Preparation and Dielectric Properties of Si₃N₄/BN(CB) Composite Ceramic

Preparation of BN-coated nano-CB

Nano-CB, with diameter around at 30 nm (Cabot, Boston, Massachusetts, USA), was used as raw materials. Boric acid (reagent grade, Xilong Chemical Co., Ltd, Shan’tou, China) and urea (reagent grade, Xilong Chemical Co.) were used to produce the BN precursor to coat the CB. The experimental procedure is presented schematically in Figure 1. Boric acid and urea with the mole ratio of 1:3 were dissolved in non-aqueous ethanol to form solution. Then the CB was dispersed into the solution. This mixture was milled by using the wet ball-milling method with agate balls and ethanol in a plastic bottle for 12 h. After drying, the BN precursor-covered particles were heated at 1,200°C in nitrogen gas for 4 h. Under this condition, the boric acid reacted with urea and BN-coated CB was obtained. After reaction, the weight content of BN coating in composite particles is 33 wt% by adjusting the ratio of boric, urea and CB. At last, keep the composite particles heat treatment 2 h at 400°C to remove the CB which has not been covered fully.

Figure 1:

Flowchart of fabrication process of BN-coated CB.

Preparation of Si₃N₄/BN(CB) composite ceramics

First, the mixture powder of α-Si₃N₄ (purity >93%, d₅₀ = 0.5 μm, Beijing Unisplendor Founder High Technology Ceramics Co., Ltd, China.), Al₂O₃ (10 wt%, Xilong Chemical Co., Ltd. China) and Y₂O₃ (5 wt%, Changsha Deli Rare Earth Chemical Co., Ltd. China) were added to the mixed acrylamide (AM) and N, N′-methylenebisacrylamide (MBAM) (Sinopharm Chemical Reagent Co., Ltd., China) system by ball-milling till solids loading up to 40 vol.%, then the BN(CB) and Si₃N₄ powers were dispersed into a premix solution. After degassing for 10–15 min in a rotary-vane pump under vacuum, the initiator and catalyst were applied to the slurry. All the above steps were operated at room temperature. Then the slurry was cast into metal mold and moved into an oven at temperature of 60–80°C. After demolding and drying, these green bodies were calcined at 1,600°C for 3 h under N₂ atmosphere. The density and open porosity of as-prepared Si₃N₄/BN(CB) composite ceramics are 2.12 g/cm³ and 3.4%, respectively.

Characterization

The chemical compositions of the coatings on CB were analyzed by X-ray photo-electron spectroscopy (XPS, K-Alpha 1063) with Al Kα radiation. Phase analysis was conducted by X-ray diffraction (XRD, D/max 2550). Scanning electron micrographs (SEM, Nova Nano SEM 230) were used for morphological characteristics. The complex permittivity (ε′, ε′′) of BN(CB) composite powder and Si₃N₄/BN(CB) composite ceramics with size of 22.86 mm × 10.16 mm × 5 mm were measured in the frequency range of 8.2–12.4 GHz at room temperature using a network analyzer (Agilent N5230A).

Microstructure and dielectric properties of BN-coated nano-CB

Before coating, the XRD analysis (Figure 2(a)) shows that the broad diffraction peaks of CB are appeared at 26.8° and 41.2°, respectively, which indicates the amorphous structure. After coating, the sharp peaks appeared at 26.6° (002), 41.5° (100), 43.8° (101), 50.2° (102), 55.2° (004) and 75.9° (110) (as shown in Figure 2(b)) correspond to the crystal structure of h-BN when the precursor-covered CB were heated at 1,200℃ for 4 h [26], which overlaps with the diffraction peaks of the CB.

Figure 2:

XRD spectra of (a) the pure CB and (b) the BN-coated CB.

XPS spectra further confirm the formation of BN coating on the CB particles, as shown in Figure 3. In Figure 3(a), the B 1s spectra with peak centered at 190.9 eV corresponds to the B–N bonding [27]. And the peak centered at 398.5 eV in N 1s spectra shown in Figure 3(b) is identified as N–B bonding [28].

Figure 3:

XPS spectra of (a) B 1s and (b) N 1s for BN coating.

Figure 4 shows the SEM images of the CB coated by BN and EDS of BN coating. As shown in Figure 4(a), nano-CBs are covered by BN coating and present obvious phenomenon of agglomeration due to their high surface energy. Figure 4(b) shows high magnification of the BN coating, which has a lot of tiny particles. The energy dispersive spectroscope (EDS) (Figure 4(c)) analysis confirms the existence of BN on CB nanoparticles. The presence of O peak may be attributed to the residual B₂O₃ in the synthetic process.

Figure 4:

(a) SEM of BN(CB); (b) SEM of high magnification of X1; (c) (EDS) analysis of Figure 4 (b).

