Flexible Hematite (α -Fe 2 O 3 )-Graphene Nanoplatelet (GnP) Hybrids with High Thermal Conductivity

. Hematite (α -Fe 2 O 3 ) has several attractive properties such as corrosion resistance, catalytic activity, sensing properties, and magnetic features but also a room-temperature stable thermal conductivity of about 16 W/m K. Its use in polymer-matrix composites as a thermal performance enhancer is rather uncommon. In this study, hematite and graphene nanoplatelet (GnP) hybrids in a rubbery latex matrix were prepared and their thermal properties were characterized. The hybrids were mechanically stabilized into freestanding films by hot-pressing them into a porous cellulosic membrane. O ptimization of total filler concentration and the α Fe 2 O 3 /GnP ratio yielded thermal interface material (TIM) films with thermal conductivity of 8.0 W/mK. Infrared measurements showed that the TIMs significantly improved heat sink cooling and demonstrated rapid heat transfer in a system simulating stacked up electronic packing.

3 large concentration of high-k fillers without significantly altering certain requirements such as conformity, ductility and adhesion [17].
In this work, we demonstrate that submicroscopic (O ~100 nm) hematite particles can be effectively used, together with GnPs, in the formulation of flexible high-k thermal nanocomposite coatings by using a distinct block copolymer with spherule-rubber morphology as the dispersive matrix medium [18]. For materials and methods, refer to the Supplementary Material. Block copolymers can be designed with varying hydrophilic and hydrophobic polymer segments with special functional groups like carboxyl, amine and hydroxyl and in solution, they form stable micellar morphology. As such, they have the ability to stabilize suspended nanoparticles based on effective hydrophilic-hydrophobic interactions or charged interactions such as Van der Waals, and hydrogen bonding [19][20][21][22]. As mentioned above, regardless of the thermal conductivity of the material, the polymer matrix has a significant limiting effect on the thermal conductivity of the composite known as the high thermal contrast effect [23]. There are two main approaches to enhance the thermal conductivity of the particle laden polymer composites. The first is the particle orientation and assembly within the polymer matrix (field structuring [23]) and the second is to use secondary additives with varying particle aspect ratios [24]. The scope of this work is the latter in which submicroscopic hematite particles and GnPs are co-dispersed in a rubbery matrix.
Rubbery acrylic copolymers have exceptional uniaxial tensile properties and strong adhesion to different surfaces including metals, ceramics and various plastics [25] Specifically, at room temperature, the polymer used herein has a Young's modulus of 31 MPa with a yield stress of 1.3 MPa; yield deformation of 3.5 %, tensile strength at break 5.7 MPa; deformation at break 420 % and storage modulus of 78 MPa [25,26]. Its morphology is in the form of aggregates of polymer spherules as shown in the TEM and AFM images of Fig. 1a and 1b. Typical spherule (particle) sizes range from 80 nm to 200 nm and the film or coating forms due to solvent evaporation causing closer particle packing [27]. Close particle packing results in severe deformation of the particles while fusing into one another forming a homogenous film structure [28]. Similarly, the average size of α-Fe2O3 particles were about 100 nm as seen in Figure 1c. The X-Ray powder spectroscopy profile of the particles are recorded in Fig. 1d. This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.
PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.0042404 Up to 60 wt.% hematite or 40 wt.% GnPs could be dispersed in the latex solution and upon drying they formed uniform crack-free coatings. As a substrate, we chose a porous cellulose membrane (see Fig. S1, Supplementary Material). Both sides of the membranes were rod-coated with the nanocomposite latex solutions and after drying, they were hot pressed into freestanding films as shown in Fig. 2a. The surface morphology of the 30 wt.% hematite-polymer composite coating before hot-press process is shown in Fig. S2   Conductivity data displays frequency dependence, which appear to follow the universal power-law [29]. Composites with 10 and 35 wt. % hematite fillers display conductivity with nearly two slopes. The first slope (<100 Hz) has virtually no dependence on frequency, whereas the This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.

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second slope (>100 Hz and 1 kHz for 25 wt.%) displays strong variation with frequency. This is known as crossover frequency. With increase in the concentrations of the hematite filler, conductivity increases, and the hopping (crossover) frequency for each curve shifts towards higher frequencies and eventually may become unnoticeable [30]. As shown in Fig. 2c, hematite has poor electrical conduction properties, however, electrical conductivity of hematite surface layers are known to be higher than its bulk. The near surface layers exhibit much stronger interaction between charge carriers than those occurring in the bulk due to deviations in stoichiometry and interaction with ambient oxygen [31,32]. In fact, this is a well-studied and documented phenomenon in physics [31]. The near-surface layer of Fe2O3 exhibits much higher deviation from stoichiometry than the bulk phase resulting in strong interaction between charge carriers. This effect has been interpreted in terms of segregation of intrinsic lattice defects to the surface, and presumably also to grain boundaries, of Fe2O3. This is the underlying reason for electrical conductivity at the surface. Moreover, according to Wang et al. [33] the surface of hematite particles are occupied with domains with distinct chemistry having unusual electronic structure due to the O3-terminated domains with a noticeable presence of states from the subsurface Fe layer.
This causes both electronic and magnetic differences at the surface of hematite. Moreover, interaction of GnPs with polarized interfaces in polymer matrix composites is known to enhance thermal conductivity [32]. GnPs and hybrid filled latex polymer composites displayed DC (direct current) conductivity above 20 wt.% concentration levels as shown in Fig. 2d. In particular, the figure displays bulk resistance (ohm/square) data for graphene, GnP and hybrids with Fe2O3/GnP:1 and Fe2O3/GnP:2 ratios. In the case of filler concentration levels below 10 wt.%, I-V curves show hysteresis (see inset in Fig. 2d) resembling mixed ionic-electronic conduction [33]. No I-V curve crossing, typical for memristor behavior, was measured for low concentrations but rather a capacitive response was registered during the I-V sweeping [34,35]. Above 25 wt.% concentration levels, the conductors displayed simple ohmic behavior due to the presence of GnPs (see inset This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.

