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Electromagnetic wave absorption properties of cement-based composites filled with graphene nano-platelets and hollow glass microspheres
Construction and Building Materials 162 (2018) 280–285
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Electromagnetic wave absorption properties of cement-based composites filled with graphene nano-platelets and hollow glass microspheres Xingjun Lv a,b,⇑, Yuping Duan b,⇑, Guoqing Chen b a
School of Civil Engineering, Dalian University of Technology, Dalian 116085, PR China Key Laboratory of Solidification Control and Digital Preparation Technology (Liaoning Province), School of Materials Science and Engineering, Dalian University of Technology, Dalian 116085, PR China b
h i g h l i g h t s Absorbing properties of cement composites were improved after doping GN and HGM. The composites include GN(0.2%) and HGM (40vol%) have the best absorbing properties.
a r t i c l e
i n f o
Article history: Received 1 June 2017 Received in revised form 31 October 2017 Accepted 6 December 2017
Keywords: Cement-based composites Graphene nano-platelets Hollow glass microspheres Microwave absorption
a b s t r a c t The cement-based composites made from graphene nano-platelets (GN) and hollow glass microspheres (HGM) were prepared and its electromagnetic waves absorbing properties were researched in this work. Results show that the absorbing properties were improved after the combination of GN and HGM. As the filling ratio of glass microspheres increases, the value of absorption peak and the bandwidth below 5 dB increases at first and decreases afterwards. In addition, several sharp peaks were obtained and the values tend to appear at high frequency. With further increase of GN, the value at absorbing peak decreases and the curves become relatively flatter. When GN is 0.2%, HGM is 40% (vol/vol) and the thickness is 20 mm, materials have the excellent absorbing properties with the average reflectivity loss being 8.2 dB in the range of 2–18 GHz and the bandwidth was 4.4 GHz below 5 dB. The thickness of sample has a significant influence on the absorbing properties. The optimal thickness is 20–30 mm with 40% (vol/vol) HGM and 0.2% (wt/vol) GN combine together. Ó 2017 Published by Elsevier Ltd.
1. Introduction As the electromagnetic environment of urban space deteriorates, many measures have been taken to reduce potential hazards to human bodies and electronic equipments. Electromagnetic shielding is the most commonly used one among these. However, electromagnetic shielding cannot get at the root of electromagnetic interference (EMI) problems as the reflection caused by shielding materials is prone to cause secondary pollution. Therefore, taking measures from buildings to make them have the electromagnetic wave (EMW) absorbing functions has a very important practical significance. In the military dimension, the ground protection engineering under the information warfare conditions not only needs ⇑ Corresponding authors at: School of Civil Engineering, Dalian University of Technology, Dalian 116085, PR China (X. Lv). E-mail addresses: [email protected] (X. Lv), [email protected] (Y. Duan). https://doi.org/10.1016/j.conbuildmat.2017.12.047 0950-0618/Ó 2017 Published by Elsevier Ltd.
higher antiknock capability, but also needs stealth and electromagnetic immunity functions to improve their ability for resist detection and electromagnetic immunity. Cement is one of the most widely used building materials. After tailoring, it can be rendered electromagnetic functions. However, the EMW absorbing properties of cement materials mainly come from the metal oxides in cement matrix; Moreover, the compact structure of the hydrated cement paste also hinders the incident electromagnetic wave transmission in the interior of the cement composite. This leads to a poor electromagnetic absorption properties of cement paste. To improve its EMW absorption, conductive or magnetic fillers must be introduced as functional inclusions to improve the electromagnetic properties of the cement pastes. Recently, researchers added porous aggregate, such as EPS [1–4], pumice [5], gelatin powder [8,9], fly ash [10] and glass [11] into cement matrix and found that porous aggregate and other wavetransparent particles with low dielectric constant can reduce the
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effective dielectric constant of cement composite and thus reduce the reflection of electromagnetic wave on the surface. This will lead the incident electromagnetic wave into the materials and increase the valid transmission distance. The published reports all show that the introduction of porous particulates have ameliorated the EMW absorption performance of the cement pastes. Hollow glass microspheres (HGM) are commonly used as a kind of filler due to its merits such as high temperature resistance, high corrosion resistance, light quality, high chemical resistance and excellent mechanical and physical properties [6,7,22]. Graphene, as a new type of carbon materials, has the two-dimensional structure made up of carbon atoms with hybrid sp2 orbital. With its high specific area, high chemical stability and high conductivity, graphene has been acted as an ideal microwave absorbing material [12–21]. These research substantiate that GN could accelerate the cement hydration and enhance strength of cement-based composite materials. In this paper, a light cement matrix absorbing plates were prepared with graphene nano-platelets as the EMW absorbent and hollow glass microspheres as the filler. Both its absorbing properties and microstructures were investigated through theoretical research and experimental examination.
