Dependences of microstructure on electromagnetic interference shielding properties of nano-layered Ti3AlC2 ceramics

The microstructure dependent electromagnetic interference (EMI) shielding properties of nano-layered Ti3AlC2 ceramics were presented in this study by comparing the shielding properties of various Ti3AlC2 ceramics with distinct microstructures. Results indicate that Ti3AlC2 ceramics with dense microstructure and coarse grains are more favourable for superior EMI shielding efficiency. High EMI shielding effectiveness over 40 dB at the whole Ku-band frequency range was achieved in Ti3AlC2 ceramics by microstructure optimization, and the high shielding effectiveness were well maintained up to 600 °C. A further investigation reveals that only the absorption loss displays variations upon modifying microstructure by allowing more extensive multiple reflections in coarse layered grains. Moreover, the absorption loss of Ti3AlC2 was found to be much higher than those of highly conductive TiC ceramics without layered structure. These results demonstrate that nano-layered MAX phase ceramics are promising candidates of high-temperature structural EMI shielding materials and provide insightful suggestions for achieving high EMI shielding efficiency in other ceramic-based shielding materials.

conductive nano-layered structures, the grain size, aspect ratios and the alignment are supposed to have a decisive influence on the EMI shielding properties 20 . For example, significantly enhanced shielding efficiency was recently reported in highly aligned graphene/polymer nanocomposites 20 . However, no studies on the dependences of microstructures on EMI shielding properties of ceramics have been reported to our best knowledge. Accordingly, it is of great interest to study the effects of microstructure on the EMI shielding properties not only to obtain excellent high-temperature EMI shielding properties in MAX phase ceramics but also to unfold the origin of high EMI SE in ceramics with unique microstructure. Compared to Ti 3 SiC 2 , Ti 3 AlC 2 exhibits much better oxidation resistance at high temperatures 23 and is therefore more suitable for high-temperature applications. Accordingly, Ti 3 AlC 2 was chosen as study object, and four typical types of nano-layered Ti 3 AlC 2 ceramics with different microstructures were fabricated by pressureless and hot-press sintering as well as their combination. As a comparison, the EMI shielding properties of highly conductive TiC ceramics without nano-layered structure were also examined. The influences of microstructure on the high-temperature EMI shielding properties were investigated systematically, and the mechanisms for high EMI SE in Ti 3 AlC 2 ceramics were presented.  Table 1). Obviously, all Ti 3 AlC 2 ceramics exhibit distinct microstructures. The pressureless-sintered sample shows certain amounts of pores, which is consistent with its relatively lower relative density around 91% as indicated in Table 1. Besides, the aspect ratio of pressureless-sintered Ti 3 AlC 2 grains is relatively low due to the absence of pressure during sintering. All hot-pressed Ti 3 AlC 2 ceramics exhibit dense microstructures and layered grains with higher aspect ratios. The distributions of grain level dimension for different Ti 3 AlC 2 ceramics were statistically calculated and exhibited in Fig. S1. The majority of grains for pressureless-sintered Ti 3 AlC 2 ceramic (TAC-A) show level dimensions in the range of 8-14 µm. The average level dimension of hot-pressed samples increases significantly with increasing sintering temperature and duration, which is consistent with previous study 24 . Both the Ti 3 AlC 2 ceramics sintered at 1350 °C for 15 min (TAC-B) and 1400 °C for 2 h (TAC-C) show broad level dimension distributions. The TAC-B shows a level dimension peak around 10 µm while the distribution peak of TAC-C locates at a higher size range around 20 µm. A followed thermal process at 1500 °C for 10 h further promotes the grain growth with a dominant percentage of grains with level dimension larger than 25 µm and a considerable percentage of grains with level dimension larger than 40 µm. All  the Ti 3 AlC 2 ceramics exhibit the same single-phase crystal structure without detectable second phases, which can be evidenced by the XRD patterns shown in Fig. S2 in the supporting information.

