Dielectric and microwave absorption properties of Ti3SiC2 powders

Accepted Manuscript Dielectric and microwave absorption properties of Ti3SiC2 powders Yi Liu, Fa Luo, Wancheng Zhou, Dongmei Zhu PII: DOI: Reference:

S0925-8388(13)01058-X http://dx.doi.org/10.1016/j.jallcom.2013.04.137 JALCOM 28431

To appear in: Received Date: Revised Date: Accepted Date:

18 January 2013 7 March 2013 20 April 2013

Please cite this article as: Y. Liu, F. Luo, W. Zhou, D. Zhu, Dielectric and microwave absorption properties of Ti3SiC2 powders, (2013), doi: http://dx.doi.org/10.1016/j.jallcom.2013.04.137

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Dielectric and microwave absorption properties of Ti3SiC2 powders Yi Liu , Fa Luo, Wancheng Zhou, Dongmei Zhu (State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, China)

Abstract: Ti3SiC2 powders are synthesized via vacuum pressureless sintering method. The dielectric properties of the powders synthesized at different temperature are investigated in the frequency range from 8.2 to 12.4 GHz(X-band), which suggests the remnant TiC has obvious influence on complex permittivity and the pure Ti3SiC2 powders have the highest dielectric loss. The dielectric and microwave absorption properties of Ti3SiC2 powders are studied. Both the real part (ε′) and imaginary part (ε″) increase with Ti3SiC2 concentration but decrease with increasing frequency, which presents the obvious frequency dispersion effect. Furthermore, the effect of thickness on the microwave absorption property of Ti3SiC2/paraffin mixture is analyzed. The mixture with 70wt% Ti3SiC2 in 2.0mm thickness presents the optimum microwave absorption property. The effective absorbing bandwidth (below -10dB) is almost in the whole frequency range and the lowest reflection loss is -33.83dB. The results show Ti3SiC2 powders are good microwave absorbents in the X-band frequency range. Key words:

Ti3SiC2

Vacuum pressureless sintering

Complex permittivity

Microwave absorption.

Corresponding author at Northwestern Polytechnical University, Xi’an 710072, PR China. Mobile phone number: +8613289327575 E-mail address: [email protected]

1.

Introduction With extensive application of electromagnetic (EM) waves, the problems of

radiation pollution and electromagnetic interference are becoming more serious. In recent years, great attention has been paid to EM wave absorbing materials that can be widely used in equipment electromagnetic shielding, aerospace industries and military technology, as well as in electronic devices and systems for wireless communication. EM wave absorbers are generally consisted of matrix and dielectric/magnetic fillers [1-3]. Absorbents such as carbon black (CB)[4, 5], carbon nano-tubes(CNTS)[6] and carbonyl-iron[7] have been widely utilized in such materials over the past decades. EM wave absorbers with magnetic fillers such as carbonyl-iron present high attenuation in broadband whereas the EM wave absorbers with CB and CNTS filler show low attenuation in narrow band. However, these two kinds of the materials cannot be applied at high temperature due to the former’s demagnetization and latter’s oxidation. In some cases, high temperature EM wave absorbers are crucial. For example, the microwave equipment used to measure the depth of molten steel level in steel making furnace needs the high temperature absorbers. In order to satisfy such special requirement, a new type of absorbent with high temperature stability and good dissipation of EM waves is needed. Ti3SiC2 is the representative and extensively studied phase in the Mn+1AXn ternary compounds (where M is an early transition metal, such as Ti or Cr, A an A group element such as Al or Si, X carbon and/or nitrogen, and n=1–3)[8, 9], which has attracted attention due to its both metallic and ceramic merits, such as high

melting point, excellent electrical conductivity(4.5×106Ω−1m−1), low density(4.52g/cm3), readily machinable and high temperature oxidation resistance[10-14]. These properties make Ti3SiC2 a good potential candidate for high temperature application. The almost pure Ti3SiC2 bulk samples were successfully synthesized through hot isostatic pressing (HIP) used Ti/SiC/C as the reactive powder mixtures [15]. After that, different methods with various reactive powders were used to synthesize Ti3SiC2, such as self-propagation sintering (SHS)[16,17], hot pressing(HP)[18], spark plasma sintering(SPS)[19], pulse discharge sintering (PDS)[20], etc. Most of the previous work has been focused on the synthesis or the mechanical, oxidation resistance properties of Ti3SiC2 and Ti3SiC2-based composites [21-23]. However, the dielectric and microwave absorbing properties of the Ti3SiC2 were rarely reported. In this paper, Ti3SiC2 powders are synthesized at different temperature. Their phase and morphology are characterized. In order to verify the Ti3SiC2 powders’ attenuation to electromagnetic waves, Ti3SiC2 powders are uniformly dispersed in paraffin with excellent transmissivity. The microwave absorption property is evaluated in the frequency range of 8.2-12.4GHz (X-band). Finally, effect of thickness on the absorption performance of the mixture of Ti3SiC2 powders and paraffin is discussed. 2. Experimental procedures Commercially available Ti (purity 99%, -200 mesh), Si (purity 99%, -300 mesh), TiC (purity 99%, -200mesh) were used as the starting materials and Al (purity 99%, -300mesh) as the