Figure 5 shows the permittivity (ε′, ε′′) of the CB with and without BN coating. For permittivity measurement, the samples were prepared by mixing the particles with molten paraffin with mass ratio of 1:4 by using the mold with dimensions of 22.86 mm × 10.16 mm × 5 mm. As shown in Figure 5, CB has high dielectric constant as an absorbing material, the permittivity (ε′, ε′′) of the CB with BN coating is obviously lower than that of the pure CB. The real part (ε′) and imaginary part (ε′′) of permittivity of the BN(CB) composite powders decrease from 9.9 to 6.7 and from 6.3 to 6.18 in the frequency of 8.2–12.4 GHz, respectively.

Figure 5:

The permittivity (εʹ εʹʹ) of the CB and BN/(CB).

Figure 6 shows the characteristic impedance of the CB particles and the BN-coated CB particles at different frequency. As shown in Figure 6, the impedance of the CB particles ranges from 0.13 to 0.15. And the impedance of the BN-coated CB particles ranges from 0.29 to 0.32 which is close to the Z₀ (Z₀ = 1, Z₀ is impedance under vacuum) compared with that of the CB particles, indicated that electromagnetic wave can enter into the BN-coated CB particles and be dissipated easily due to the high conductivity of CB particles [29]. Therefore, we added BN(CB) as a kind of efficient absorbent into Si₃N₄ to improve the materials with good absorbing properties in the following study.

Figure 6:

Characteristic impedance of the CB particles and the BN(CB) particles at different frequency.

Microstructure and dielectric properties of Si₃N₄/BN(CB) composite ceramics

XRD analysis (Figure 7(a)) shows the characteristic peaks of BN, CB and Si₃N₄ were detected in the Si₃N₄/BN(CB) composite ceramics. The diffraction peaks of the BN and CB overlap with each other. The diffraction peaks at 26.6° and 41.5° are assigned to h-BN, and the diffraction peaks of CB are corresponding to 26.8° and 41.2°. Figure 7(b) shows a new substances of β-SiC phase in the Si₃N₄/CB composite ceramics which was generated by Si₃N₄ reacting with CB at 1,600°C. This fact revealed that the BN interphase could effectively prevent reaction between CB and Si₃N₄ under the environment of high temperature.

Figure 7.

XRD of (a) Si₃N₄/CB composite ceramics; (b) Si₃N₄/BN(CB) composite ceramics.

Both SEM (Figure 8(a) and (b)) and EDS analysis (Figure 8(c)) results indicated that the BN(CB) were uniform distributed in the Si₃N₄ ceramic, which were about several microns. EDS shows no CB existence due to BN coated its surface.

Figure 8:

(a) SEM of Si₃N₄/BN(CB) composite ceramics; (b) SEM of high magnification of Si₃N₄/BN(CB) composite ceramics and (c) (EDS) analysis at X2.

Figure 9 shows the effect of different mass content of BN(CB) on absorption properties of the composite ceramic. It can be seen from Figure 9(a), the εʹʹ value of Si₃N₄ ceramics without BN(CB) is almost zero which is because Si₃N₄ as a wave-transmitting material cannot absorb microwaves. After adding BN(CB) composite powders, the ε′ values of the Si₃N₄/BN(CB) composite ceramics are lower than that of the pure Si₃N₄ ceramics, whereas the εʹʹ values of the Si₃N₄/BN(CB) composite ceramics are higher than that of the pure Si₃N₄ ceramics. The dielectric loss tangent (tgδ = ε″/ε′) of Si₃N₄/BN(CB) composite ceramics (Figure 9(b)) shows as a function of frequency. The tgδ of the composite ceramic increased as the mass content of BN(CB) increases. The tgδ of the composite ceramic was about 0.43 when the content of BN(CB) increased to 15 wt%.

Figure 9:

Dielectric properties of Si₃N₄/BN(CB) composite ceramics: (a) ε′, ε″ and (b) tgδ.

Figure 10 shows the schematic diagram of absorbing of Si₃N₄/BN(CB) composite ceramics. The porosity of Si₃N₄/BN(CB) composite ceramics is so small that the effect of porosity on dielectric constant of Si₃N₄/BN(CB) composite ceramics could be ignored [30]. As an interface layer, BN coating with excellent electrical insulating property not only prevented the reaction of Si₃N₄ with CB, but also improved impedance matching between CB and free space, which made electromagnetic wave to enter into the BN-modified CB easily and be dissipated. When incorporating the BN(CB) into Si₃N₄ substrate (a wave-transmitting material), the electromagnetic wave can pass through Si₃N₄ ceramics easily, then it passed through the BN coating and can be absorbed by CB. Meanwhile as the BN(CB) content increased, more radar wave can be absorbed and converted into heat energy, so the dielectric loss tangent of Si₃N₄/BN(CB) composite ceramics can increase gradually in agreement with results shown in Figure 9(b).

Figure 10:

The schematic diagram of absorbing of Si₃N₄/BN(CB) composite ceramics.