范 宇馳

English

博士学位論文の要旨及び審査結果の要旨 (Summary of Thesis(DR))othe

范, 宇馳

Tohoku University Repository (TOUR) / 東北大学機関リポジトリ

Processing of Graphene/Al2O3 Nanocompositeswith Unique Electrical and MechanicalProperties著者 范 宇馳号 58学位授与機関 Tohoku University学位授与番号 工博第004879号URL http://hdl.handle.net/10097/59009          ??? ???? ?         ? ??? ? ? ?         ?????????????         ??????????????????? ?????4??1?????????? ?????????????????????????? ??? ? ? ? ? ?         Processing of Graphene/Al2O3 Nano-composites with Unique Electrical and Mechanical Properties(?????/??????????????????????????)? ? ? ? ?????? ?? ?? ? ? ? ? ?         ?? ?????? ?? ? ?????? ?? ?             ?????? ?? ??? ? ? ? ? ?         Graphene/metal oxide hybrids and composites have received great attention due to the synergetic effects provided by the combination of metal oxides and graphene, which can prominently improve the performance comparing with individual metal oxides. These novel composites have broad applications in the field of drug delivery, lithium ion battery, super capacitors and transparent conductors, etc. However, the main obstacle lying in the preparation of graphene/metal oxide composites is to disperse graphene homogeneously throughout the oxides, since the agglomeration would deteriorate the electrical, optical and magnetic properties of the composites. To solve this problem, considerable efforts have been made but challenges still remain. On one hand, most of current methods to prepare graphene/metal oxide composites are based on an in situ growth strategy,[4] As a result it is very difficult to control the quality and morphology of metal oxides at will because of the presence of graphene. In this sense it would be more desirable that an ex situ method could be adopted. Specifically, graphene (or graphene oxide (GO)) and metal oxides are prepared separately at first and then combined together through certain ways. Thereby the preparation of metal oxide could be optimized easily and many commercially available oxide powders with h???? ???????? ?????? ??? ?-Al2O3 and TiO2) can be exploited directly for optimizing the performance of composites. On the other hand, many of those methods include the usage of surfactants which could either assist the dispersion of metal oxide or facilitate the formation of special structure. However, the residue of surfactants will probably harm the performance of composites. Moreover, removing these surfactants often requires multiple washings or harsh conditions such as heating at high temperature, which is not feasible for many metal oxides.? 135 ?Although many graphene/metal oxide hybrids have been reported, very few works have been carried out to further densify those hybrids to obtain composites and study the properties in a filler/matrix system. In our previous work, fully dense graphene nanosheet/Al2O3 composites were fabricated by ball milling and spark plasma sintering (SPS) for the first time. It has been proved that the electrical properties of graphene based composites were far better than those of carbon nanotube based composites. Albeit the dispersion of graphene in the matrix is quite well, the percolation threshold is still higher than that of corresponding carbon nanotube based composite since the average thickness of graphene flakes are not thin enough to be qualified as graphene according to the classic definition. Like the agglomeration of carbon nanotubes, thick graphene sheets are also harmful especially for the mechanical properties of composites.An ex situ strategy is introduced to prepare GO/metal oxide hybrids without the assistance of surfactant. By controlling the electrostatic interaction between GO and metal oxide particles, the corresponding hybrid material with homogenous dispersion of GO can be fabricated. As an example, GO/Al2O3 hybrids were obtained with GO composition in a range from 1.23 to 1.92wt.% and from 2.88 to 11.5wt.% by using two kinds of methods respectively. For FG/Al2O3 composites, it is found by XPS and Raman spectra that the sintering treated FG flakes possess very high quality. In addition, the average thickness of as-prepared graphene is around 1.2nm when the graphene content is low, but the thickness of graphene flakes slightly increases when graphene content becomes high. The percolation threshold of FG/Al2O3 composite is as low as 0.38 vol.% and the electrical conductivity can reach 103 Sm-1 when graphene content is only 2.35 vol.%. More interestingly, the charge carrier type is p-type when graphene content is low and changes to n-type when graphene content increases to high level. We deduce that the p-type composite with low graphene content arises from the doping induced by alumina matrix. Firstly, the intrinsic carrier type of thermally reduced GO should be electron. It has been reported that the chemically reduced and annealed GO films (composed of 3-5 layers graphene) exhibit graphite-like semi-metals characteristics and the electron mobility is generally higher than the hole mobility in vacuum. Thereby it can be inferred that the intrinsic carrier type of as-prepared graphene is more resembling to that of few layers graphene, since the C 1s peak of XPS spectra show that the reduction level of graphene in this work is better than the reported graphene film. Secondly, the positive hall coefficient presumably arises from the doping induced by the matrix. The influence of substrate to properties of graphene has been noticed since the discovery of graphene. It is found that when the interaction between graphene and substrate is weak, such as the situation that micromechanical cleaved graphene on different substrates, the influence of substrates is negligible. However, ? 