ISO/TS 11308:2020

TECHNICAL ISO/TS
SPECIFICATION 11308
Second edition
2020-04
Nanotechnologies — Characterization
of carbon nanotube samples using
thermogravimetric analysis
Reference number
©
ISO 2020
© ISO 2020
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ii © ISO 2020 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and abbreviated terms . 2
5 Principles of TGA . 3
5.1 Measurement . 3
5.2 Exothermic and endothermic reactions . 3
6 Sampling . 3
6.1 Sample pan selection . 3
6.2 Sample size . 4
6.3 Sample compaction . 4
7 Test method . 5
8 Data interpretation and results . 5
8.1 General . 5
8.2 Non-carbon content . 6
8.3 Constituents. 7
8.4 Thermal stability . 7
8.5 Homogeneity . . 7
8.6 Purity . 7
8.7 Quality . 7
9 Uncertainties . 8
10 Test report . 8
Annex A (informative) Case studies .10
Annex B (informative) Effects of operating parameters on TGA analysis .19
Bibliography .22
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
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ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
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iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 229, Nanotechnologies.
This second edition cancels and replaces the first edition (ISO/TS 11308:2011), which has been
technically revised. The main change compared with the previous edition is as follows:
— a generalization has been made from single-walled carbon nanotubes to all forms of carbon
nanotubes (including multi-wall).
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/ members .html.
iv © ISO 2020 – All rights reserved

Introduction
Carbon nanotubes (CNTs) are allotropic forms of carbon with cylindrical nanostructures. As a result
of their geometric structures, these materials exhibit unique mechanical, thermal and electrical
[1][2][3][4][5]
properties . CNTs are synthesized by several different methods, including pulsed laser
vaporization, arc discharge, high pressure disproportionation of carbon monoxide and chemical
[6][7][8]
vapour deposition (CVD) . These processes typically yield a heterogeneous mixture of CNTs and
impurities, often requiring post-synthesis purification. Commonly observed impurities include other
forms of carbon [e.g. fullerenes, amorphous carbon, graphitic carbon, single-wall carbon nanotubes
(SWCNTs) and multi-wall carbon nanotubes (MWCNTs) outside the desired size or chirality range],
as well as residual metallic catalyst nanoparticles. Purification can be accomplished using gaseous,
[9][10][11][12]
chemical or thermal oxidation processes .
Thermogravimetric analysis (TGA) measures changes in the mass of a material as a function of
temperature and time, which provides an indication of the reaction kinetics associated with structural
decomposition, oxidation, pyrolysis, corrosion, moisture adsorption/desorption and gas evolution.
By examining the reaction kinetics for a given sample, the relative fraction of different constituents
present can be either quantitatively or qualitatively determined.
TGA is one of a number of analytical techniques that can be used to assess impurity levels in samples
[14][15][16][17][18][19][20][21][22]
containing CNTs . For CNT-containing samples, TGA is typically used to
quantify the level of non-volatile impurities present (e.g. metal catalyst particles). TGA is also used to
assess thermal stability of a given sample, providing an indication of the type(s) of carbon materials
present. Recent advances in TGA instrumentation enable better resolution during analysis. However,
TGA alone is not specific enough to conclusively quantify the relative fractions of carbonaceous products
within the material. Therefore, the information obtained from TGA should be used to supplement
information gathered from other analytical techniques in order to achieve an overall assessment of the
[23][24][25][26][27][28][29]
composition of a CNT sample .
TECHNICAL SPECIFICATION ISO/TS 11308:2020(E)
Nanotechnologies — Characterization of carbon nanotube
samples using thermogravimetric analysis
1 Scope
This document gives guidelines for the characterization of carbon nanotube (CNT)-containing samples
by thermogravimetric analysis (TGA), performed in either an inert or oxidizing environment. Guidance
is provided on the purity assessment of the CNT samples through a quantitative measure of the types
of carbon species present as well as the non-carbon impurities (e.g. metal catalyst particles) within the
material.
In addition, this technique provides a qualitative assessment of the thermal stability and homogeneity
of the CNT-containing sample. Additional characterization techniques are required to confirm the
presence of specific types of CNT and to verify the composition of the metallic impurities present.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO/TS 80004-3, Nanotechnologies — Vocabulary — Part 3: Carbon nano-objects
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO/TS 80004-3 and the
following apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1
primary oxidation temperature
T
ox'
temperature at which the most intense peak occurs in the first derivative thermogravimetric curve
3.2
thermal stability
dimensional stability of a solid material heated under specified conditions
Note 1 to entry: Thermal stability can be affected by the relative fraction of material constituents (3.4) in the
sample as well as physical characteristics of the nanotube materials, such as diameter, length, defect state or
surface treatment.