2.2.2. Preparation of cement-based microwave absorbing composite The GN and HGM were first mixed in a mixer-agitator with water cement ratio (w/c) of 0.4 and stirred for 1 min. Then the cement was added and stirred for another 4 min. The stirred cement composites were injected into a mould and scraped to make the surface flat. Then the mould was dismantled after they were maintained for 24 h. After moulding, the specimens were conserved in a concrete-curing room for 28 days and then used for electromagnetic and density testing. The size of mould used for absorbing performance test was 200 200 mm with a thickness of 10, 20 and 30 mm. Strength test size was 40 40 160 mm3. The mix proportions of each sample are listed in Table 4.
2. Experimentation 2.1. Materials (1) Cement: ordinary Portland cement P.O42.5R, produced by Dalian Onoda Cement Co., LTD. Its chemical composition is shown in Table 1. (2) HGM: Type T32, produced by Sino Steel Maanshan Mining Research Institute Co., LTD. Its particle size, true density and compressive strength are shown in Tables 2 and 3. (3) Graphene nano-platelets (GN), produced by American Cheap Tubes Inc. through chemical vapor deposition method. Its average thickness, diameter and specific surface areas are about 8–10 nm, 2 lm and 600–750 m2/g, respectively. Its morphology is shown in Fig. 1.
Fig. 1. TEM image of graphene nano-platelets (GNs).
Table 4 Mix proportion and bulk density of each sample.
2.2. Preparation 2.2.1. Preparation of GN dispersed suspension To improve the water solubility of GN and enable them to be dispersed better in water, the GNs was mixed with 300 ml deionized water, and then put in water bath of 30 °C with ultrasonic treatment for 30 min. Then the mixture was dealt with magnetic stirring for 30 min for later use.
Table 1 Chemical composition of the cement matrix. Component
CaO
SiO2
Al2O3
Fe2O3
SO3
MgO
Na2O
Content (wt%)
61.10
21.46
5.25
2.90
2.51
2.08
0.77
Sample
HGM (vol%)
GN (wt%)
Thickness (mm)
Bulk density (g/cm3)
1# 2# 3# 4# 5# 6# 7# 8# 9# 10#
0 40 0 20 40 60 40 40 40 40
0 0 0.2 0.2 0.2 0.2 0.1 0.3 0.2 0.2
20 ± 0.1 20 ± 0.1 20 ± 0.1 20 ± 0.1 20 ± 0.1 20 ± 0.1 20 ± 0.1 20 ± 0.1 10 ± 0.1 30 ± 0.1
1.75 ± 0.02 1.28 ± 0.02 1.70 ± 0.02 1.61 ± 0.02 1.26 ± 0.02 0.97 ± 0.02 1.26 ± 0.02 1.26 ± 0.02 1.26 ± 0.02 1.26 ± 0.02
Table 2 Chemical composition of HGM. Component
CaO
SiO2
Al2O3
Fe2O3
MgO
Na2O
Others
HGM (wt%)
>8.0
>67.0
0.5–2.0
>0.15
>2.5
>14.0
2.0
Table 3 The physical properties of HGM. Colour
Particle size (lm)
Density (g/cm3)
Compressive strength (MPa)
White
10–90
0.32
12–15
Fig. 2. Schematic of the experimental setup for arched testing in the anechoic chamber.
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2.3. Test methods The electromagnetic parameters of each specimen were tested with an Agilent 8720B vector network analyzer (VNA) in a micro wave anechoic chamber. The reflection loss of each specimen was tested by the arched testing method in the frequency range of 2–18 GHz, as is shown in Fig. 2. Vacuum dehydration at 80 °C for 48 h was carried out before the electromagnetic testing, to avoid the influence caused by moisture. The microstructure of each specimen was characterized by field emission scanning electron microscope (FESEM, NOVA NANOSEM450). Strength tests were done refer to Method of Testing Cement-Determination of Strength (GB/T 17671-1999).
3. Results and discussion 3.1. Microwave absorption performances 3.1.1. The influence of HGM contents The EMW absorbing properties of the cement-based composites with a filling ratio of GN at 0.2% and different contents of HGMs are shown in Fig. 3. It is clearly shown that after the introduction of GN into the cement matrix, the absorption peak at different frequency has certain increment and the absorption peak has a little shift to higher frequency band. With the increase of the HGM filling ratio, the value of absorption peak and the frequency bandwidth below 5 dB increases at first and decreases afterwards. In addition, several sharp peaks are obtained and the values of them tend to appear at high frequency. The average reflectivity loss of pure cement sample 1# is 3.5 dB with an absorbing peak of 29.8 dB at 13.5 GHz. The total effective absorption bandwidth with the absorption value below 5 dB is 4.4 GHz. After 0.2% GN was added, the average reflectivity loss of sample 3# turns to 6.8 dB, and the absorbing peak becomes 22.4 dB at 4.7 GHz. As to the effective absorption bandwidth, it increases to about 6.5 GHz with an absorption value below 5 dB, which is much wider than that of the pure cement. With the further introduction of HGM, the absorption performances of the samples 4#, 5# and 6# have much great improvement. As can be seen from Fig. 3, the absorption values of these three samples are all superior to 5 dB when the frequency is higher than 4 GHz. Their effective absorption bandwidth reaches to 12.5 GHz, 14.8 GHz and 14.8 GHz, respectively. For pure cement, due to its compact structure and the impedance mismatching between the air space and the sample, most
Fig. 3. Influence of HGM contents on the reflection loss of the cement composites.