sintering additive. The synthesis route was similar to one reported by Zou[11]. The loose bulks prepared at different temperature were grinded into fine powders and sieved with 100 meshes. In order to study the dielectric characteristic of the powders, they were thoroughly mixed with the molten paraffin matrix, characteristic of high transparency to microwave. Then the homogeneous mixture in full flow state was poured into the flange and cool down at room temperature. The testing specimen had a size of 22.86mm length, 10.16mm width and 2.58mm thickness after it was removed from the flange. After that, the dielectric properties of the mixture with different Ti3SiC2 concentration were investigated; the ratios of fillers were 55wt%, 60wt%, 65wt%, 70wt% and 75wt%, respectively. Finally, microwave absorption property of the 70wt% Ti3SiC2/paraffin mixture in different thickness was analyzed. The XRD patterns of the samples were obtained by a Cu Kα radiation (X-Pert Diffractometer, Philips, the Netherlands). The morphology of the samples were observed by scanning electron microscopy (Model JSM-6360, JEOL, Tokyo, Japan). The complex permittivities of Ti3SiC2/paraffin mixture were measured using the rectangle waveguide method in the frequency range from 8.2 to 12.4 GHz by a vector network analyzer (Agilent Technologies E8362B: 10MHz-20GHz). The reflection losses (RL) of the samples were calculated via transmission line theory from the measured dielectric parameters.

3. Results and discussions 3.1 Phase analysis Fig.1a shows the XRD patterns of the samples prepared at 1250 -1400

in

vacuum for 2h. As can be seen from the patterns, narrow diffraction peaks indicate the samples are highly crystallized. Ti3SiC2 occurs in all the samples but impurity, TiC,

exists at the same time. The main peak of Ti5Si3 and another peak of Ti3SiC2 (2θ=40.78 degree) are too close to be distinguished from each other in the XRD patterns. The content of Ti3SiC2 increases firstly then decreases as the synthesizing temperature elevated from 1250

to 1400 . Ti3SiC2 in the sample prepared at 1350

is maximum and the impurity TiC is least. The relative content of Ti3SiC2 and TiC can be roughly calculated by the following experimental formula in the mixture of Ti3SiC2 and TiC system:[24]: WTSC =

WTC =

1 .8 1.8 + I TC ( 200 ) I TSC (104 )

ITC ( 200 ) ITSC (104 ) 1.8 + ITC ( 200) ITSC (104)

(1)

(2)

Where WTSC is the mass fraction of Ti3SiC2 in this system, ITC(200) and ITSC(104) are the integrated diffraction intensity of TiC at 2θ=41.86 degree and Ti3SiC2 at 2θ=39.65 degree, respectively. The Ti3SiC2 and TiC content of the samples after sintering at 1350

are 96.3% and 3.7% according to the formula, while after sintering at 1400 ,

the ratios turn to 90% and 10%, respectively. The main reason is that the powders react sufficiently and more Ti3SiC2 forms as the temperature increases from 1250

to

1350 , but a little of Ti3SiC2 decomposed into TiCx (Here Cx replaced C because the atomic ratio of Ti and C is 3 to 2 in Ti3SiC2, the subscript x in the new formed TiCx is not exactly 1) and Si above 1350 , which have been reported in reference [25] The peak of Si is not detected because of minor mass. Fig.1b shows the variation of relative intensity of the main peaks of TiC (200) phase in the samples synthesized from 1250

to 1400 . The intensity of the highest Ti3SiC2 (104) diffraction peak is

defined as 100, and I was the relative intensity of TiC (200) to the Ti3SiC2 phase. The value of I was the lowest when the sintering temperature is 1350 . 3.2 Microstructure Fig.2 shows the morphology of original samples prepared at 1250 , 1350 1400

and

in vacuum for 2h. Granular TiC, fine laminar Ti3SiC2 and Ti5Si3 are observed

in sample prepared at 1250 , bigger and more Ti3SiC2 particles are found in the sample sintered at 1350