Results and Discussion
The total SE (SE T ) of EMI shielding materials can be characterized as follows 25 : in which SE T , SE A SE R and SE M denote the total SE, SE due to reflection, absorption loss and multiple reflections, respectively. SE T , SE R and SE A were calculated from the S-parameters as follows 26 , Figure 2a-c compare the frequency dependences of room-temperature SE T , SE R and SE A respectively of Ti 3 AlC 2 ceramics with different microstructures. The SE T exhibits notable differences among various Ti 3 AlC 2 ceramics. The pressureless sintered Ti 3 AlC 2 ceramic exhibits the lowest SE T around 30 dB, which is comparable to those of most ceramic-based composites 5,11,27 . The hot-pressed Ti 3 AlC 2 ceramics exhibit remarkably enhanced SE T compared to pressureless sintered sample. Further, the room-temperature SE T of hot-pressed samples increases monotonically with the increase of grain size. TAC-D ceramic with the largest grain size shows the highest SE T over 40 dB at the whole Ku-band, which means over 99.99% of the incident radiation could be effectively blocked with only 0.01% transmission. The room-temperature EMI SE T of TAC-D ceramic is higher than most ceramic-based EMI shielding composites containing conductive particles or carbon nano-fillers [3][4][5][6]8,27 , while the preparation of Ti 3 AlC 2 ceramics is much less complicated. Moreover, the high SE T of TAC-D ceramic is almost independent of the frequency. In Equation (1), the SE A can be regarded as the energy dissipation of the electromagnetic microwave in the absorber and therefore the multiple reflection SE M in single phase Ti 3 AlC 2 ceramics actually is included in absorption because the re-reflected waves could get absorbed or dissipated within the material 1 . Accordingly, the SE T could be expressed as, In order to have a deeper understanding on the origin of microstructure-dependent EMI shielding properties in Ti 3 AlC 2 ceramics, the dependences of microstructure on the reflection loss SE R and absorption loss SE A of Ti 3 AlC 2 ceramics were investigated and the results were exhibited in Fig. 3b and c respectively. It can be seen that the SE R of Ti 3 AlC 2 ceramics is almost independent of their microstructure. All Ti 3 AlC 2 ceramics with various microstructures show similar SE R values between 10 and 14 dB, especially, the SE R of all the hot-pressed ceramics exhibit almost the same values at the whole Ku-band frequency range. The reflection loss of EMI shielding materials arises mainly from the impedance mismatch between the sample and the free space 25 . The extremely high electrical conductivity (~10 6 S/m) of Ti 3 AlC 2 ceramics could lead to a huge impedance mismatch and therefore give rise to a high reflection loss. The pressureless sintered Ti 3 AlC 2 ceramics exhibit inferior electrical conductivity due to high porosity (Table 1), accordingly, its SE R is relatively lower. While for dense Ti 3 AlC 2 ceramics, the influence of grain size on the conductivity is negligible and therefore the SE R becomes almost independent of the microstructure. All Ti 3 AlC 2 ceramics exhibit high absorption loss SE A (Fig. 2c) which is much larger than SE R , indicating that Ti 3 AlC 2 ceramics are typically absorption-dominant EMI shielding materials. Specially, high SE A of 30 dB which is almost 3 times larger than SE R was obtained for TAC-D ceramic at 12.4 GHz. Being different from SE R , the absorption loss shows obvious grain size dependence and becomes the major contribution to the variation of SE T among Ti 3 AlC 2 ceramics with different microstructures, as demonstrated in Fig. 2c. Hot-pressed Ti 3 AlC 2 ceramics exhibit higher SE A than pressureless sintered sample. For hot-pressed Ti 3 AlC 2 ceramics, the SE A increases gradually with increasing grain size. Therefore, it can be concluded that dense and coarse-grained microstructure with higher aspect ratios is more favorable for high EMI attenuation and the total EMI shielding in nano-layered Ti 3 AlC 2 ceramics.