of the incident wave will be reflected at the surface of the sample, which will lead to a poor absorbing property. After the introduction of GN (sample 3#), the microwave absorption can have an improvement at certain degree due to the absorbing characteristics of GN. However, the impedance matching cannot be ameliorated, so the improvement is not obvious. On the contrary, the hollow glass microsphere (HGM) is a kind of wave-transparent material with silicon dioxide as its main composition. It will generate lots of porous structures after being added into the cement matrix. The low electromagnetic parameters of HGM and its porous structure can be able to reduce the electromagnetic parameters of the composites and then form the complicated channels for the transmission of the incident electromagnetic waves. The transmission channels can guide the incident wave going inside the composite materials and then be attenuated accordingly. Moreover, the cement paste adherent to the surface of HGMs will form a kind of structure with closed-cell microspheres. The electromagnetic waves can be scattered and depleted, even produced resonant loss, when they are incident into the scattering cross sections. In addition, when the electromagnetic waves transmit from one microsphere to another, it leads to multiple scattering and multiple reflections. That also leads to the increase of transmission distance and energy loss. After the incorporation of GN and HGM, the lattice vibration caused by the interaction between electric dipole generated on the surface of GN and HGMs also increase the attenuation properties of the composites [22,23]. According to references [24], if the intensity of an incident electromagnetic wave was I0, then the intensity at the position x from the incident surface can be expressed as:
k jvj2 IðxÞ ¼ I0 exp n x 6p 4
where k ¼ 2pf
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
0 l0 l
!
ð1Þ
was the propagation constant of electro-
magnetic wave in medium, k 6jvpj ¼ rSC was the scattering cross section of a microsphere. It can be seen that scattering loss and the scattering cross section of microspheres are related to the microsphere volumes in the matrix. Increasing the scattering cross section could increase the loss. It can also be verified from the experiments that the absorbing properties improved a lot as the increase of HGM. According to the energy conservation equation [25], the absorption coefficient of materials is A = 1 (R + T), among which R and T are the reflection coefficient and transmission coefficient of the incident electromagnetic waves, respectively. The absorbing property is determined by transmission function (impedance matching) and absorptive function (loss property) of the incident wave. Only when (R + T) reaches its minimum value, can the material get its maximum absorption coefficient. As the HGM content increases, the electromagnetic impedance matches well with the space, which leads to the decrease of R and the increase of T. When the HGM content increases to 40 vol%, the absorbing property of the composite gets its best values. However, the adding amount of HGM in cement matrix has a limit value. When the addition exceeds its limit value, the cement composite will turn to wave transparent and so its absorbing property would decrease. 4
2
3.1.2. The influence of GN contents Fig. 4 shows the absorbing properties of cement-based absorbing materials with different content of GN and 40 vol% HGM. As seen in Fig. 4, with the increase of GN, the absorbing peak decreases. When the GN content is 0.1%, the sample 7# has four absorbing peaks of 31.8 dB, 22.4 dB, 19.1 dB and 19.3 dB at 5.5 GHz, 9 GHz, 12.6 GHz and 16.2 GHz, respectively. With the increase of GN content, the absorbing peaks of the samples decrease gradually. When GN is increased to 0.3%, the absorbing
X. Lv et al. / Construction and Building Materials 162 (2018) 280–285
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properties, but a larger thickness shifts the absorbing peaks to lower frequency bands. As to the samples 9#, 5# and 10# with the thickness of 10 mm, 20 mm and 30 mm, respectively, the effective absorption bandwidth below 5 dB are 3.8 GHz, 14.8 GHz and 13.8 GHz, respectively. According to the principle of reflection method [25], when an electromagnetic wave arrives at the surface of a monolayer absorbing plate with a normal incidence, the reflection loss can be expressed as:
R ¼ 20 lg jZ 1=Z þ 1j
Z¼
Fig. 4. Influence of GN contents on the reflection loss of the cement composites.
