode capacitors.
9.5 Dielectric Strength—Test Method D149,usingthe
7.1.3 ClassC-2—Class C-2 block mica and mica films have
short-time test with ⁄4-in. (6.4-mm) diameter electrodes in oil.
a high order of Q value (1500 min at 1 MHz in capacitors) and
are suitable for use in all sizes and types of silver and foil
9.6 Weight Loss on Heating—Preheat the test specimens in
electrode molded and clamped unit capacitors similar to those
an oven at 121°C (250°F) for a minimum time of 2 h and then
specified for Class C-1 mica. However, a certain percentage of
weigh. Then heat the specimens in the oven at 600°C (1110°F)
capacitors made with Class C-2 block mica and films may
for 5 min and reweigh. Calculate the percentage loss in weight
show a somewhat higher temperature rise in transmitter types
after heating based on the weight of the specimen at the end of
than capacitors made with Class C-1 block mica or mica films.
the 2-h preheating period.
7.1.4 ClassC-3—Class C-3 block mica and mica films have
9.7 Thickness [Uniformity (Films)]—Judge splitting quality
the lowest Q value (200 min at 1 MHz in capacitors) of the
by the uniformity of thickness of films split from mica by
three classes covered by this specification. Such Q value,
viewing between crossed polaroids.
however, is sufficiently high to permit this mica to be classed
as a low-loss insulating material. This mica is particularly
9.8 Visual Qualities—See Classification D351.
suitable for use in foil electrode molded and clamped type
9.8.1 Air Inclusions—Reflected daylight or equivalent.
capacitors (Note 2) used in less critical circuits for blocking
9.8.2 Waves, Buckles, Ridges, and so forth—Reflected day-
light or its equivalent where distortion of parallel and vertical
lines of reflected image, such as a window frame, can be
Coutlee, K. G., “Judging Mica Quality Electrically,” Transactions, Am. Inst.
Electrical Engrs., Vol 64, 1945. judged.
D748−14
TABLE 1 Electrical, Physical, and Visual Quality Requirements of Natural Block Mica and Mica Films for Use in Capacitors
Class C-1 Class C-2 Class C-3
Electrical Properties:
A
Conducting paths none none none
B B B
Q value or dissipation factor at 1 MHz E-1 E-2 E-3
average
Dielectric strength, min 1000 1000 1000
H
V/mil at 60 Hz single 850 850 850
CCC
Dielectric constant
Physical Properties:
Weight loss on heating (5 min at 600 °C), max % 0.2 0.2 0.2
D
Thickness uniformity (mica films) best best intermediate
EE
Temperature coefficient of capacitance and retrace
F
Visual Qualities:
H H H
Air A very slight very slight very slight
I F I
G
slight slight slight
inclusions
B
J J J
H medium medium medium
C
A nearly nearly nearly
flatslight flatslight flatslight
B
medium medium medium
C
K
Waves heavy heavy heavy
D
Cracks none none none
Tears none none none
Pin holes none none none
Stones none none none
Buckles none none none
Ridges none none none
A
This applies to the conductivity of visible spots and stains only and excludes air stains or air inclusions.
B
The Q values or dissipation factor Q5 of block mica or mica films suitable for use in capacitors shall fall within specified electrical quality groups,
S D
dissipation factor
based on end-use requirements, designated E-1 Special, E-1, E-2, or E-3. These quality groups shall conform to the Q or dissipation factor values prescribed in Table 2
or the corresponding scale readings of the vacuum-tube voltmeter when tested by the rapid, direct-reading method described in Appendix X1.
C
As the dielectric constant of natural block mica suitable for use in capacitors is fairly uniform, no value is specified.
D
Until definite values can be specified, the permissible amounts of such defects shall be agreed upon between the purchaser and the manufacturer.
E
It has been found that the temperature coefficient of capacitance and retrace of capacitors made with Classes C-1, C-2, and C-3 mica are more dependent on such factors
as electrical and mechanical design and manufacturing technique than on any differences in the mica itself.
F
See Classification D351.
G
The amount of air inclusions shall not exceed the specified limits for each subclass. The permissible amount of air inclusions shall be stated by suffixing the appropriate
letter (in other words, either A, B, C, or D) to the required electrical quality class.
H
Few and tiny in one fourth of usable area (must not contain air chains, and so forth).
I
In one half of usable area (must not contain air chains, and so forth).
J
In two thirds of usable areas.
K
The waviness shall not exceed the specified limits for each subclass. The permissible amount of waves shall be stated by suffixing the appropriate letter (in other words,
either A, B, C or D) to the letter denoting the amount of permissible air inclusions. For example, Class C-1 B A block mica or mica films denotes:
C-1 B A
Best electrical quality slight air inclusion in one half usuable area nearly flat waves
9.8.3 Cracks, Tears, Pinholes, and Stones—Determine the a statement of precision or bias. No activity has been planned
presence of such mechanical defects as cracks, tears, pinholes, to develop such information.
and stones by the spark coil test as prescribed in AnnexA1,or
by visual inspection as judged by transmitted daylight or its
11. Keywords
equivalent.