3.3
homogeneity
degree to which a property or a constituent (3.4) is uniformly distributed throughout a quantity of
material
EXAMPLE Primary oxidation temperature.
3.4
constituent
component present in a carbon-nanotube-containing sample
Note 1 to entry: A carbon-nanotube-containing sample is often comprised of different carbon and non-carbon
materials and is identified by oxidation peaks in the thermogravimetric analysis curve and by residual weight.
3.5
monotypic
material consisting of only one type of carbon nanomaterial
Note 1 to entry: When multi-walled carbon nanotubes are present, monotypic samples may contain a narrow
range of diameters and lengths.
Note 2 to entry: A typical carbon nanotube sample comprises several types of carbon nanomaterials, including
amorphous carbon, fullerenes, single-wall carbon nanotubes and multi-wall carbon nanotubes.
3.6
purity
measure of the fraction (percentage weight or mass fraction) of impurities within a given sample
Note 1 to entry: Thermogravimetric analysis alone cannot conclusively quantify the relative fractions of any
and all carbonaceous products within the material. It can, however, quantify the level of non-volatile (e.g. metal
catalyst) impurities, which is one measure of purity.
3.7
quality
overall assessment of the carbon nanotube sample
Note 1 to entry: A high quality carbon nanotube material is taken as a material with high purity (3.6), structural
integrity and homogeneity (3.3).
Note 2 to entry: Thermogravimetric analysis can partly contribute to the quality assessment of carbon-nanotube-
containing material by providing its residual weight and oxidation temperature.
Note 3 to entry: A carbon nanotube material may have a high purity level (i.e. a net mass fraction of 100 %)
but it may have a considerable amount of damage, which can alter or destroy its physical properties, thereby
deteriorating the quality of the material.
4 Symbols and abbreviated terms
CNT carbon nanotube
CVD chemical vapour deposition
DSC differential scanning calorimetry
DTGC derivative thermogravimetric curve (sometimes known as the “derivative weight loss
curve”)
HiPco high pressure CO conversion
MWCNT multi-wall carbon nanotube
SWCNT single-wall carbon nanotube
TGA thermogravimetric analysis
T oxidation temperature
ox
W residual mass of the sample after heating
res
2 © ISO 2020 – All rights reserved

5 Principles of TGA
5.1 Measurement
At the basic functional level, TGA measures the change in mass of a material as a function of
temperature as it is heated at a specified rate. In order to accomplish this, TGA requires the precise
measurements of mass, temperature and temperature change. The change in mass of a material is
related to the composition of the material and its thermal reactivity with the atmospheric conditions
of the measurement. TGA analysis of CNT samples is most commonly performed in an oxidizing
atmosphere, but can also be done with inert, reducing or other atmospheric conditions to probe
different thermal reaction kinetics. Mass loss relative to an increase in temperature can result from
the removal of absorbed moisture, solvent residues, chemically bound moieties and/or the thermal or
oxidative decomposition of product.
TGA alone cannot identify the volatile materials released during heating; analytical techniques such
as mass spectrometry (MS), gas chromatography (GC) and Fourier-transform infrared spectroscopy
(FTIR) can be combined with TGA in order to identify volatile materials by coupling the appropriate
instrumentation to the exhaust. Similarly, with respect to CNT-containing materials, TGA cannot by
itself identify the different carbon forms present within the material. What it does do is determine the
temperature at which the carbon species oxidize, which can be indicative of the identity of the species,
as well as provide a quantitative measure of the non-volatile component.
When a CNT-containing sample is subjected to elevated temperatures in the presence of air, the carbon
species present will oxidize into gaseous compounds such as CO or CO . The residue comprises non-
volatile materials, which for the most part are metal impurities.
5.2 Exothermic and endothermic reactions
Some CNT-containing samples have been observed to undergo combustive reactions during TGA
analysis, resulting in rapid burning of material, possibly catalysed by residual metals in the sample.
Such events are distinguished by a difference in temperature between the sample and the reference,
[30]
but also a rapid change in sample mass with little or no change in reference temperature .
6 Sampling
6.1 Sample pan selection
Sample pan size and type will vary depending on the instrument being used and the temperature range
of interest. There is no restriction on the sample pan size so long as it is compatible with the instrument
and it is capable of accommodating the required amount of CNT sample with minimal compaction (see
6.3 and the references within for a discussion of sample compaction). Aluminium, ceramic or platinum
pans may be used depending on the experimental temperature range. Aluminium pans are the least
likely to catalytically oxidize the CNT material, but they do not cover as wide of a temperature range.