peaks of sample 8# turn to 12.7 dB, 12.6 dB, 10.7 dB and 10.5 dB, respectively. Due to discussed in Fig. 3, the filling of HGMs can improve the absorbing properties of the cement composites due to the ameliorated impedance matching. Graphene is a new type of carbon material with high specific area and high conductivity, which endows it as an ideal microwave absorbing material. However, when graphene is used alone, its high electrical conductivity always leads to a poor absorbing property. A slight addition in cement can improve its absorption; however, with the further filling of GN, the microwave absorption will be decreased according, as shown in Fig. 4. Furthermore, it can be seen that the content variations of GN only change the absorbing peak values with a same matching frequency. It’s because that the loss mechanism of GN on electromagnetic waves is physical loss than structure loss. The physical loss has important effect on absorbing performance in some particular band, but it does not change the matching frequency. 3.1.3. The influence of sample thickness Fig. 5 shows the reflection loss of cement-based absorbing materials with different thickness when GN is 0.2% and HGM is 40 vol%, from which it can be seen that the thickness has a great influence on the microwave absorbing performances. The samples with thickness of 10 mm and 30 mm have comparable absorption
Zi ¼ Zo
rffiffiffiffiffi lr 2pd pffiffiffiffiffiffiffiffiffi tanh j lr er k0 er
ð2Þ
ð3Þ
where Z presents the normalized input impedance, d expresses the thickness, and k0 is the wavelength of electromagnetic wave in a vacuum. It can be seen from Eqs. (2) and (3) that the scattering loss of absorbing panels has direct relationship with volume rate of HGM and the thickness of specimen. In addition, the absorbing property is a limitless multiplication with the change of thickness. Substantially, increasing the thickness of materials is to increase the amount of absorbing component and the propagation distance of electromagnetic waves in materials, consequently can increase the loss probability and response time of absorbing mechanism in materials. Therefore absorbing properties can be improved by increasing the thickness of materials. When HGM is 40 vol% in the materials and the thickness increases from 10 mm to 20 mm, its absorbing properties would be improved a lot in the range of 2–18 GHz, and the bandwidth below 5 dB increases from 3.8 GHz to 14.8 GHz. The thickness not only has an impact on the interference effect of the incident electromagnetic waves, but also influences the impedance matching of the absorbing materials. With the increase of thickness, the wave impedance of composite changes a lot. Although there is still effective transmission channels for the electromagnetic waves in the materials, the absorbing properties do not show a significant improvement since the change of electromagnetic wave transmission on the surface leads to lots of reflection of the incident wave. So, when the thickness increases to 30 mm, its absorbing properties no longer increase accordingly. For a microwave absorbing material, its matching thickness is related to its electromagnetic parameters and the matching frequency. It can be expressed as Eq. (4).
d¼
c 2pf l00
ð4Þ
From Eq. (4), it is clear that only with a certain thickness can the material get its optimum absorbing performance at a certain frequency. For the cement matrix composite with 40 vol% HGM and 0.2 wt% GN, its optimum thickness is 20–30 mm in the frequency range of 2–18 GHz.
3.2. Microstructures
Fig. 5. Influence of specimen thickness on the reflection loss of the cement composites.
The microstructures of the composite with 40 vol% HGM and 0.2 wt% GN (sample 5#) were examined with SEM and the results are displayed in Fig. 6. It can be seen that the HGM particulates are dispersed in the cement matrix uniformly without obvious aggregates, as is in Fig. 6(a). The diameters of the HGM particulates are distributed in the range of 10–90 lm and some have broken during the stirring process. The GN platelets can also be seen in the matrix, as is shown in Fig. 6(b).
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Fig. 6. Microstructures of the cement composites with 0.2% GN and 40 vol% HGM.
the New Century Excellent Talents in University [No. NCET-130071], the Fundamental Research Funds for the Central Universities (DUT14YQ201, DUT15LAB24, DUT17JC24).
References
Fig. 7. The compressive strengths and flexural strength.
3.3. Mechanical strength The compressive strengths and flexural strength of the cement matrix composites are illustrated in Fig. 7. It is clearly shown that the compressive strength of the composite varies with the filling ratios of GN and HGM. 4. Conclusion The cement-based composites filled with graphene nanoplatelets (GN) and hollow glass microspheres (HGM) were prepared and its electromagnetic wave absorbing properties were studied. As the filling ratio of glass microspheres increases, the value of absorption peak and the effective absorption bandwidth below 5 dB increases at first and decreases afterwards. When GN is 0.2%, HGM is 40 vol% and the thickness is 20 mm, the composite has the excellent absorbing properties with the average reflectivity loss being 8.2 dB in the range of 2–18 GHz and the effective absorption bandwidth reach 4.4 GHz. For cement based composites, the addition of GN improves the strength and decreases with the increase of the amount of HGM. Acknowledgments The authors acknowledge the Supported by Program for the National Natural Science Foundation of China (No. 51577021),
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Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Electromagnetic wave absorption properties of cement-based composites filled with graphene nano-platelets and hollow glass microspheres Xingjun Lv a,b,⇑, Yuping Duan b,⇑, Guoqing Chen b a
School of Civil Engineering, Dalian University of Technology, Dalian 116085, PR China Key Laboratory of Solidification Control and Digital Preparation Technology (Liaoning Province), School of Materials Science and Engineering, Dalian University of Technology, Dalian 116085, PR China b
h i g h l i g h t s Absorbing properties of cement composites were improved after doping GN and HGM. The composites include GN(0.2%) and HGM (40vol%) have the best absorbing properties.