136 ?when there is a strong interaction between graphene and substrate SiC substrate has no graphitic electronic properties and acts as a buffer layer that causes the subsequent graphene layer performs like isolated but doped graphene. In FG/Al2O3 composite, the interaction between FG and matrix is strong because of the residual stress that stems from the difference of contraction after cooling. Graphene has a negative in-plane coefficient of linear thermal expansion from room temperature to 2000K, while Al2O3 has a positive one, which means during the cooling procedure graphene has a trend of expansion but the matrix will contract. As a result, great pressure will be generated and lead to very firm contact between FG and matrix. In addition, when exposed to environment of low oxygen partial pressure at high temperature, oxygen vacancies and aluminum interstitials are promoted as main point defects. These positively charged point defects act as electron acceptor to make graphene hole doped. Thus it can be inferred that the doping level depends on the concentration of oxygen vacancy and aluminum interstitials on the grain surface, but quantitative investigation will not be given here. Although only the outermost layers are doped by matrix, the inner layers also can be affected through charge transfer between layers. Therefore the Hall coefficient remains positive when most of flakes contain a few graphene layers, such as the situation in the composite with graphene content of 0.66 vol.% (Figure 5a and b). Finally, the hall coefficient changes to negative when the graphene layers become thick. The electrical conductivity of composite can be enhanced by filler material in two ways. One is through establishing new conductive path in the matrix and the other is through increasing the cross area of formed path, which is the thickness of graphene flakes in the case of graphene based composite. Because of the appropriate strategy adopted in this study, the dispersion of FG in the matrix is homogeneous so that the former way is dominant when graphene content is low. Since new interfaces between FG and matrix continue forming in this procedure, the composites persist to be p-type. After graphene content becomes high, the conductivity will mainly increase through the second mechanism, such as the situation in the composite with graphene content of 1.56 vol.% (Figure 5c and d). Since the total amount of hole-type carrier keeps unchanged statistically, the doping level will decrease with increasing graphene content and eventually the carrier type will perform as n-type. We believe this FG/Al2O3 carrier type tunable composite is very promising for semiconductive materials applied in harsh environments (radiation, high temperature, corrosion, etc.)Since FG/Al2O3 composites have excellent electrical properties, it is critical to assess the influence of graphene on the composite’s mechanical properties like hardness and elastic modulus, as well as the mechanical reliability after processing at elevated temperature. Unfortunately only few works demonstrate ? 137 ?the effect of graphene as second phase on the microstructure and mechanical behavior of composites, apart from several reports focusing on the toughening effect. To obtain reliable hardness of ceramics, one should avoid introducing crack during measurement. For alumina it requires the applied force as low as 200mN, which makes the indent too shallow to be measured optically in a traditional micro-indentation. Thus the nanoindentation that measures the hardness and elastic modulus by sensing the penetration depth of indenter was exploited in this work. Combining with analysis of the microstructure, the influence of two-dimensional FG on the mechanical properties of ceramic composite is interpreted.In this research, fully dense (relative density>98%) FG/Al2O3 nanocomposites with homogenously dispersed FG have been prepared by SPS under different sintering temperatures. It is found that graphene has great ability to restrain grain growth. By adding only 1.2 vol% of graphene, the average grain size is 10 times finer than that in monolithic alumina sintered under identical condition. Grain size has a significant effect on the properties of ceramics and their composites. In thermoelectric materials, it is found that nano-sized grains introduce numerous boundaries and interfaces that scatter phonons and reduce lattice thermal conductivity. For restraining grain growth in ceramic materials, inclusions (pores or second phases) are always considered as effective agent. It was estimated by C. Zener that there is a definite relation between the number and size of inclusions and the minimum curvature that can move. For instance, if there is 1 vol.