Ceramic and platinum pans are more likely to have adsorbed contaminants that can lead to inconsistent
or even erroneous data. Table 1 provides guidance to some of the available pans. Standard manufacturer
recommended procedures should be followed to remove any residual material from previous samples
prior to conditioning. To remove adsorbed moisture, it is recommended that the pans be conditioned by
heating to at least 300 °C in an air environment prior to introduction of the CNT sample. If the pans are
not used immediately after conditioning, they should be stored in a dry box or desiccator until loading
to prevent the reintroduction of moisture.
Table 1 — Sample pan selection guidance for TGA of CNTs
Pan material Temperature range Comments
Aluminium pans are considered disposable by manufacturers,
which is beneficial in eliminating contamination. However, the low
Aluminium Ambient to 600 °C
maximum temperature means they can only be used when it is
certain that all the CNT material will burn off before 600 °C.
Porous ceramic will easily pick up moisture from the atmosphere,
so it is especially important to condition the pans for consistent
Ambient to 1 200 °C
Ceramic results. Ceramic pans are capable of use above 1 200 °C;
and above
however, most TGA instruments are limited to an upper tempera-
ture of 1 200 °C.
It is important that the platinum pan is rated for high temperature
Platinum Ambient to 1 000 °C
use otherwise the maximum temperature is only 750 °C.
6.2 Sample size
The controlling factor in the selection of sample size is the bulk density of the CNT material. As-
produced samples can be more difficult to accommodate in TGA pans because of their lower bulk
density, whereas purified materials are condensed during the purification processes. If the minimum
recommended amount of the sample is too fluffy for the available TGA pan, slight compacting with a
spatula may be used to contain the sample within the pan.
Additional details on sampling can be found in Reference [31].
For dry powder samples:
a) use a minimum of 1 mg of sample or the amount recommended for the instrument by the
manufacturer for the specific pan size being used, whichever is greater;
b) load samples into pan and store in a dry environment for at least 48 h prior to measurement; if
possible, load samples in a controlled environment such as a glove box or nano-hood to prevent the
airborne release of nanotubes;
c) weigh samples at an ambient temperature on a microbalance.
6.3 Sample compaction
Compaction of a powder using a press is a common method of preparation for TGA and DSC
measurements of powder samples. The effects of high pressure compaction of SWCNT samples have
[31]
been investigated and it has been found that compaction in a KBr pellet die (such as those commonly
used for the preparation of samples for infrared spectroscopy) can influence the observed oxidation
temperatures, though no effect was found on the residual mass values. Details of necessary provisions
concerning compaction are described further in B.3.
The following rules regarding sample compaction apply:
a) do not use high pressure sample compaction (e.g. with a pellet die);
b) slight compaction by low pressure pressing with a spatula is acceptable to contain the full sample
within the pan.
4 © ISO 2020 – All rights reserved

7 Test method
The following procedure outlines the minimum requirements for obtaining TGA data that will allow
for a reliable characterization of CNT samples. At a minimum, this procedure should be repeated three
times for each sample. A typical run usually takes approximately 2 h, depending on the ramp rates used.
a) Calibrate the TGA instrument according to the manufacturer’s protocols to ensure the proper
temperature and weight measurement.
b) Prepare the TGA instrument by first taring an empty sample pan in the TGA balance. On a
microbalance, weigh and record a minimum of 1 mg of sample and transfer into the pan. If possible,
perform all weighing and transfer operations in a dry environment. The samples should be kept in
a desiccator for 48 h to remove any retained moisture. If the equipment allows it, store the loaded
pan in the same way. Alternatively, if the tare function is not available in the instrument (as may be
the case with older instrumentation), weighing the sample in a dry environment is acceptable.
c) Transfer the sample, or the loaded pan, to the TGA instrument for analysis.
d) Set the maximum temperature for the scan based on the anticipated composition of the sample and
the sample pan being used. If the sample is unknown, set the maximum temperature to 900 °C or
the highest temperature that can be used with the sample pan selected.
e) Set the temperature ramp rate to a constant rate between 1 °C/min and 10 °C/min for the entire
temperature range. Slower heating rates allow more time for reactions to occur.
NOTE 1 If there are known or suspected organic impurities in the sample, the sample can first be heated
to 120 °C with an inert gas flow and held at this temperature for 30 min prior to heating over the entire
temperature range. This additional heating step can also be used if the sample is unknown to ensure that
there is sufficient time for evolution of any organic impurities.
f) Set the gas flow rate based on the manufacturer’s recommendation for the particular TGA
instrument. If no recommendation is given, set the flow rate to no less than 100 ml/min.
NOTE 2 The flow rate needed for successful operation will be dependent on the instrument geometry. If
no flow rate is recommended by the manufacturer, the most important consideration is that the flow rate
provides an optimal burn rate of the sample while minimizing any buoyancy effects on the sample weight
determination.
