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Standard-compliant element analysis of silicon carbide and mixtures containing silicon carbide with

阅读：6103 发布时间：2015-5-15

Elemental analyzers are important tools for quality control of a
wide range of products. A variety of matrices, such as ceramics,
coal, steel or soil, can be analyzed for their element
concentrations with different types of instruments. The product
range of Eltra GmbH, located near Düsseldorf, Germany,
comprises analyzers for C, H, N, S, O and thermogravimetry which
allow for the standard-compliant determination of carbon in
various chemical bondings, as well as oxygen and nitrogen in SiC
and in materials containing SiC. The requirements which have to
be fulfilled for a standard-compliant analysis may, however, vary
greatly, depending on the desired parameters.
1 Introduction
Silicon carbide has a high melting point of 2,700 °C and is therefore an
important raw material for refractory and ceramic products. Another
characteristic of SiC is its resistance against chlorine and strong acids,
also at high temperatures. Thanks to a hardness of 9.6 Mohs, it is also
used in the metallurgical industry for the production of abrasives and
polish.
The European standard series EN ISO 21068 (2008) regulate the
chemical analysis of silicon carbide and raw materials containing silicon
carbide. Part 1 deals with sampling, part 2 with the chemical analysis of
carbon, silicon and loss of ignition and part 3 covers metal analysis and
determination of oxygen and nitrogen concentrations.
Eltra combustion analyzers are well suited for the quality control of
refractory and ceramic products which contain silicon carbide. This article
outlines the possibilities and limits of elemental analyzers when dealing
with these materials.
2 Thermogravimetric parameters
The determination of thermogravimetric parameters such as, for
example, loss of ignition, is described in the second part of ISO 21068.
Thermogravimetry is based on the continuous recording of mass changes
as a function of a combination of time, temperature and atmosphere. The
standard clearly defines the use of muffle furnaces and balances for this
process. All methods described in the standard use a defined sample
container made of steel, ceramic material or platinum which is preheated at the prescribed temperature between 250 °C and 1050 °C. The sample weight is not always defined (e. g. loss on drying LOD250) or
ranges from 2 – 5 g to 1 kg (change of mass in air at 200 °C and 400
°C). After weighing the sample and applying the defined temperature
program (e. g. for LOI850, heating up to 850 °C and maintaining for 3 h),
the hot crucibles need to cool down in the desiccator and are then
weighed.
Thermogravimetric analyzers which are equipped with a combination of
furnace and balance considerably simplify the manual procedure. Usually,
these analyzers have an interior chamber which can be heated up to
1,000 °C and a separate weighing cell in the analysis chamber which is
connected with the furnace by a ceramic pedestal. A rotating carousel
places up to 19 different samples, one after the other, on the pedestal to
be weighed. The market offers thermogravimetric analyzers for both
small sample quantities (e. g. 20 mg) and quantities of 5 g or more (such
as Eltra’s Thermostep, fig. 1). By using an empty reference crucible,
thermal buoyancy is compensated and measurements can be carried out
reliably even at high temperatures. The ceramic crucibles usually have a
volume of 12 ml, allowing for sample weights of up to 5 g. This, however,
is not very practical due to the high filling level. So far, the DIN EN ISO
21068-2 standard does not mention automated thermogravimetric
analysis.

If a thermogravimetric analyzer is used for the determination of LOI850
(loss on ignition at 850 °C), the specifications of the standard can be met
to a high degree, if not 100 %.
For a series of tests the standard Euronorm CRM No. 781-1 was weighed
into crucibles which had been preheated to 850 °C in Eltra’s Thermostep.
Figure 2 shows a typical measurement curve, table 1 contains the corresponding results. The standard was first dried at 105 °C over night
and then 3 crucibles were filled with 1 g each. The LOI850 value measured
with the Thermostep correlates well with the value of free carbon
stipulated in the standard.

