# The Effect of Nanosized Carbon Black on the Physical and Thermomechanical Properties ofAl2O3–SiC–SiO2–C Composite

Authors: Mohamad Hassan Amin; Mohsen Amin- Ebrahimabadi; Mohamad  Reza Rahimipour
Journal: Journal of Nanomaterials
 (2009)

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## Abstract

The effects of using nanosized carbon black in the range of 0–10 weight percentages on the physical and thermomechanical properties ofAl2O3–SiC–SiO2–graphite refractory composites were investigated. Nanosized carbon black addition improved the relative heat resistance and oxidation resistance of composites. The bulk density of the composites is reduced with increasing carbon black (CB) content. Increase in CB content first causes an increase in the apparent porosity, but at more than 3 wt% amount of CB, a decrease of apparent porosity was observed. The cold crushing strength (CCS) increased with increasing CB content in samples fired at 800∘C and in samples fired at 1500∘C when the content is increased to 3 wt%, but the CCS decreased with increasing CB content in samples fired at 1500∘C when the CB content was less than 3 wt%. The composite without CB exhibits the highest value of CCS at firing temperature of 1500∘C.


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## Body

## 1. Introduction

Al2O3–SiC–SiO2 refractory composites
have been widely used as a castable refractory in several applications in the iron and
steel making industry. However,  Al2O3-based castables have poor resistance to corrosion (especially in a basic slag
environment) and spalling  [1]. The growing demand and application of
these castable materials encourage researchers and producers to investigate
their physical, chemical, and thermomechanical properties.The successful development of oxide carbon bricks in
the 1970 s, by adding carbon to oxide brick formulations, suggested that
introducing carbon (graphite) into current oxide based castables would give
improved properties [1]. In recent
years, one of the most significant trends in refractory technology has been
associated to the development of carbon-containing refractory castables. Al2O3–SiC–SiO2–C
refractory composites are commonly used in the iron and steel making industry,
especially in blast
furnace iron/slag runners. With the advent of quality steel making
processes, it became essential to opt for good quality refractories, especially
monolithic castables which were suitable in the presence of molten slag and
metal [2].  The use of graphite as a carbon source has many
advantages in refractory applications [3]. It has a high melting point and promotes the improvement of the corrosion
resistance of the refractory, mainly due to its lower wettability by metal and
slag. Thermal shock resistance of the refractory is improved due to the low
thermal expansion, high thermal conductivity, and low modulus of elasticity
promoted by the graphite addition. On the other hand, these composites have a
few disadvantages, such as oxidation of graphite and reduction of fracture
strength and Young’s modulus because of the presence of graphite flakes.Recent research
has revealed that replacing graphite flakes with nanocarbon powder such as in situ formation of nanofibres in the
matrix of C-containing refractories can markedly improve the refractories
properties and performance [4, 5]. In spite of these studies, the influence of nanosized
carbon black on properties of Al2O3–SiC–SiO2–graphite
refractory composites is not yet clearly understood. This will be addressed in
the present paper by measuring behavior of a typical Al2O3–SiC–SiO2–graphite castable.


## 2. Experimental
Procedure

Alumina–SiC–SiO2–graphite
refractory composites were obtained from a mixture of tabular alumina grains
(Alfa-tab-30,
Silkem, Slovenia), with grain
sizes up to 5 mm, reactive
alumina (PFR 20, Pechiney, France), silicon carbide (EC6-F220, Treibacher, Austria-Brazil), microsilica (971, Elkem Co.,
Norway). All composites contained 2 wt% natural
Chinese flake graphite
(Great Wall Group) with an average flake size of 100 μm, an ash content of 5 wt% and density of 2.26 gr/cm3. A
modified Andreasen grain size distribution (Dinger and Funk) [6],  with maximum grain size of 5 mm and
modulus n  =  0.8, was used for all the composites and the tabular alumina
fractions were varied to obtain the specified grain size distribution. The composite mixed with
amounts
of  0, 1,  2, 3, 4, 5, 6, 7, 8, 9,  and 10 wt% nanoparticles of carbon black (CB), with average size
about 20 nm, and liquid phenolic Novalac resin was used as binder in amount of
0.1% weight also for all composites.Raw materials were weighed (each batch 10 kg) and then dry mixed for
4 minutes in a Hobart mixer. After water addition,
wet mixing was continued for another 4 minutes. The mixture was poured into the
moulds of 50 mm×  50 mm ×  50 mm
without vibration. After demoulding, the
samples were cured for 24 hours at room temperature and cured samples were dried at
110°C/18 h. Then, all compositions were heat-treated in
a reducing atmosphere up to 1450°C for 5 hours  to remove the volatile
species and to minimize their influence.Bulk density
and apparent porosity of samples measured according to ASTM C20-97, and cold
crushing strength (CCS) of the samples was measured according to ASTM C133-97.Relative heat
resistance was determined by the method according to [7]. In this method,
prismatic samples are subjected to abrupt thermal cycling with heating to 850°C
and cooling on water-cooled plate. After the chosen number of cycles, the
samples are tested for bending strength. Relative heat resistance Rrel is evaluated by the formula (1)Rrel=[σF3C(σF7C+0.1)σF]1/2, where σF is ultimate bending strength after
gradual roasting at 850°C, σF3C is ultimate bending strength after three
heating-cooling cycles, and σF7C is ultimate bending strength after seven
heating-cooling cycles. The values of σF, σF3C and σF7C were determined for three pairs of samples.
Samples that withstood thermal cycling were tested for ultimate bending
strength.The
samples were oxidized isothermally at 1500°C for 5  hours, in an
electric furnace by natural convection of air and fraction of weight loss was
calculated (Ox%) according to the equation: (2)Ox%=I.C−R.CI.C×100, where I.C is
the initial weight of carbon before oxidation, and R.C is the weight of residual
carbon after complete oxidation. Carbon contents of samples were determined
using the high-frequency combustion-infrared absorption method (CS-400, LECO).
Observation of the microstructure was performed on polished faces using a
scanning electron microscope or SEM (LEO-460) equipped with an energy
dispersive spectroscopy (EDS) system (beryllium window). Transmission electron microscopy
images were obtained using a Philips TEM operating at the voltage of 200 kV.


