Controlled Preparation and Thermal Conductivity of Highly Oriented Graphite Blocks

doi:10.15541/jim20160480

Abstract

Abstract:

Using cheap and available natural flake graphite with high crystallinity and mesophase pitch as raw materials, graphite block materials with high bulk density, highly preferred orientation and high thermal conductivity, were prepared by a hot-press molding at mild temperatures and subsequent carbonization and graphitization treatments at high temperatures. XRD, SEM and PLM analyses indicate that the prepared graphite block has a highly preferred structural orientation, the graphitic layers of the graphite flakes inside are clearly oriented perpendicular to the hot-pressing direction. The components of various sized flake graphite and different types of pitch binders, and the proportions of raw materials, as well as the preparation process parameters (including hot-pressing temperature and pressure, heat treatment temperature, etc.), have great influences on the room-temperature in-plane thermal conductivity of the resultant graphite blocks. Graphitized blocks prepared from 86wt% NG (+32 mesh) and 14wt% AR mesophase pitch through hot-pressed at 500℃ at a fixed pressure of 10 MPa for 5 h and subsequently underwent 1000℃ carbonization and 2800℃ graphitization, have a good comprehensive thermophysical property. Their bulk density reaches above 1.91 g/m³, and room-temperature in-plane thermal conductivity is measured as high as 550 W/(m·K), and improved to 620 W/(m·K) after 3000℃ graphitization treatment.

Key words: graphite block, high orientation, controlled preparation, thermal conductivity property

CLC Number:

TB303

YUAN Guan-Ming, XUE Zheng, CUI Zheng-Wei, DONG Zhi-Jun, LI Xuan-Ke, ZHANG Zhong-Wei, WANG Jun-Shan. Controlled Preparation and Thermal Conductivity of Highly Oriented Graphite Blocks[J]. Journal of Inorganic Materials, 2017, 32(6): 587-595.

Figures/Tables 15

Fig. 1 SEM images of natural graphite flakes (a) before and (b) after coated by mesophase pitch binder

Fig. 2 (a) Optical photos, (b) textural diagram and (c) XRD patterns of a highly oriented graphite block after heat treatment at 2800℃

Fig. 3 Typical SEM images of a graphite block after heat treatment at 2800℃ showing various planes (a-A, b-B, c-C)

Fig. 4 Typical PLM micrographs of graphite block after heat treatment at 2800℃ showing the cross-section of side C

Table 1 Physical properties of graphite blocks made with different pitch binders

Pitch binder	ρ/ (g·cm^-3)	σ/ (μΩ·m)	α/ (mm²·s^-1)	λ/ (W·m^-1·K^-1)
WG	1.76	2.02	294.01	388.1
NP	1.78	1.62	309.74	413.5
MP	1.83	1.27	385.66	529.3
SC	1.78	1.31	363.01	484.6
AR	1.91	1.45	395.52	551.6

Table 2 Basic physical properties of different pitch binders

Pitch binder	Softening point/℃	Ash content/%	Volatile content/%	Carbon yield/%	TI/%	QI/%
WG	110	0.12	48.65	50.56	31.5	11.2
NP	82	0.004	55.31	44.65	1.3	1.0
MP	275	0.02	21.67	78.24	69.4	42.5
SC	280	0.03	19.52	80.25	67.8	20.6
AR	266	0.001	22.03	77.95	66.9	50.8

Table 3 Microcrystal parameters of the different pitch binder-derived graphite particles after 2800℃ graphitization treatment

Pitch binder	d₀₀₂	L_a/ nm	L_c/ nm	g/ %
WG	0.3367	73.1	20.4	84.88
NP	0.3365	86.4	24.5	87.21
MP	0.3357	316.7	25.0	96.51
SC	0.3366	79.2	22.7	86.05
AR	0.3356	475	26.5	97.67

Fig. 5 Typical PLM micrographs of different pitch binder-derived graphite particles after heat treatment at 2800℃ a-WG; b-NP; c-MP; d-SC; e-AR

Fig. 6 Room-temperature thermal conductivity and thermal diffusivity of graphite blocks heat treated at 2800℃ versus initial weight fraction of mesophase pitch

Fig. 7 Ideal structure of stacked natural graphite flakes and pitch binder derived carbon particles in a graphite block

Table 4 Room-temperature thermal diffusivity and thermal conductivity of graphite blocks prepared with different particle- sized graphite flakes

Thermal property	Particle sizes of graphite flakes/ mesh
Thermal property	80-40	40-32	+32	+20
α/(mm²·s^-1)	343.8	360.8	395.5	366.1
λ/(W·m^-1·K^-1)	484.7	514.2	551.6	510.7

Table 5 Influence of hot-pressing temperature on the physical properties of graphite blocks

Hot-pressing temperature/℃	ρ/ (g·cm^-3)	α/(mm²·s^-1)	λ/(W·m^-1·K^-1)
300	1.66	331.36	412.5
500	1.91	395.52	551.6
650	1.95	415.13	607.1

Fig. 8 Room-temperature thermal conductivity of graphite blocks versus heat treatment temperature

Table 6 Physical properties of 2800℃ treated graphite blocks before and after doping

Dopants	ρ/(g·cm^-3)	σ/(μΩ·m)	α/(mm²·s^-1)	λ/(W·m^-1·K^-1)
Undoped	1.91	1.45	395.52	551.6
5wt% Si	1.79	1.90	307.21	412.4
5wt% Ti	1.89	1.52	296.60	420.4
3wt%Si+ 5wt%Ti	1.65	2.25	229.20	283.6

Fig. 9 Thermophysical properties of graphite blocks and copper versus testing temperatures under non-oxygen atmosphere

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