By the formation of a 1:1 cocrystal of caffeine and methyl gallate, we demonstrated that powder compaction properties could be profoundly improved. The selection criterion for cocrystal exhibiting superior compaction properties was the presence of slip planes in crystal structure. Bulk cocrystal was prepared by suspending powders of the two pure compounds in ethanol. Fine powders of similar particle size distribution were compressed. Within the whole range of compaction pressure, the tablet tensile strength of methyl gallate was very poor (<0.5 MPa) and severe lamination and sticking occurred in almost all tablets. Tabletability of caffeine was acceptable at <150 MPa. However, at >180 MPa, severe lamination of caffeine tablets suddenly occurred. Tablet tensile strength dropped sharply at >240 MPa. In contrast, the tabletability of the cocrystal was excellent over the entire pressure range. Tablet tensile strength of the cocrystal was ∼2 times that of caffeine at <200 MPa, and the ratio gradually increased with increasing pressure, e.g., ∼8 fold at 350 MPa. Poor tablet tensile strength was always associated with high elastic recovery and low plasticity. The good plasticity and tabletability of the cocrystal validated the selection criterion, i.e., the presence of slip planes in crystal structure.
Along
with the technology evolution for dense integration of high-power,
high-frequency devices in electronics, the accompanying interfacial
heat transfer problem leads to urgent demands for advanced thermal
interface materials (TIMs) with both high through-plane thermal conductivity
and good compressibility. Most metals have satisfactory thermal conductivity
but relatively high compressive modulus, and soft silicones are typically
thermal insulators (0.3 W m–1 K–1). Currently, it is a great challenge to develop a soft material
with the thermal conductivity up to metal level for TIM application.
This study solves this problem by constructing a graphene-based microstructure
composed of mainly vertical graphene and a thin cap of horizontal
graphene layers on both the top and bottom sides through a mechanical
machining process to manipulate the stacked architecture of conventional
graphene paper. The resultant graphene monolith has an ultrahigh through-plane
thermal conductivity of 143 W m–1 K–1, exceeding that of many metals, and a low compressive modulus of
0.87 MPa, comparable to that of silicones. In the actual TIM performance
measurement, the system cooling efficiency with our graphene monolith
as TIM is 3 times as high as that of the state-of-the-art commercial
TIM, demonstrating the superior ability to solve the interfacial heat
transfer issues in electronic systems.
With
the increasing integration of devices in electronics fabrication,
there are growing demands for thermal interface materials (TIMs) with
high through-plane thermal conductivity for efficiently solving thermal
management issues. Graphene-based papers consisting of a layer-by-layer
stacked architecture have been commercially used as lateral heat spreaders;
however, they lack in-depth studies on their TIM applications due
to the low through-plane thermal conductivity (<6 W m–1 K–1). In this study, a graphene hybrid paper (GHP)
was fabricated by the intercalation of silicon source and the in situ growth of SiC nanorods between graphene sheets based
on the carbothermal reduction reaction. Due to the formation of covalent
C–Si bonding at the graphene–SiC interface, the GHP
possesses a superior through-plane thermal conductivity of 10.9 W
m–1 K–1 and can be up to 17.6
W m–1 K–1 under packaging conditions
at 75 psi. Compared with the current graphene-based papers, our GHP
has the highest through-plane thermal conductivity value. In the TIM
performance test, the cooling efficiency of the GHP achieves significant
improvement compared to that of state-of-the-art thermal pads. Our
GHP with characteristic structure is of great promise as an inorganic
TIM for the highly efficient removal of heat from electronic devices.
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