a r t i c l e
i n f o
Article history: Received 1 June 2017 Received in revised form 31 October 2017 Accepted 6 December 2017
Keywords: Cement-based composites Graphene nano-platelets Hollow glass microspheres Microwave absorption
a b s t r a c t The cement-based composites made from graphene nano-platelets (GN) and hollow glass microspheres (HGM) were prepared and its electromagnetic waves absorbing properties were researched in this work. Results show that the absorbing properties were improved after the combination of GN and HGM. As the filling ratio of glass microspheres increases, the value of absorption peak and the bandwidth below 5 dB increases at first and decreases afterwards. In addition, several sharp peaks were obtained and the values tend to appear at high frequency. With further increase of GN, the value at absorbing peak decreases and the curves become relatively flatter. When GN is 0.2%, HGM is 40% (vol/vol) and the thickness is 20 mm, materials have the excellent absorbing properties with the average reflectivity loss being 8.2 dB in the range of 2–18 GHz and the bandwidth was 4.4 GHz below 5 dB. The thickness of sample has a significant influence on the absorbing properties. The optimal thickness is 20–30 mm with 40% (vol/vol) HGM and 0.2% (wt/vol) GN combine together. Ó 2017 Published by Elsevier Ltd.
1. Introduction As the electromagnetic environment of urban space deteriorates, many measures have been taken to reduce potential hazards to human bodies and electronic equipments. Electromagnetic shielding is the most commonly used one among these. However, electromagnetic shielding cannot get at the root of electromagnetic interference (EMI) problems as the reflection caused by shielding materials is prone to cause secondary pollution. Therefore, taking measures from buildings to make them have the electromagnetic wave (EMW) absorbing functions has a very important practical significance. In the military dimension, the ground protection engineering under the information warfare conditions not only needs ⇑ Corresponding authors at: School of Civil Engineering, Dalian University of Technology, Dalian 116085, PR China (X. Lv). E-mail addresses: [email protected] (X. Lv), [email protected] (Y. Duan). https://doi.org/10.1016/j.conbuildmat.2017.12.047 0950-0618/Ó 2017 Published by Elsevier Ltd.
higher antiknock capability, but also needs stealth and electromagnetic immunity functions to improve their ability for resist detection and electromagnetic immunity. Cement is one of the most widely used building materials. After tailoring, it can be rendered electromagnetic functions. However, the EMW absorbing properties of cement materials mainly come from the metal oxides in cement matrix; Moreover, the compact structure of the hydrated cement paste also hinders the incident electromagnetic wave transmission in the interior of the cement composite. This leads to a poor electromagnetic absorption properties of cement paste. To improve its EMW absorption, conductive or magnetic fillers must be introduced as functional inclusions to improve the electromagnetic properties of the cement pastes. Recently, researchers added porous aggregate, such as EPS [1–4], pumice [5], gelatin powder [8,9], fly ash [10] and glass [11] into cement matrix and found that porous aggregate and other wavetransparent particles with low dielectric constant can reduce the
281
X. Lv et al. / Construction and Building Materials 162 (2018) 280–285
effective dielectric constant of cement composite and thus reduce the reflection of electromagnetic wave on the surface. This will lead the incident electromagnetic wave into the materials and increase the valid transmission distance. The published reports all show that the introduction of porous particulates have ameliorated the EMW absorption performance of the cement pastes. Hollow glass microspheres (HGM) are commonly used as a kind of filler due to its merits such as high temperature resistance, high corrosion resistance, light quality, high chemical resistance and excellent mechanical and physical properties [6,7,22]. Graphene, as a new type of carbon materials, has the two-dimensional structure made up of carbon atoms with hybrid sp2 orbital. With its high specific area, high chemical stability and high conductivity, graphene has been acted as an ideal microwave absorbing material [12–21]. These research substantiate that GN could accelerate the cement hydration and enhance strength of cement-based composite materials. In this paper, a light cement matrix absorbing plates were prepared with graphene nano-platelets as the EMW absorbent and hollow glass microspheres as the filler. Both its absorbing properties and microstructures were investigated through theoretical research and experimental examination.