11.1 block mica; capacitor; classes; dielectric stren
...
This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: D748 − 00 (Reapproved 2011) D748 − 14
Standard Specification for
Natural Block Mica and Mica Films Suitable for Use in Fixed
Mica-Dielectric Capacitors
This standard is issued under the fixed designation D748; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope Scope*
1.1 This specification covers natural block mica and mica films (cut and uncut) suitable for use in the manufacture of fixed
mica-dielectric capacitors, based on electrical, visual, and physical properties as determined by tests specified herein.
1.2 The values stated in inch-pound units are to be regarded as the standard.
2. Referenced Documents
2.1 ASTM Standards:
D149 Test Method for Dielectric Breakdown Voltage and Dielectric Strength of Solid Electrical Insulating Materials at
Commercial Power Frequencies
D351 Classification for Natural Muscovite Block Mica and Thins Based on Visual Quality
D1082 Test Method for Dissipation Factor and Permittivity (Dielectric Constant) of Mica
2.2 ASTM Adjuncts:
Test electrodes
Grid test electrodes
3. Terminology
3.1 For definitions pertaining to this specification see Classification D351.
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. There may be It is possible that other
requirements will be necessary to identify particular characteristics. These will be added to the specification as their inclusion
becomes generally desirable, and the necessary test data and methods become available. Natural It is possible that natural block
mica and mica films that do not conform to the requirements of this specification for capacitor use may well be 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 specification 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 of spots 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.
This specification is under the jurisdiction of ASTM Committee D09 on Electrical and Electronic Insulating Materials and is the direct responsibility of Subcommittee
D09.19 on Dielectric Sheet and Roll Products.
Current edition approved April 1, 2011Nov. 1, 2014. Published April 2011December 2014. Originally approved in 1943. Last previous edition approved in 20052011 as
{1
D748 – 00 (2011). (2005) . DOI: 10.1520/D0748-00R11.10.1520/D0748-14.
Coutlee, K. G., “Electrical Quality Classification of Raw Mica by a Rapid, Direct-Reading Test Method,” Proceedings,ASTM, Vol 46, 1946, p. 1486.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D748 − 14
4.3 In capacitor fabrication, one or more pieces of cut film or block mica having lower than required electrical and physical
properties may will possibly prevent meeting the end requirements of the capacitor. It is therefore required that each piece of block
(cut) or film (cut or uncut), or both, be tested for the electrical requirements and inspected for the visual requirements listed in this
specification.
5. Forms
5.1 This specification covers the following three forms of natural mica, suitable for use in the manufacture of mica-dielectric
capacitors:
5.1.1 Form 1—Full-knife trimmed natural block mica 0.007 to 0.035 in. (0.18 to 0.89 mm) in thickness.
NOTE 1—A certain percentage of block mica thinner than 0.007 in. may be accepted under this specification, upon agreement between the purchaser
and the manufacturer.
5.1.1.1 It is acceptable for a certain percentage of block mica thinner than 0.007 in. to be accepted under this specification, upon
agreement between the purchaser and the manufacturer.
5.1.2 Form 2—Half-knife trimmed natural block mica 0.007 to 0.035 in. in thickness.
5.1.3 Form 3—Films (cut or uncut) split or manufactured from natural block mica in any range from 0.0008 up to and including
0.006 in. 0.006 in. (0.020 to 0.15 mm).0.15 mm).
6. Grades (Sizes)
6.1 For grades see Classification D351.
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 isare 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.
NOTE 1—Based on commericalcommercial experience Class C-1 block mica or mica films are satisfactory for the manufacture of all of these types
of mica capacitors. However, it has been found that some “medium to heavy” air-stained mica maywill produce a slightly lower yield of highest 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 maywill also adversely influence the application of silver electrodes. In addition, there is some
possibility that excessive waviness maywill cause cracked laminations in molded capacitors due to the high molding pressures employed and it may be
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 may result from excessive amounts of either air inclusions or waviness.capacitors.
7.1.3 Class C-2—Class C-2 block mica and mica films have a high order of Q value (1500 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 similar to those specified
for Class C-1 mica. However, a certain percentage of capacitors made with Class C-2 block mica and films may show a somewhat
higher temperature rise in transmitter types than capacitors made with Class C-1 block mica or mica films.
7.1.4 Class C-3—Class C-3 block mica and mica films have the lowest Q value (200 min at 1 MHz in capacitors) of the three
classes covered by this specification. Such Q value, however, is sufficiently high to permit this mica to be classed as a low-loss
insulating material. This mica is particularly suitable for use in foil electrode molded and clamped type capacitors (Note 32) used
in less critical circuits for blocking and coupling purposes where high Q value, high stability, and low temperature coefficient are
not required.
NOTE 2—Experience has shown that silver electrode molded capacitors made with Class C-3 mica, which contained slightly conducting spots and stains
but contained “very slight” air inclusions and “nearly flat” waves, had temperature coefficient and capacitance stability characteristics just as good as that
obtained with capacitors made with the best Class C-1 mica.