except for only smaller unreacted TiC. Once the nuclears

form, the higher the sintering temperature is, the easier the mass transmission process occurs, which contributes to growth of grains and the reaction of the powders . After sintering at 1400 , not only the laminar Ti3SiC2 particles can be seen in the sample, but also a part of melt zone is showed, which can be ascribed to the decomposition of Ti3SiC2 into Si(l) and TiCx in accordance with the above discussion. 3.3 Dielectric properties 3.3.1 Effect of sintering temperature on dielectric properties Fig.3 shows the real (ε′) part, imaginary (ε″) part and dielectric loss (tanδ) for the mixture with powders sintered from 1250

to 1400

in the frequency range of

8.2-12.4 GHz, and the content of fillers is fixed 70wt%. It can be seen that the real (Fig.3a) and imaginary (Fig.3b) parts decrease firstly then increase with the sintering temperature increase from 1250

to 1400 . The complex permittivity of Sample A

with the powders synthesized at 1250 powders prepared at 1350

are the highest, while the Sample C with

has the lowest complex permittivity. The variation of

permittivity of the samples in Fig.3a and b depends on the characteristic of its constituents. Differences among Ti5Si3, TiC and Ti3SiC2 powders govern dielectric

properties of the above mixtures. The density and conductivity of the intermediate phase Ti5Si3 are similar to those of Ti3SiC2, but TiC shows very different properties from Ti5Si3 and Ti3SiC2. Although the theory density of TiC (4.9g/cm3) is as much as the Ti3SiC2 (4.52 g/cm3), the electrical conductivity of TiC (30×108 S/m) is much higher than Ti3SiC2 (4.5×106 S/m). The content variation of Ti5Si3 and Ti3SiC2 in the samples has a little influence on dielectric properties of the mixtures. Therefore, difference of the complex permittivity can mainly be ascribed to the remnant TiC. The variation of the permittivity of the samples is in a good consistence with the XRD peak intensity of TiC in Fig.1b, which verifies the above deduction. Moreover, we analyze the dielectric loss (tanδ) of the samples by the equation tan δ = ε " , which reflects the attenuation capability of the absorbers for ε' microwave (Fig.3c). Sample A has the lowest tanδ value (0.28-0.2) though the imaginary part is the highest, while Sample C with pure Ti3SiC2 powders has the highest dielectric loss (0.43-0.24), Simple B and D are in the middle. For the microwave absorbing materials, which require the impedance of the absorbers matching the free space impedance, so that the microwave can enter and subsequently be attenuated inside the material. In another word, EM parameters should meet the equation µ ≈ ε then the dielectric loss is larger. As the increasing complex permittivity of the mixture goes away from matching with free space, much of the wave will be reflected which leads to the lower dielectric loss of the absorbers. 3.3.2 Effect of Ti3SiC2 content on dielectric properties Maximum Ti3SiC2 content was obtained at 1350

and it was selected as an

absorbent to investigate its dielectric properties. Fig.4 shows the variation of real (ε′) and imaginary (ε″) part of the mixture with different Ti3SiC2 concentration in the frequency range from 8.2 to 12.4 GHz. With the Ti3SiC2 content increased from 55 to 75wt%, both the real (Fig.4a) and imaginary parts (Fig.4b) increase across the whole frequency range. The complex permittivities decrease with increasing frequency, which is an obvious frequency dispersion effect and crucial for broad band and high absorptivity. With the increasing content of the Ti3SiC2, more interfaces are formed between Ti3SiC2 powders and paraffin matrix. The enhancement of interaction between the Ti3SiC2/matrix and the improvement of interfacial polarization result to the higher real part. The real part ε′ is the function of frequency and can be illustrated by the Debye Theory [26]:

ε ' = ε ∞ + (ε s − ε ∞ )

(1 + ω 2τ 2 )

(2)

Where ω is the angular frequency, εs is the static (zero frequency), ε∞ is the relative dielectric permittivity at light frequency and τ represents the polarization relaxation time relates to temperature. The increase of ωτ with the frequency leads to the decrease of ε′, as shown in Fig.4a. As aforementioned, the imaginary part, which depends upon the electrical conductivity of the mixtures, denotes ability to dissipate electromagnetic wave energy closely concerns to the volume fraction of the Ti3SiC2 powders. According to the percolation theory [27], the dc conductivity of composite following the equation: σ = σ p (v − vc ) x

(3)