2.2.2. Preparation of cement-based microwave absorbing composite The GN and HGM were first mixed in a mixer-agitator with water cement ratio (w/c) of 0.4 and stirred for 1 min. Then the cement was added and stirred for another 4 min. The stirred cement composites were injected into a mould and scraped to make the surface flat. Then the mould was dismantled after they were maintained for 24 h. After moulding, the specimens were conserved in a concrete-curing room for 28 days and then used for electromagnetic and density testing. The size of mould used for absorbing performance test was 200 200 mm with a thickness of 10, 20 and 30 mm. Strength test size was 40 40 160 mm3. The mix proportions of each sample are listed in Table 4.
2. Experimentation 2.1. Materials (1) Cement: ordinary Portland cement P.O42.5R, produced by Dalian Onoda Cement Co., LTD. Its chemical composition is shown in Table 1. (2) HGM: Type T32, produced by Sino Steel Maanshan Mining Research Institute Co., LTD. Its particle size, true density and compressive strength are shown in Tables 2 and 3. (3) Graphene nano-platelets (GN), produced by American Cheap Tubes Inc. through chemical vapor deposition method. Its average thickness, diameter and specific surface areas are about 8–10 nm, 2 lm and 600–750 m2/g, respectively. Its morphology is shown in Fig. 1.
Fig. 1. TEM image of graphene nano-platelets (GNs).
Table 4 Mix proportion and bulk density of each sample.
2.2. Preparation 2.2.1. Preparation of GN dispersed suspension To improve the water solubility of GN and enable them to be dispersed better in water, the GNs was mixed with 300 ml deionized water, and then put in water bath of 30 °C with ultrasonic treatment for 30 min. Then the mixture was dealt with magnetic stirring for 30 min for later use.
Table 1 Chemical composition of the cement matrix. Component
CaO
SiO2
Al2O3
Fe2O3
SO3
MgO
Na2O
Content (wt%)
61.10
21.46
5.25
2.90
2.51
2.08
0.77
Sample
HGM (vol%)
GN (wt%)
Thickness (mm)
Bulk density (g/cm3)
1# 2# 3# 4# 5# 6# 7# 8# 9# 10#
0 40 0 20 40 60 40 40 40 40
0 0 0.2 0.2 0.2 0.2 0.1 0.3 0.2 0.2
20 ± 0.1 20 ± 0.1 20 ± 0.1 20 ± 0.1 20 ± 0.1 20 ± 0.1 20 ± 0.1 20 ± 0.1 10 ± 0.1 30 ± 0.1
1.75 ± 0.02 1.28 ± 0.02 1.70 ± 0.02 1.61 ± 0.02 1.26 ± 0.02 0.97 ± 0.02 1.26 ± 0.02 1.26 ± 0.02 1.26 ± 0.02 1.26 ± 0.02
Table 2 Chemical composition of HGM. Component
CaO
SiO2
Al2O3
Fe2O3
MgO
Na2O
Others
HGM (wt%)
>8.0
>67.0
0.5–2.0
>0.15
>2.5
>14.0
2.0
Table 3 The physical properties of HGM. Colour
Particle size (lm)
Density (g/cm3)
Compressive strength (MPa)
White
10–90
0.32
12–15
Fig. 2. Schematic of the experimental setup for arched testing in the anechoic chamber.
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2.3. Test methods The electromagnetic parameters of each specimen were tested with an Agilent 8720B vector network analyzer (VNA) in a micro wave anechoic chamber. The reflection loss of each specimen was tested by the arched testing method in the frequency range of 2–18 GHz, as is shown in Fig. 2. Vacuum dehydration at 80 °C for 48 h was carried out before the electromagnetic testing, to avoid the influence caused by moisture. The microstructure of each specimen was characterized by field emission scanning electron microscope (FESEM, NOVA NANOSEM450). Strength tests were done refer to Method of Testing Cement-Determination of Strength (GB/T 17671-1999).
3. Results and discussion 3.1. Microwave absorption performances 3.1.1. The influence of HGM contents The EMW absorbing properties of the cement-based composites with a filling ratio of GN at 0.2% and different contents of HGMs are shown in Fig. 3. It is clearly shown that after the introduction of GN into the cement matrix, the absorption peak at different frequency has certain increment and the absorption peak has a little shift to higher frequency band. With the increase of the HGM filling ratio, the value of absorption peak and the frequency bandwidth below 5 dB increases at first and decreases afterwards. In addition, several sharp peaks are obtained and the values of them tend to appear at high frequency. The average reflectivity loss of pure cement sample 1# is 3.5 dB with an absorbing peak of 29.8 dB at 13.5 GHz. The total effective absorption bandwidth with the absorption value below 5 dB is 4.4 GHz. After 0.2% GN was added, the average reflectivity loss of sample 3# turns to 6.8 dB, and the absorbing peak becomes 22.4 dB at 4.7 GHz. As to the effective absorption bandwidth, it increases to about 6.5 GHz with an absorption value below 5 dB, which is much wider than that of the pure cement. With the further introduction of HGM, the absorption performances of the samples 4#, 5# and 6# have much great improvement. As can be seen from Fig. 3, the absorption values of these three samples are all superior to 5 dB when the frequency is higher than 4 GHz. Their effective absorption bandwidth reaches to 12.5 GHz, 14.8 GHz and 14.8 GHz, respectively. For pure cement, due to its compact structure and the impedance mismatching between the air space and the sample, most
Fig. 3. Influence of HGM contents on the reflection loss of the cement composites.