8. Electrical and Physical Properties, and Visual Qualities
8.1 Natural block mica and mica films shall conform to the requirements as to electrical and physical properties and visual
qualities as prescribed in Table 1. Visual qualities not covered in this specification are permitted provided mica meets the electrical
and physical requirements.
Coutlee, K. G., “Judging Mica Quality Electrically,” Transactions, Am. Inst. Electrical Engrs., Vol 64, 1945.
D748 − 14
TABLE 1 Electrical, Physical, and Visual Quality Requirements of Natural Block Mica and Mica Films for Use in Capacitors
Class C-1 Class C-2 Class C-3
Electrical Properties:
A
Conducting paths none none none
B B B
Q value or dissipation factor at 1 MHz E-1 E-2 E-3
average
Dielectric strength, min 1000 1000 1000
H
V/mil at 60 Hz single 850 850 850
C C C
Dielectric constant
Physical Properties:
Weight loss on heating (5 min at 600 °C), max % 0.2 0.2 0.2
D
Thickness uniformity (mica films) best best intermediate
E E
Temperature coefficient of capacitance and retrace
F
Visual Qualities:
H H H
Air A very slight very slight very slight
I F I
G
slight slight slight
inclusions
B
J J J
H medium medium medium
C
A nearly nearly nearly
flatslight flatslight flatslight
B
medium medium medium
C
K
Waves heavy heavy heavy
D
Cracks none none none
Tears none none none
Pin holes none none none
Stones none none none
Buckles none none none
Ridges none none none
A
This applies to the conductivity of visible spots and stains only and excludes air stains or air inclusions.
B
The Q values or dissipation factor Q5 of block mica or mica films suitable for use in capacitors shall fall within specified electrical quality groups,
S D
dissipation factor
based on end-use requirements, designated E-1 Special, E-1, E-2, or E-3. These quality groups shall conform to the Q or dissipation factor values prescribed in Table 2
or the corresponding scale readings of the vacuum-tube voltmeter when tested by the rapid, direct-reading method described in Appendix X1.
C
As the dielectric constant of natural block mica suitable for use in capacitors is fairly uniform, no value is specified.
D
Until definite values can be specified, the permissible amounts of such defects shall be agreed upon between the purchaser and the manufacturer.
E
It has been found that the temperature coefficient of capacitance and retrace of capacitors made with Classes C-1, C-2, and C-3 mica are more dependent on such factors
as electrical and mechanical design and manufacturing technique than on any differences in the mica itself.
F
See Classification D351.
G
The amount of air inclusions shall not exceed the specified limits for each subclass. The permissible amount of air inclusions shall be stated by suffixing the letter
appropriate letter (in other words, either A, B, C, or D as the case may be, D) to the required electrical quality class.
H
Few and tiny in one fourth of usable area (must not contain air chains, etc.).and so forth).
I
In one half of usable area (must not contain air chains, etc.).and so forth).
J
In two thirds of usable areas.
K
The waviness shall not exceed the specified limits for each subclass. The permissible amount of waves shall be stated by suffixing the letter appropriate letter (in other
words, either A, B, C or D, as the case may be, D) to the letter denoting the amount of permissible air inclusions. For example, Class C-1 B A block mica or mica films
denotes:
C-1 B A
Best electrical quality slight air inclusion in one half usuable area nearly flat waves
9. Test Methods
9.1 The properties enumerated in this specification shall be determined in accordance with the following:
9.2 Grading According to Size—Classification D351.
9.3 Electrical Conductivity—See Annex A1.
NOTE 3—For the purpose of this specification, electrical conductivity in spotted and stained areas of block mica is revealed when visible sparking or
glowing takes place inside or on the surface of the mica in the vicinity of the test probe, and not by actual puncture of the mica by the high-potential
current. If actual puncture of the test specimen does take place this indicates the presence of mechanical faults, such as pinholes, tears, or cracks which
extend completely through the mica. While this test method had been found suitable for controlling conductivity in spots and stains and dielectric
weakness due to mechanical faults in block mica, an even greater factor of safety will be realized if this flash test is applied directly to capacitor films.
In this instance the purpose of the test is to detect dielectric weakness due to any cause.
9.4 Q Value or Dissipation Factor—Test Method D1082 at 1 MHz, or by the rapid, direct-reading method described in
Appendix X1.
NOTE 4—In cases of dispute arising from borderline cases of Q value or dissipation factor and dielectric strength, the test specimens shall be baked
for a minimum period of 2 h at a temperature of 121°C (250°F), and tested immediately upon cooling to room temperature.
9.5 Dielectric Strength—Test Method D149, using the short-time test with ⁄4-in. (6.4-mm) diameter electrodes in oil.
D748 − 14
9.6 Weight Loss on Heating—Preheat the test specimens in an oven at 121°C (250°F) for a minimum time of 2 h and then weigh.
Then heat the specimens in the oven at 600°C (1110°F) for 5 min and reweigh. Calculate the percentage
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