Article introduction
Cold plasma surface modification was put forward in 1960s. It is widely used in metallurgy, chemistry, electronics, medicine, environmental protection and other fields, and has high efficiency in the process of treatment. This treatment will only affect the depth of several nanometers to dozens of nanometers on the surface of the material, without changing the overall characteristics of the material.
In this paper, the literature about cold plasma surface treatment of inorganic fillers at home and abroad is summarized in three ways
(1) The surface of the powder was treated by plasma produced by medium gas, and the active sites were produced by plasma bombardment. Then the powder was treated by traditional surface treatment. Because the properties and characteristics of the active sites of this method need to be studied, there are few reports about this method;
(2) The modified material is deposited on the surface of the inorganic powder by plasma, and a continuous porous polymer film is formed on the surface of the powder;
(3) The results show that the living sites of the monomers are produced by the glow plasma produced by the medium gas, and the polymerization of the monomers on the powder surface is initiated at a suitable temperature.
Next, we will arrange some typical literatures to introduce the application of cold field plasma in the surface modification of inorganic powder and the influence of modified powder on the properties of composite.
Cold plasma modified BN filler and thermal conductivity of silicone rubber BN composite
The flaky morphology and high surface of h-BN particles can lead to poor affinity between filler particles and siloxane matrix, and it is difficult to process into high filled Sir / BN. The current solution to this problem is to use coupling agent to modify the filler surface organically. However, due to its low surface activity, h-BN particles are difficult to be fully modified by traditional surface treatment. In this paper, the surface of h-BN was modified by plasma. The LSR state of liquid polydimethylsiloxane on the surface of h-BN was characterized by HRTEM and XPS. The microstructure and thermal conductivity of the composite were analyzed.
? test process:
LSR and h-BN were dissolved into Hexane at a mass ratio of 3:100. After a few minutes, the mixture was dried in vacuum at 80 ℃ to remove hexane. Then it was dried completely at 160 ℃ for 2 hours to obtain powder l-bn. Then it was treated in plasma processor for 1, 2, 3 hours, until lp1-bn, lp2-bn, lp3-bn. The LSR in incomplete contact was removed by extraction of the four powders at 120 ℃ with hexane, and the LSR was labeled as l-e-bn, lp1-e-bn, lp2-e-bn and lp3-e-bn. L-bn, lp1-bn, lp2-bn and lp3-bn were mixed with methyl vinyl silicone rubber, methyl silicone oil, triallylisocyanurate, 2,5-dimethyl-2,5-di (tert butyl peroxy) hexane respectively. The final material was obtained by holding pressure for 20 min at 170 ℃ in standard mould.
Figure 1: experimental flow chart
? powder material characterization:
Figure 2 shows the SEM image of h-BN particles. H-BN particles are mainly composed of flake particles, with an average diameter of 10 μ m and a thickness of 1 μ M. Fig. 3A shows the HRTEM image of the surface of h-BN particles. There is no coating on the surface of h-BN, and the crystal surface of the powder is clearly visible. Fig. 3B shows the HRTEM image of lp3-e-bn, which shows the ultra-thin coating on the surface of h-BN filler. The thickness of the coating is 0.5 ~ 2 nm.
Figure 2: SEM of h-BN
Figure 3: HRTEM (a) unprocessed h-BN, (b) HRTEM image of lp3-e-bn
Fig. 4 is the XPS wide scan of different powders. The XPS test shows that the LSR material without plasma treatment is coated on the surface by physical adsorption, and the plasma treatment must be passed before it can be firmly coated on the BN surface. The longer the time is, the greater the solid coating amount is, and the plasma treatment has no effect on the structure of h-BN.
Figure 4: XPS wide scan spectra of different h-BN samples: (a) lp3-e-bn, (b) lp2-e-bn, (c) lp1-e-bn, (d) le-bn, (E) l-bn, (f) untreated h-BN
Figure 5 shows the hydrophilicity test after powder compaction. The contact angle of h-BN is 81 °, the contact angle of lp3-e-bn is 94 °. The surface energy of h-BN is 42.38 Mn / m, the surface energy of lp3-e-bn is 31.61 Mn / M and the surface energy of Sir is 18.09 Mn / m. The greater the difference between the two substances, the worse the affinity.
Figure 5: hydrophilicity test (a) untreated h-BN, (b) lp3-e-bn
? performance characterization of heat conducting gasket
Fig. 6 is the SEM of silicone rubber sheet filled with different fillers. SEM shows that h-BN and l-bn have obvious sheet particles with obvious interface, while lp3-bn does not. It shows that plasma treatment can improve the interface compatibility, and the distribution of the three is different.
Figure 6: SEM photos of fracture surfaces of Sir / BN (100 / 300) composites filled with different h-BN particles: (a) untreated h-BN, (b) l-bn (h-BN of LSR coating) and (c) lp3-bn
Figure 7 shows the thermal conductivity related test of heat conducting gasket. Under the same conditions, lp3-bn has the highest thermal conductivity, and the addition limit of h-BN is 53%, while the addition amount of lp3-bn is more than 60%, which has an upward trend. The results show that plasma modification has little effect on the distribution of fillers in the composite, enhances the binding ability between modifiers and fillers, improves the compatibility between fillers and matrix interface, reduces the interface thermal resistance and improves the thermal conductivity of the material.
Figure 7: comparison of thermal conductivity of Sir / BN (100 / 300) composite filled with different h-BN particles
? summary:
LSR was first coated with h-BN by solution dispersion method, and then plasma treatment was carried out in argon atmosphere. A layer of 0.5 ~ 2 nm coating was formed on the surface of the inorganic powder by this way. After Soxhlet extraction, the film was still tightly coated on the surface of the material. Plasma modification improves the interface interaction between h-BN and matrix, but hardly affects the distribution of h-BN in composite. The plasma modification of h-BN greatly improves the thermal conductivity of Sir / BN composite.
The above research results are from: Tuo Ji, Li Qun Zhang, Wen Cai Wang, et al. Cold plasma modification of boron nitrode fillers and its effect on the thermal conductivity of silicon rubber / boron nitrode composites [J]. Polymer composites, 2012, 33 (9): 1473 – 1481
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