In order to further validate the significant contribution of the nano-layered structure to the absorption loss, the EMI shielding properties of highly conductive TiC ceramics without nano-layered structures were characterized and compared with those of Ti 3 AlC 2 ceramics. Figure S3 shows the fracture surfaces of TiC ceramics sintered at different temperatures. Both TiC ceramics exhibit different microstructures from Ti 3 AlC 2 ceramics without layered structures. It is apparent that the grain size of TiC-B is much larger than that of TiC-A. The frequency dependences of the EMI SE for different TiC ceramics were shown in Fig. 3. It is interesting to notice that, being different to that of Ti 3 AlC 2 ceramics, the change of grain size did not bring significant variation of SE T for TiC ceramics. Especially, the SE A for fine-and coarse-grained TiC ceramics are almost the same. Compared to Ti 3 AlC 2 ceramics, the TiC ceramics shows much lower SE A while higher SE R . The microstructure-independent SE A of TiC ceramics varies in the range of 18-25 dB at the whole frequency range. The SE R has almost equal contribution to the total shielding for both TiC ceramics. The different shielding properties between Ti 3 AlC 2 and TiC further confirms that the nano-layered structure is responsible for the superior and microstructure-dependent SE A in Ti 3 AlC 2 ceramics. The reflection loss of Ti 3 AlC 2 ceramics is considerably lower than that of TiC ceramics although both ceramics exhibit similar electrical conductivity as demonstrated in Table 1.
Similar to most EMI shielding materials with two-dimensional nano-layered structures, the high SE A of nano-layered Ti 3 AlC 2 ceramics arises mainly from two major attenuation mechanisms: the high electrical/dielectric loss and layered architecture 1,5,19,20,22 . Figure 4 presents the real (ε ′ ) and imaginary (ε″) parts of permittivity as well as the dielectric loss tan defined as 9 of TAC-D ceramic measured at room-temperature. It can be seen that Ti 3 AlC 2 ceramics exhibit moderate real permittivity around 15 while significantly higher imaginary part of ~40. Consequently, the dielectric loss of Ti 3 AlC 2 ceramics reaches a considerably high value around 2.5. The high electrical/dielectric loss of Ti 3 AlC 2 ceramics arises from the abundant free electrons and a large quantity of unpaired defects 16 . The conduction current owing to the skin effect as well as the eddy current will contribute to the ohmic loss, and the unpaired defects will contribute to the dielectric loss. Compared to the electrical/dielectric loss, the multiple reflections also played a significant role in achieving high SE A in nano-layered structures such as graphene and MAX phases, and the multiple reflections are more likely to display microstructure dependences in ceramics. Ti 3 AlC 2 ceramics exhibit layered grains and therefore abundant flat grain boundaries. When the incident EMI radiations encounter those flat grain boundaries, a portion of the EMI radiations gets immediately reflected due to the impedance mismatch of adjacent layers with the others penetrating through these interfaces. The penetrated EMI radiations can be reflected back and forth between the adjacent grain boundaries. These multiple reflections could increase the length of EMI transmission pathway by a large scale and enhance the attenuation of EMI accordingly 1,7,8,20 . In coarse grains with high aspect ratios, a higher degree of multiple reflections can be expected due to the large surface area of a single grain boundary and thereby a higher absorption loss. Consequently, the origin of microstructure dependent EMI shielding effectiveness can be ascribed to the enhanced degree of multiple reflections with increasing level dimensions of layered Ti 3 AlC 2 grains. Figure 5 shows the frequency dependences of SE T at different temperatures up to 600 °C for different Ti 3 AlC 2 ceramics. It can be seen that the SE T of all Ti 3 AlC 2 ceramics only shows a slight decrease with increasing temperature. The variations of SE T from room-temperature to 600 °C for all Ti 3 AlC 2 ceramics is less than 10%. A high SE T around 38 dB was still maintained at 600 °C for TAC-D ceramic, demonstrating that the EMI shielding properties of Ti 3 AlC 2 ceramics have excellent thermo-stability and is well suitable for high-temperature EMI shielding applications. Figure S4 shows the frequency dependences of SE R and SE A of different Ti 3 AlC 2 ceramics measured at   Fig. 2 and Fig. S4, it can be easily observed that for all Ti 3 AlC 2 ceramics the SE R kept almost the same with increasing temperature to 600 °C. Although the electrical conductivity of Ti 3 AlC 2 ceramics tend to decrease with increasing temperature, the giant impedance mismatch between Ti 3 AlC 2 ceramics and the air made the SE A almost unaffected by the slight decrease of electrical conductivity. The decrease of SE T solely arises from the decrease of SE A with increasing temperature. The SE A decrease equally for all Ti 3 AlC 2 ceramics and TAC-D ceramic still exhibits the highest SE A around 26 dB in the Ku-band frequency range at 600 °C. The slight decrease of absorption loss with increasing temperature can be ascribed to the less internal friction of dipole reorientation at high temperatures.