## 3. Results and
Discussion

Figure1 shows the
transmission electron microscope (TEM) image of used CB. It is seen that nanoparticles are in spherical
shape. Size measurement on the electron micrographs gave values in the range 5–30 nm, with an average size around 20 nm.Figure 1
Typical electron micrographs of used
CB.Figure2 shows the
dependence of the relative heat resistance of composites on amounts of CB as
additive.Figure 2
Dependence of the relative heat resistance of
composites versus the amount of CB. Bars refer to standard deviation.The results showed that the relative
heat resistance of composites increased
with the amount of
carbon black. This can be correlated with the filling of pores
and voids by CB particles beside gels; so that
the structure of the castable is not compacted and stresses can be relaxed
under the influence of heating cycles [7]. Castable without CB contains two sources of
gel content, namely, alumina gel (the hydration product of hydratable alumina)
and silica gel, which fills the pores and voids; however, castables containing
CB show the higher relative heat resistance owing to the filling of pores and voids by CB
particles beside gels, resulting in
compact structure, because the large density difference between CB and
oxide raw materials. This result is attributed to the ability of the
compressible carbon phase to absorb the expansion of alumina grains. The
probability of  any cracks formed fails to propagate because of the grain
mismatch and consequent poor bonding between CB and ceramic phase [3].The above claim was confirmed
by investigating the microstructure investigation, as shown in Figure3. Figure 3(a)
exhibits an SEM photomicrograph of a composite without CB additive. Some minor
phases can be observed between the alumina grains. These phases have a tendency
to occupy of porous. Figure 4 shows the typical elemental analysis patterns of
these phases. EDS analysis of phases A and S confirms that these phases are
based on alumina and silica, respectively, and phase M has similar stochiometric
composition of mullite. The Al and Si elements are not observed in the X-ray
spectroscopy of the phases C in Figures 3(a) and 3(b). It is concluded that
these phases are just based on carbon.SEM micrographs of
composites (×30), (a) sample without CB and (b)
sample containing 7 wt% CB.
(a)(b)X-ray spectroscopy of
the minor phases in Figure2.
(a)(b)(c)Figure5 shows the
effect of CB content on the composite’s apparent porosity and density. As can
be seen, the bulk density of the composites is reduced with increasing CB
content in samples, due to the lower density of the CB compared with the
density of the other components.Figure 5
The effect of various
amounts of CB on apparent porosity (AP) and bulk density (BD) of composite.
Bars refer to standard deviation.Increase in CB
content first causes an increase in the apparent porosity, but at larger
amounts of CB, a decrease of apparent porosity was observed. The porosity is
reduced by the influence of CB addition, which improves the packing efficiency.
On the other hand,
the porosity is increased by the oxidation of carbon and CO and CO2 gases exiting from the body in
the low amount of CB;  but gas pressure of CO evolved during C oxidation prevents further oxidation
[8]. This claim was
confirmed by the measuring of the amount of residual carbon in samples on
weight percent of CB as additive, as shown in Figure 6.Figure 6
The residual carbon
content (R.C) of composite (wt%) and Ox% versus CB% as additive. Bars refer to
standard deviation.Figure6
shows the oxidation percentage and mass% change of the residual carbon
content (R.C) of composite with CB addition after heating at 1500°C
for 5 hours  in natural air. For the sample without CB, the content graphite was
almost completely oxidized; however, for the sample with CB, a large unoxidized
zone was still observed. The enhancement of gas pressure was considered to occur
by the oxidation of carbon turning to CO and CO2. As a result, the
pores were filled with condensed gases, and  O2 would be expected to
diffuse only a short distance. Consequently, carbon reacted with oxygen just at
the original carbon site or the pores nearby and oxidation of carbon was
reduced. It is considered that the enhancement of gas pressure can effectively
suppress the diffusion of oxygen into the refractories,  and the oxidation rate
was decreased. This result confirmed that the addition of CB effectively
improved the oxidation resistance of the refractories.When used in blast furnace
iron/slag runners, refractory composite castables are subjected to a high
impact of iron/slag melt during tapping of blast furnace. The effect of this
phenomenon on the refractory lining of runners is crushing of the lining,
especially at the impact area of the main trough and the tilting runner.  Thus knowledge of crushing behaviour is very
important to prevent structural spalling and extensive cracking. To establish how
the CB content influences the cold crushing strength (CCS) of refractory
composite castables, CCS was measured
after heat treatment at
800°C and 1500°C.Figure7 shows the variation of CCS
as a function of the CB content at different firing temperature. It can be seen
that the CCS increased with increasing CB content in samples fired at 800°C,
but in samples fired at 1500°C, an increase in CB content first
causes a decrease in the CCS; however, when the content is increased to 3 wt%,
the CCS increases gradually as the CB content increases.Figure 7
The CCS of composite versus
CB% as additive after heat treatment at 800°C and 1500°C. Bars refer to standard deviation.Ceramic matrix composites
fracture occurs by one of the following mechanisms, depending on the pore
structure: catastrophic fracture in dense and tough specimens, with the
formation of one or several transcrystallite cracks; stepped fracture in
specimens with a high concentration of microcracks and pores, with the
formation of numerous transcrystallite cracks; gradual slow fracture in
specimens with low strength.  The
formation of cracks on the surface of the castables depends on their
composition.The
increase in CCS in samples fired at 800°C with CB% is
almost linear, following a typical composite mixture rule. This can be correlated with the filling of
pores and voids by CB particle in castables containing CB (see Figure 3(b)). It seems that, in this
situation, the ceramic matrix composites fracture occurs by gradual slow
fracture in specimens with low strength. One of the reasons invoked to explain
this effect is crack deflection along the nanosized carbon black particles, so
the average stress intensity at its tip is reduced, because the stress is no
longer always normal to the crack plane.The composite
without CB exhibits the highest value of CCS at firing temperature of 1500°C
due to the formation of mullite phase (see Figure 3(a)) that causes the growth of
elongated needle-shaped mullite crystals which strengthens the structure at
high temperatures [9]. Because of
the high reactivity of microsilica and reactive alumina, the formation of
mullite phase occurs at
temperatures above 1300°C, and the porosity is
reduced so there is a significant amount of increase in CCS compared to
specimens fired at 800°C.Also, by
comparing Figures5 and 7, one can immediately see the correlation between the
apparent porosity and CCS of specimens fired at 1500°C.
Therefore, the variation of the CCS values of specimens fired at 1500°C
can also be represented as function of the apparent porosity. Increase in CB
content of specimens fired at 1500°C, first causes
an increase in the apparent porosity and, as a result,
the CCS value decreased because fracture occurs by stepped fracture
mechanism, with the formation of numerous transcrystallite cracks. But at a
larger amount of CB, the CCS decreases with the increasing of CB%. The reason
for this variation of the CCS values can be attributed to the decrease of porosity. Pores are usually quite deleterious to the strength of
ceramics not only because they reduce the cross-sectional area over which the
load is applied, but more importantly because they act as stress concentrators.
Typically the strength and porosity have been related by following empirical
relationship: (3)σp=σ0e−BP, where P, σp, and  σ0  are, respectively, the volume fraction porosity and the strength of the
specimen with and without porosity; B is a constant that depends on the
distribution and morphology of the pores [10].