NOTE 3 Air or oxygen is used as the gas for oxidative decomposition studies while an inert gas such
as nitrogen or argon is used for pyrolytic decomposition studies. Other gases can be used for different
situations.
NOTE 4 Cooling rate is instrument dependent.
g) After completion of the measurement, record the residual mass (W ) value for each scan at room
res
temperature on a microbalance.
h) Record the oxidation temperature (T ) for each peak within a scan. The overall T for each species
ox ox
attributing to the TGA curve is determined from the mean value of the three runs. T for the
ox
particular species is then documented as the mean value plus and minus the standard deviation.
8 Data interpretation and results
8.1 General
The following are guidelines for the interpretation of the TGA curves and the type of information used
to evaluate CNT-containing materials. An example of TGA curves for four different CNT samples (each
produced by a different method) is shown in Figure 1.
Key
X temperature (in °C)
Y1 weight (in %)
Y2 deriv. weight (in %/°C)
[32]
Figure 1 — TGA curves for CNT samples produced by different methods
8.2 Non-carbon content
The non-carbon content of the CNT-containing sample is assessed through the W value. This value is
res
acquired from both the TGA data and a microbalance value. From the TG curve, W is recorded as the
res
mass remaining above 800 °C. This value is compared to the microbalance weight in order to assess if
there is a systematic error due to buoyancy effects caused by the air flow in the instrument. W can
res
be expressed as either the actual weight that remains or as a percentage of the original weight of the
sample. To report the non-carbon content of a CNT-containing sample, W will be expressed as the
res
mean percentage weight together with the standard deviation from at least three TGA measurements.
NOTE The determination of non-carbon content in the original CNT sample from W can be inaccurate as
res
some contributors to the residual will change oxidation state during heating, resulting in either a decrease or
increase in weight. W will, however, still provide a good approximation to the overall non-carbon impurity
res
component of the CNT material, especially when comparing samples from the same production method (see A.2)
that will likely have the same impurities.
It is recommended that the user verify the oxidative stability of the metals used as catalysts by
conducting TGA analysis of the catalyst at identical heating and air flow rates. These measurements
will establish whether W measures metals, metal oxides or a mixture of the two. If the catalyst
res
composition is unknown, the largest temperature range should be used.
6 © ISO 2020 – All rights reserved

8.3 Constituents
Most production methods for CNTs are not 100 % efficient in the conversion of the source carbon to
a single species. Single-wall production methods do not eliminate multi-wall tubes and vice versa.
Additional forms of carbon such as fullerenes or amorphous carbon also cannot be fully eliminated at
the time of production. The presence of multiple constituents can be qualitatively determined from TGA
[34]
data by determining the number of oxidation peaks present in the DTG curve . While it is difficult to
assign any particular carbon form to a specific oxidation peak, especially as the number of constituents
in the sample increases, it is commonly agreed upon that multiple peaks arise due to the presence of
different carbon types (see A.3).
NOTE TGA data from relatively pristine samples have shown that single, double and multi-wall CNTs
have unique ranges of oxidation temperatures, which can be used to distinguish between the different CNT
[34]
constituents if it is known that there is minimal overlap with any non-CNT carbon constituents .
8.4 Thermal stability
The thermal stability of a sample of CNT material is the dimensional stability of a solid material heated
under specified conditions. Thermal stability of a sample of CNT material can be affected by the relative
fraction of material constituents in the sample as well as physical characteristics of the nanotube
materials, such as diameter, length, defect state or surface treatment. The parameter used to assess
thermal stability is the primary oxidation temperature, which is the temperature at which the highest
fraction of carbon content oxidizes (see A.4 and Reference [30]).
8.5 Homogeneity
The homogeneity of CNT materials is established in TGA by the constituency, thermal stability and
scatter in the T and W values of multiple runs (see A.5 and Reference [30]). A material is considered
ox res
sufficiently homogeneous if it meets pre-defined quantitative limits for the following criteria.
The TGA data from a minimum of three runs:
— should produce the same set of oxidation peaks (same constituency);
— should have a similar primary T (thermal stability) from run to run;
ox
— should have T and W values with a narrow standard deviation (see Annex A).
ox res
If all three conditions are not met, the material cannot be considered homogeneous.
8.6 Purity
The purity of a either a SWCNT or a MWCNT sample is established by the mass fraction of the desired
CNTs relative to both the carbon and non-carbon impurities within the material. TGA alone can only
reliably provide purity assessment relative to the non-carbon impurities through the W value. A
res
material with lower residual values is therefore considered a material with better purity relative to
the non-carbon impurity content. To clearly define the overall purity of a material, TGA results must be
coupled with information from other techniques (see A.6).
8.7 Quality
As with purity assessment, the quality assessment of a CNT sam
...