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for instance, demonstrated about 120% elongation capability, rendering it reasonably foldable for certain applications (see Fig. 2a).
Thermal interface materials connect different thermal elements for efficient thermal management. The connections should minimize thermal contact resistance for active heat transfer.
Thermal contact resistance occurs due to imperfect interface separation (local gaps) derived from mismatches in thermal expansion. The interfacial mismatch in thermal expansion also causes thermal stress, which can harm electronic components. The localized interfacial gaps will significantly increase the contact resistance, reduce the effective heat dissipation and accumulate heat within the devices. The minimization of interfacial contact resistance is correlated with the thermal expansion or the coefficients of thermal expansion (cte) of the thermal interface material [37,38]. Measured thermal conductivity and the cte values of the hematite, GnP and hybrid filled composites are shown in Figure 3. The maximum filler ratio was 55 wt.%. Comparison of thermal conductivity of Fe2O3 and GnP filled composites (Fig. 3a) indicates that at after 5 wt.% concertation levels, on average, the thermal conductivity GnP filled composites is larger by sevenfold. At 50 wt.% GnP filled composite has 6.5 W/mK thermal conductivity compared to 0.87 W/mK of hematite filled composite. The hybrid system with Fe2O3/GnP of 0.3 however (Fig. 3b), features a higher thermal conductivity of 8.0 W/mK at the same 50 wt.% concentration. Note that the enhancement is achieved for the through-plane heat conduction mode because graphene is horizontally oriented within the polymer composite (see Fig. S4, Supplementary Material). This boost in thermal conductivity may be attributed to the synergetic effect known as the "bridge-link effect" of hybrid fillers [39]. Moreover, based on other detailed experimental works [40], enhanced thermal conductivity due to hybrid filling of polymers was explained by the better connectivity offered by structuring the filler with high aspect ratio (i.e. GnPs) in the composite. Such conclusions were also drawn from mathematical modeling studies such that the presence of two dissimilar particulate fillers collectively contribute to the enhancement in the thermal conductivity value of such hybrid filler composites [41].
This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset. PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.0042404 This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.

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increased by about 10×10 -5 / o C when the temperature was increased to 75 o C within the concentration range studied. In the case of hybrid composites shown in Fig. 3d [45]. It has been demonstrated that microstructural features significantly influence the cte of graphene thermoset composites [41].

Changes in graphene dispersion, agglomeration, and alignment within the polymer matrix along
This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.

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with the large differences between the elastic coefficients of graphene and the polymer, anisotropic cte values are reported for similar composites also supported by effective medium calculations using Green's function method [46].
In Figs. 4a and 4b, two distinct thermal experiments were presented in order to compare the performance of some of the selected TIM films. In the first experiment in Fig.4a, resistances between the TIM films and the surfaces. In the heat sink cooling experiment, the TIM film is in contact with an aluminum plate and a carbon coated heat sink (Fig. 4a) and the metal blocks in Fig. 4b are stainless steel. Lim et al. [48] argued that contact resistance between This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.

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nanocarbons and metals depend on the wettability of the metal surfaces and their work functions in a complex manner. Cola et al. [49] also indicated that thermal resistance across metalnanocarbon interfaces is a complex phenomenon affected by local deformations, Van der Waals forces, roughness of the nanocarbon agglomerates, and the pressure. As seen in Fig. 2b, the TIM films do not have a very smooth surface texture and, most of the dispersed GnPs are coated with the polymer latex that introduces an extra contact resistance within the TIM film itself.
Even though single layer graphene has excellent thermal conductivity of about 5300 W/m K [45], outperforming other nanocarbon materials [50], the GnPs are more like graphite flakes with much lower thermal conductivity [51,52]. With proper dispersion, alignment and using specific polymer matrices that can tolerate large concertation of GnPs without compromising flexibility and workability, conformal TIM films or coatings can be produced with thermal conductivities approaching 10 W/mK [53][54][55]. Hybrid systems can help reduce bulk contact resistance between the polymer and a particular filler. In this study, since the near-surface layer of α-Fe2O3 exhibits much higher deviation from stoichiometry than the bulk phase resulting in strong interaction between the surrounding charge carriers, it can significantly enhance the thermal conductivity of GnP-polymer composites even by itself it is not an effective thermal filler.
However, hematite filled rubbery latex matrix thermal conductivity can be improved at least by three times compared to the plain polymer enabling other effective filler like GnPs to function better. Extensive size and weight reductions associated with the latest developments in electronic technologies are accompanied by increased power consumption levels. Estimates indicate that in the near future, the heat flux at the die level will exceed 100 Wcm −2 . If such high levels of heat are not effectively dissipated or transmitted at the interfaces, the life, reliability, and performance of electronic components and devices will be jeopardized. This work intended to address this issue by developing cost-effective, paper-thin hybrid composites that can function as high-k thermal conductors at various interfaces.

Supplementary Material
See supplementary material for experimental methods and materials, SEM, TEM images of the composites, FTIR, Raman and thermo-gravimetric characterization of hematite and GnP powders and mechanical properties of some selected composites.
This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset. PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.0042404

Data Availability
The data that supports the findings of this study are available within the article and its