of the incident wave will be reflected at the surface of the sample, which will lead to a poor absorbing property. After the introduction of GN (sample 3#), the microwave absorption can have an improvement at certain degree due to the absorbing characteristics of GN. However, the impedance matching cannot be ameliorated, so the improvement is not obvious. On the contrary, the hollow glass microsphere (HGM) is a kind of wave-transparent material with silicon dioxide as its main composition. It will generate lots of porous structures after being added into the cement matrix. The low electromagnetic parameters of HGM and its porous structure can be able to reduce the electromagnetic parameters of the composites and then form the complicated channels for the transmission of the incident electromagnetic waves. The transmission channels can guide the incident wave going inside the composite materials and then be attenuated accordingly. Moreover, the cement paste adherent to the surface of HGMs will form a kind of structure with closed-cell microspheres. The electromagnetic waves can be scattered and depleted, even produced resonant loss, when they are incident into the scattering cross sections. In addition, when the electromagnetic waves transmit from one microsphere to another, it leads to multiple scattering and multiple reflections. That also leads to the increase of transmission distance and energy loss. After the incorporation of GN and HGM, the lattice vibration caused by the interaction between electric dipole generated on the surface of GN and HGMs also increase the attenuation properties of the composites [22,23]. According to references [24], if the intensity of an incident electromagnetic wave was I0, then the intensity at the position x from the incident surface can be expressed as:
k jvj2 IðxÞ ¼ I0 exp n x 6p 4
where k ¼ 2pf
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
0 l0 l
!
ð1Þ
was the propagation constant of electro-
magnetic wave in medium, k 6jvpj ¼ rSC was the scattering cross section of a microsphere. It can be seen that scattering loss and the scattering cross section of microspheres are related to the microsphere volumes in the matrix. Increasing the scattering cross section could increase the loss. It can also be verified from the experiments that the absorbing properties improved a lot as the increase of HGM. According to the energy conservation equation [25], the absorption coefficient of materials is A = 1 (R + T), among which R and T are the reflection coefficient and transmission coefficient of the incident electromagnetic waves, respectively. The absorbing property is determined by transmission function (impedance matching) and absorptive function (loss property) of the incident wave. Only when (R + T) reaches its minimum value, can the material get its maximum absorption coefficient. As the HGM content increases, the electromagnetic impedance matches well with the space, which leads to the decrease of R and the increase of T. When the HGM content increases to 40 vol%, the absorbing property of the composite gets its best values. However, the adding amount of HGM in cement matrix has a limit value. When the addition exceeds its limit value, the cement composite will turn to wave transparent and so its absorbing property would decrease. 4
2
3.1.2. The influence of GN contents Fig. 4 shows the absorbing properties of cement-based absorbing materials with different content of GN and 40 vol% HGM. As seen in Fig. 4, with the increase of GN, the absorbing peak decreases. When the GN content is 0.1%, the sample 7# has four absorbing peaks of 31.8 dB, 22.4 dB, 19.1 dB and 19.3 dB at 5.5 GHz, 9 GHz, 12.6 GHz and 16.2 GHz, respectively. With the increase of GN content, the absorbing peaks of the samples decrease gradually. When GN is increased to 0.3%, the absorbing
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properties, but a larger thickness shifts the absorbing peaks to lower frequency bands. As to the samples 9#, 5# and 10# with the thickness of 10 mm, 20 mm and 30 mm, respectively, the effective absorption bandwidth below 5 dB are 3.8 GHz, 14.8 GHz and 13.8 GHz, respectively. According to the principle of reflection method [25], when an electromagnetic wave arrives at the surface of a monolayer absorbing plate with a normal incidence, the reflection loss can be expressed as:
R ¼ 20 lg jZ 1=Z þ 1j
Z¼
Fig. 4. Influence of GN contents on the reflection loss of the cement composites.