Conclusions
Various Ti 3 AlC 2 ceramics with distinct microstructures were prepared by pressureless sintering and hot-pressing. Their EMI shielding properties were characterized at Ku-band frequency range from room temperature to 600 °C. It was found that the EMI shielding properties of Ti 3 AlC 2 ceramics display remarkable microstructure dependences. High EMI shielding effectiveness over 40 dB was obtained in coarse-grained Ti 3 AlC 2 ceramic and the excellent shielding properties were well maintained at high temperatures up to 600 °C. The variations of EMI SE T with changing microstructure dominantly arises from the variation of absorption loss SE A which is related to the high electric/dielectric loss and more importantly the nano-layered structure of Ti 3 AlC 2 . Coarse Ti 3 AlC 2 grains with high aspect ratio are more favourable for high multiple reflections of EMI waves and consequently more absorption loss. Highly conductive TiC ceramics without nano-layered structure exhibit almost microstructure-independent shielding properties and much lower absorption loss than that of Ti 3 AlC 2 ceramics. These results indicate that Ti 3 AlC 2 ceramic are promising high-temperature EMI shielding materials.

Methods
Ti 3 AlC 2 powder (purity ≥ 98%, ∼300 mesh) was purchased from Forsman (Beijing) Scientific Co., Ltd. Four types of Ti 3 AlC 2 ceramics referred as TAC-A, TAC-B, TAC-C and TAC-D respectively were prepared by different sintering techniques at different temperatures. In the case of TAC-A, the Ti 3 AlC 2 powder was first cold-pressed into pellets of 50 mm in diameter and 2 mm in thickness under a uniaxial pressure of 50 MPa, and then pressureless-sintered in Ar atmosphere at 1450 °C for 120 min; In the case of TAC-B, TAC-C and TAC-D, the Ti 3 AlC 2 powder was placed in a 50 mm diameter graphite die and hot-pressed in Ar atmosphere under different conditions: TAC-B was hot-pressed at 1350 °C for 15 min; TAC-C was hot-pressed at 1400 °C for 120 min; TAC-D was hot-pressed at 1400 °C for 120 min followed by a pressureless sintering in Ar atmosphere at 1450 °C for 500 min. During the hot-press sintering a uniaxial pressure of 30 MPa was applied. Two distinct types of TiC ceramics with 50 mm in diameter and 2 mm in thickness were prepared by hot-pressing at 1800 °C for 15 min (TiC-A) and 1950 °C for 2 h (TiC-B) respectively using commercial TiC powders (∼100 nm, Forsman (Beijing) Scientific Co., Ltd.). A uniaxial pressure of 60 MPa was applied during sintering. The bulk density of the prepared ceramics was measured using the Archimedes' method. Crystalline phases were characterized by X-ray diffraction (XRD). For microstructure characterization, the Ti 3 AlC 2 samples were polished using SiC papers and diamond suspensions down to 0.5 µm. The well-polished surfaces were further etched in an acid solution with a volume ratio of HF: HNO 3 :H 2 O = 1:1:3 for 45 s. The morphology of different specimens was observed by field emission scanning electron microscopy (FE-SEM). The grain size distribution of Ti 3 AlC 2 ceramics was analysed on SEM images (with a magnification of 1000×) of polished and etched surfaces using ImageJ software. In order to ensure the accuracy of measurement, at least 1000 grains from the polished and etched surfaces were considered to statistically obtain the grain level dimension distributions.
For EMI SE characterization, specimens with dimensions of 22.86 mm × 10.16 mm × 1.00 mm were cut and polished. The magnitudes of complex scattering parameters (S-parameters) that correspond to reflection (S 11 or S 22 ) and transmission (S 21 or S 12 ) in the frequency range of 12.4~18 GHz (Ku band) were determined through wave-guide method using a vector network analyzer (Agilent N5230A). For accuracy of measurement, the device is carefully calibrated with Through-Reflect-Line (TRL) approach. The high-temperature measurement of SE was performed in a waveguide heated by an inner heater at a rate of 10 °C/min. The temperature range is 25-600 °C and each temperature spot was stabilized for 10 min in order to ensure the accuracy of measurement.