## 4. Conclusions

Based on the
above results, the following conclusions were drawn when nanosized carbon
black was added as an additive to the Al2O3–SiC–SiO2–C refractory composites.(1)
The relative heat resistance of
composites increased with the amount of carbon black because in the presence of CB, the pores and voids were filled
by CB particles beside gels, so that the structure of the castable is not
compacted and stresses can be relaxed under the influence of heating cycles.(2)
The bulk
density of the composites is reduced with increasing CB content in samples due
to the lower density of the CB compared with the density of the other
components.(3)
Increase in
CB content first causes an increase in the apparent porosity, because of
oxidation of carbon and
exiting of CO and CO2 gases from the body. At more than 3 wt% amount of CB, a decrease of
apparent porosity was observed, because the addition of nanosized carbon black
improves the oxidation resistance of Al2O3–SiC–SiO2–C refractory composites,
and the pores were filled with CB.(4)
The addition of CB
effectively improved the oxidation resistance of the refractories, because CB
plays a role as antioxidant.(5)
The CCS increased
with increasing CB content in samples fired at 800°C
and in samples fired at 1500°C when the
content is increased to 3 wt%, because the crack deflected along the nanosized
carbon black particles, so the average stress intensity at its tip is reduced.(6)
The
composite without CB exhibits the highest value of CCS at firing temperature of
1500°C,
due to the formation of mullite phase that causes the growth of elongated
needle-shaped mullite crystals which strengthens the structure at high
temperatures.(7)
The CCS
decreased with increasing CB content in samples fired at 1500°C
when the CB content was less than 3 wt%, the reason for this variation of the
CCS values can be attributed to the increasing of porosity.



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*Source: 325674-2009-03-26.xml*