An advantage of the thermogravimetric method is the fact that the
samples, after having been heated, are not submitted to ambient air;
however, due to the selected weighing and measurement process it only
represents an approximation to the ISO standard. Currently the method
cannot be adapted to further parameters which is partly due to the
required high temperature (LOI1050) resp. to the large sample weights
(LOI200 with 1 kg sample volume). Due to the missing reference material,
validation of the process remains a difficult task.
3 Determination of the carbon content
3.1 Determination of total carbon
Part 2 of the ISO 21068 series also describes the analysis of silicon
carbide and its carbon content. A careful differentiation has to be made
between the total carbon content and the SiC-bound carbon content.
Depending on the relation of the two bondings, different analysis
methods are stipulated in the standard. There are various ways to
determine the total carbon content in silicon carbide samples, which differ
in the combustion method (resistance or induction furnace) and in the
detection method. Coulometric, gravimetric and conductometric methods
don’t require calibration. The carbon content can be determined by
measuring the weight, the electric charge or the conductivity. For these
detection methods the requirement of chemicals is high; moreover, the
market doesn’t offer adequate instruments.
Alternative procedures permitted by the standard include the use of
elemental analyzers with induction furnace (Eltra CS-800) or resistance
furnace (Eltra CS-580) as well as detection with infrared cells. A
combination of the two combustion technologies and a combined usage of
the detectors have been realized in Eltra’s CS-2000 analyzer (fig. 3).
Fig. 3: ELTRA’s CS-2000 features the unique Dual Furnace Technology
Resistance furnaces with ceramic tube operate at lower temperatures
than induction furnaces. The ceramic combustion tubes allow for a
maximum temperature of 1,500 °C. To safely oxidate the total carbon in
the silicon carbide to CO2, resistance furnaces require the addition of
accelerators such as lead borate or tin powder. Due to a temperature of
approx. 2,500 °C the oxidation of silicon carbide tends to take place more
quickly and more reliably in an induction furnace; moreover, the
measurement results show fewer variations (see table 2). Whereas the
accelerators for the resistance furnaces are used for oxidation (lead
borate) or local temperature rise (tin powder), the metal accelerators
used in induction furnaces are required to ensure combustion in the first
place, as silicon carbide has no proper electric conductivity.
A clearer definition of the calibration methods for the various procedures
would be a welcome extension of the standard. Though [1-2] mention
suitable calibration materials, the calibration with graphite or calcium
carbonate is only described in detail for the detection method with
thermal conductivity cells (chapter 5.4.5). The results shown in table 2
were obtained by calibration with graphite.
3.2 Determination of free carbon
This parameter can be easily determined with elemental analyzers. The
standard stipulates the use of an analyzer with resistance furnace and
infrared detection; however the direct determination is limited (chapter
6.4.1). As the upper limit for the free carbon content is given with 2 %,
analysis solely by combustion, for example of the Euronorm standard
CRM 781-1 with an informative value of 37.22 % is not permitted. Thus,
characterization of free carbon by direct combustion is practically limited
to pure silicon carbide. To measure very low concentrations, the standard
specifies the use of a quartz tube (as is used, for example, in Eltra’s CW-
800 analyzer). The analyzer described in chapter 3.1 of this article is not
suitable for this application; due to the usage of different furnaces,
ceramic tubes cannot simply be exchanged for quartz tubes. Due to their
permeable structure ceramic tubes are not suited for the determination of
very low concentrations. Table 3 shows the results of a standardcompliant analysis of the reference material BAM-S003 and additionally
of the Euronorm 781-1 standard, carried out with Eltra’s CW-800. The
standard was also measured with an analyzer with ceramic combustion
tube (Eltra CS-580). The results in table 3 clearly support the usage of a
furnace with quartz tube when analyzing low carbon concentrations. If
the concentrations are sufficiently high, analyzers with ceramic tubes can
also provide meaningful measurement results. In this case, the
calibration of the elemental analyzer is of great importance. Whereas
higher concentrations can be easily calibrated with graphite (100 %
carbon) or pure calcium carbonate (12 % carbon), low concentrations can
only be calibrated either with an expensive reference material or a
synthetic carbon standard.
3.3 Determination of the silicon carbide content
The SiC content can be determined from the difference between total
carbon and free carbon content; however, this is only permissible when
the free carbon content is 50 % or less of the total carbon content. This
applies, for example, to the BAM S-003 standard but not to CRM 781-1.
If the relation between free and bound carbon is more than 25 %, a
direct analysis is possible. If the CRM 781-1 sample is freed of surface
carbon (table 3), it can be analyzed directly with an induction or
resistance furnace (table 4).
4 Analysis of oxygen and nitrogen
The third part of the DIN EN 21068 series deals with the determination of
oxygen and nitrogen. In contrast to the determination of carbon species,
the sample needs to be heated in an inert gas flow (helium) to measure
its oxygen and nitrogen content. The inert gas fusion method has been
established for many years and is also used for analyzing metals. The
stipulated temperature of approx. 2,800 °C can only be realized in an
electrode/impulse furnace. An upper and a lower electrode apply voltage
to the graphite crucible containing the sample which has previously been
purged with inert gas. The analysis quality is strongly influenced by the
power line available (approx. 5 kW recommended) and the purity of the
flux used (nickel baskets or capsules). These materials are basically used
for melting point reduction to release the gasses contained in the silicon
carbide from the molten sample.
Table 5 shows the results for the BAM S-003 standard obtained with
Eltra’s ONH-2000 analyzer. The analysis was carried out with pre-edged
nickel baskets and a 5 kW power line. After calibration with a certified
steel standard, good compliance with the certified/informative values was
achieved. Primary substances (such as KNO3) can also be used for
calibration; however, these substances need to be dissolved and diluted
first, and are then dried in nickel capsules. The measuring principle does
not allow for direct analysis of liquid standards. The inert gas fusion
procedure can be easily applied to the analysis of other ceramics with
high nitrogen (e. g. Si3N4) or oxygen (e. g. SiO2, ZrO2) content.
5 Conclusion
A comprehensive analysis of silicon carbide and mixtures containing
silicon carbide in accordance with the DIN EN ISO 21068 standard series
involves sophisticated technical equipment. If the determination of
metals, not elaborated on in this article, is taken into account as well, the
additional use of spectrometers (ICP, OES, XRF) is called for. The market
offers analyzers for standard-compliant analysis of carbon
concentrations; however, different analysis specifications require different
configurations (induction furnace, combustion in ceramic or quartz glass
tube). Oxygen and nitrogen are easily and reliably determined with
analyzers using inert gas fusion. For a future update of the standard a
more extensive treatment of thermogravimetric methods would be highly
recommendable. Modern TGA analyzers not only reduce human errors
through automization but, thanks to flexible use of carrier gas and
temperatures, provide manifold possibilities to characterize SiC and
related products.
Author:
Dr. Andre Klostermeier
Product Manager
ELTRA GmbH
Phone: +49 (0)2104/2333-300
: a.klostermeier@eltra.com
Literature
[1] Römp Chemie Lexikon, 10th edition (2002)
[2] EN ISO 21068 (2008), Parts 1, 2, 3

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