peaks of sample 8# turn to 12.7 dB, 12.6 dB, 10.7 dB and 10.5 dB, respectively. Due to discussed in Fig. 3, the filling of HGMs can improve the absorbing properties of the cement composites due to the ameliorated impedance matching. Graphene is a new type of carbon material with high specific area and high conductivity, which endows it as an ideal microwave absorbing material. However, when graphene is used alone, its high electrical conductivity always leads to a poor absorbing property. A slight addition in cement can improve its absorption; however, with the further filling of GN, the microwave absorption will be decreased according, as shown in Fig. 4. Furthermore, it can be seen that the content variations of GN only change the absorbing peak values with a same matching frequency. It’s because that the loss mechanism of GN on electromagnetic waves is physical loss than structure loss. The physical loss has important effect on absorbing performance in some particular band, but it does not change the matching frequency. 3.1.3. The influence of sample thickness Fig. 5 shows the reflection loss of cement-based absorbing materials with different thickness when GN is 0.2% and HGM is 40 vol%, from which it can be seen that the thickness has a great influence on the microwave absorbing performances. The samples with thickness of 10 mm and 30 mm have comparable absorption
Zi ¼ Zo
rffiffiffiffiffi lr 2pd pffiffiffiffiffiffiffiffiffi tanh j lr er k0 er
ð2Þ
ð3Þ
where Z presents the normalized input impedance, d expresses the thickness, and k0 is the wavelength of electromagnetic wave in a vacuum. It can be seen from Eqs. (2) and (3) that the scattering loss of absorbing panels has direct relationship with volume rate of HGM and the thickness of specimen. In addition, the absorbing property is a limitless multiplication with the change of thickness. Substantially, increasing the thickness of materials is to increase the amount of absorbing component and the propagation distance of electromagnetic waves in materials, consequently can increase the loss probability and response time of absorbing mechanism in materials. Therefore absorbing properties can be improved by increasing the thickness of materials. When HGM is 40 vol% in the materials and the thickness increases from 10 mm to 20 mm, its absorbing properties would be improved a lot in the range of 2–18 GHz, and the bandwidth below 5 dB increases from 3.8 GHz to 14.8 GHz. The thickness not only has an impact on the interference effect of the incident electromagnetic waves, but also influences the impedance matching of the absorbing materials. With the increase of thickness, the wave impedance of composite changes a lot. Although there is still effective transmission channels for the electromagnetic waves in the materials, the absorbing properties do not show a significant improvement since the change of electromagnetic wave transmission on the surface leads to lots of reflection of the incident wave. So, when the thickness increases to 30 mm, its absorbing properties no longer increase accordingly. For a microwave absorbing material, its matching thickness is related to its electromagnetic parameters and the matching frequency. It can be expressed as Eq. (4).
d¼
c 2pf l00
ð4Þ
From Eq. (4), it is clear that only with a certain thickness can the material get its optimum absorbing performance at a certain frequency. For the cement matrix composite with 40 vol% HGM and 0.2 wt% GN, its optimum thickness is 20–30 mm in the frequency range of 2–18 GHz.
3.2. Microstructures
Fig. 5. Influence of specimen thickness on the reflection loss of the cement composites.
The microstructures of the composite with 40 vol% HGM and 0.2 wt% GN (sample 5#) were examined with SEM and the results are displayed in Fig. 6. It can be seen that the HGM particulates are dispersed in the cement matrix uniformly without obvious aggregates, as is in Fig. 6(a). The diameters of the HGM particulates are distributed in the range of 10–90 lm and some have broken during the stirring process. The GN platelets can also be seen in the matrix, as is shown in Fig. 6(b).
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Fig. 6. Microstructures of the cement composites with 0.2% GN and 40 vol% HGM.
the New Century Excellent Talents in University [No. NCET-130071], the Fundamental Research Funds for the Central Universities (DUT14YQ201, DUT15LAB24, DUT17JC24).
References
Fig. 7. The compressive strengths and flexural strength.
3.3. Mechanical strength The compressive strengths and flexural strength of the cement matrix composites are illustrated in Fig. 7. It is clearly shown that the compressive strength of the composite varies with the filling ratios of GN and HGM. 4. Conclusion The cement-based composites filled with graphene nanoplatelets (GN) and hollow glass microspheres (HGM) were prepared and its electromagnetic wave absorbing properties were studied. As the filling ratio of glass microspheres increases, the value of absorption peak and the effective absorption bandwidth below 5 dB increases at first and decreases afterwards. When GN is 0.2%, HGM is 40 vol% and the thickness is 20 mm, the composite has the excellent absorbing properties with the average reflectivity loss being 8.2 dB in the range of 2–18 GHz and the effective absorption bandwidth reach 4.4 GHz. For cement based composites, the addition of GN improves the strength and decreases with the increase of the amount of HGM. Acknowledgments The authors acknowledge the Supported by Program for the National Natural Science Foundation of China (No. 51577021),
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