% of inclusions, the grains can only grow to approximately 100 times of the inclusion size. In fact, the resisting effect of inclusion becomes more tangible when the dihedral angle between ??????????????????????????????????????????????????????????????????????????tween 120º and 180º for crystalline additives, such as ZrO2 particles in Al2O3 ceramics. However, dihedral angle of lower than 120º is hardly to realize for sphere-like inclusions during sintering because of the difficulty in fitting of surface energy. Similar effect of low dihedral angle can be achieved by using inclusions with large aspect ratios. For instance, one dimensional material exerts more prominent effect of retarding grain growth along the direction of long axis compared with sphere-like inclusions if they have same volume. As a typical case, MWCNTs with good dispersion in matrix can form a restraining force along the tube, especially when MWCNTs lie along the three-grain junctions since they pin the movement of boundaries at the junctions. In addition, CNTs benefit from their feature of readily bending which makes them easy to fit the curvature of grain surface. To create the largest effective area of grain boundary intersected by smallest volume of inclusions, the ideal situation is two-dimensional solid material. As the first two-dimensional material discovered by human race, graphene is rigid when bearing tensile force while soft when bearing bending force. It is one-atom-thick and ? 138 ?of extremely large surface area, which perfectly matches the requirements of ideal inhibitor of grain growth. Like CNT, graphene is easy to deform, creating a contact resembling the structure of 0º dihedral angle. However, compared with CNT, graphene is more efficient because it simply blocks all the possible routes ofmass transport between intersected grains, no matter it crosses a three-grain junction or not. Moreover, with the thickness of one atom graphene is impermeable to any atoms even as small as helium. Therefore, when grain size is much smaller than graphene, theoretically the diffusion crossing graphene cannot happen and so as the grain growth, in spite of the probably decreased effective size by holes on FG which are induced by strong acid reaction and heating process.In addition, the morphology of grain in composite is changed during sintering process. The unusual behavior of grain growth has been analyzed by TEM and HRTEM observation. The hardness and elastic modulus of alumina and composites are characterized by nanoindentation. It is surprised that FG/Al2O3 composites are almost as hard as monolithic alumina at low sintering temperature of 1573K even if graphene content is as high as 1.2vol.%, benefiting from the greatly decreased grain size. However, at higher sintering temperature, the hardness of composites decreases further but the change in elastic modulus is very limited. The decline of harness and elastic modulus mainly arise from the sliding feature of FG, low modulus of reduced graphene oxide in both in-plane and out-of-plane directions. However, it is worth to note that the hardness of FG/Al2O3 composite with graphene content of 0.6 vol.% is even higher that of monolithic alumina, and the composite is already above the percolation threshold of electrical conductivity. Thus the FG/Al2O3 composite is very promising to be used as structural material possessing electrical functions, such as protecting cover with microwave shielding ability.? 139 ?Figure 1. TEM and HRTEM images of FG/Al2O3 composites with graphene content of 0.66 vol.% (a and b) and 1.56 vol.% (c and d). The arrows indicate the residual FG outside the polishing surface and the yellow dash lines indicate the overlapped FG.? 140 ?Figure 2. (a) Electrical conductivi????????? ????????????????????????????????????????????????????????????????), ??????? ??????????????????????? ?????-???????????????? ??? ???????????????? ??????????? ????????? ???????????? ?????plotted against filler volume fraction (?).? 141 ?Figure 3. SEM images of fracture surface of alumina (a1 and a2) and FG/alumina composites with graphene content of 0.6vol.% (b1 and b2) sintered at 1573 and 1673K, respectively. Insets in a1 and b1 are photographs of as-sintered alumina and composite respectively.Figure 4. Indentation hardness (a) and elastic modulus (b) of monolithic alumina and composites with increasing graphene content prepared at different sintering temperatures.? 142 ?? 143 ?

Processing of Graphene/Al2O3 Nanocomposites with Unique Electrical and Mechanical Properties

https://tohoku.repo.nii.ac.jp/?action=repository_action_common_download&item_id=98047&item_no=1&attribute_id=18&file_no=1

グラフェン/アルミナ焼結ナノ複合材料の作製と電気的機械的特性評価

Abstract

Similar works

Full text

Available Versions

Tohoku University Repository (TOUR) / 東北大学機関リポジトリ