Where δp and v is the conductivity and the volume fraction of the filler, respectively, vc is the critical volume fraction of the conducting component or the percolation threshold, x is the constant relates to the dimension of the composite. Eq.3 shows the conductivity of the composite increases with the enhancement of volume fraction as the structure is fixed. Fig.5 demonstrates the dispersion of Ti3SiC2 powders in paraffin matrix with different content. The distance between Ti3SiC2 powder is long when the volume is low, which is not enough to form the conductive network. However, the powders get closer with the increasing volume fraction which leads to enhancement of the conductivity as well as the imaginary part of the mixture. The frequency dependence of the imaginary part in Fig.4b can be explained by the following equation according to the free electron theory [2]:

ε " = δ 2πε f 0

(4)

Where δ is the electrical conductivity, f is frequency and ε0 is the dielectric constant in vacuum. The conductivity of Ti3SiC2 originates from its free electrons has no relationship with other factors , so the imaginary part ε〞decrease with the increasing frequency. 3.4 Microwave absorption properties According to the transmission line theory, the reflection loss of the single layer mixture including Ti3SiC2 powders can be calculated from the measured complex permittivity using the following equation [28]: RL(dB ) = 20 log

Z0 =

µ0

ε0

( Z in − Z 0 )

( Z in + Z 0 )

(5) (6)

Z in = Z 0

µr

⎡ 2π ⎤ ε r tanh ⎢⎣ j c µ r ε r fd ⎥⎦

(7)

where Zin and Z0 are the input impedances of absorber and air, respectively. εr and µr are the relative complex permittivity and permeability of absorber, where µr=1 for nonmagnetic Ti3SiC2. f is the microwave frequency and d is the thickness of the absorber, c is the velocity of light. 3.4.1 Effect of Ti3SiC2 content on microwave absorption properties The calculated reflection losses of the mixtures with different Ti3SiC2 content in 2.0mm thickness are shown in Fig.6. The reflection loss of S1 and S2 are higher than those of S3, S4 because the higher content of Ti3SiC2 in S3 and S4 can turn more microwave energy to heat by leakage conductance. The lowest reflection loss peak of S4 is -33.83 dB at 9.73 GHz which due to combination of attenuation and interference between incident wave and back wave. The effective absorption bandwidth (below -10dB) is almost in the whole frequency range from 8.2-12.4 GHz. 3.4.2 Effect of thickness on microwave absorption properties The reflection losses of S4 in different thickness are shown in Fig.7. It is evidently observed that the matching frequency shifts to the lower frequency region as the absorber thickness increases from 1.8mm to 2.4mm. Electric thickness, almost 1/4 , that offers interference between incident wave and back wave, increases with decreasing frequency. Combined with Fig.6, we find the mixture with 70wt% Ti3SiC2 in a thickness of 2.0mm has the optimum microwave absorption property. 4. Conclusions

Ti3SiC2 powders are synthesized by pressureless sintering in vacuum. The

dielectric properties of the powders are investigated in the frequency range of 8.2-12.4 GHz. The remnant TiC powders have obvious influence on complex permittivity of the Ti3SiC2 powders and paraffin mixture. One layer absorbent with 70wt% Ti3SiC2 content in a thickness of 2.0mm has the desired absorbing bandwidth and lowest reflection loss. Pure Ti3SiC2 powders are required in high temperature wave absorbers for better microwave absorption property. Acknowledgements

This work was supported by National Natural Science Foundation of China, (No.51072165)

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Figure Captions Fig.1 (a) XRD patterns of Ti/Si/2TiC/0.2Al powders after sintering at different temperature in vacuum for 2h; (b) The variation of relative intensity of TiC main peak in the samples after sintering from 1250

to 1400

Fig.2 The SEM morphology of the samples after sintering at (a) 1250 , (b) 1350 and (c) 1400

in vacuum for 2h

Fig.3 The real part (a), imaginary part (b) and dielectric loss (c) of the powders synthesized at different temperature: Sample (A): 1250

(B) 1300

(C) 1350

(D)

1400 Fig.4 The real part (a) and imaginary part (b) of Ti3SiC2/paraffin mixture with different Ti3SiC2 content: S1:55wt%; S2:60wt%; S3:65wt%; S4: 70wt% and S5:75wt% Fig.5 The schematic of different contents of Ti3SiC2 particles disperse in paraffin matrix: (L) low content; (R) high content Fig.6 The reflection loss of the single layer Ti3SiC2/paraffin mixture in 2.0mm thickness: S1:55wt%; S2:60wt%; S3:65wt%; S4: 70wt% and S5:75wt% Fig.7 The reflection loss of S4 in different thickness

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Highlights: We synthesized Ti3SiC2 powders at different temperature. The powders with higher TiC had larger complex permittivity but lower dielectic loss. Thickness and filler content have influence on dielectric and absorption properties. Pure Ti3SiC2 powders are good microwave absorbents in X-band.