Abstract
The highly thermo-conductive but electrically insulating film, with desirable mechanical performances, is extremely demanded for thermal management of portable and wearable electronics. The integration of boron nitride nanosheets (BNNSs) with regenerated cellulose (RC) is a sustainable strategy to satisfy these requirements, while its practical application is still restricted by the brittle fracture and loss of toughness of the composite films especially at the high BNNS addition. Herein, a dual-crosslinked strategy accompanied with uniaxial pre-stretching treatment was introduced to engineer the artificial RC/BNNS film, in which partial chemical bonding interactions enable the effective interfiber slippage and prevent any mechanical fracture, while non-covalent hydrogen bonding interactions serve as the sacrifice bonds to dissipate the stress energy, resulting in a simultaneous high mechanical strength (103.4 MPa) and toughness (10.2 MJ/m3) at the BNNS content of 45 wt%. More importantly, attributed to the highly anisotropic configuration of BNNS, the RC/BNNS composite film also behaves as an extraordinary in-plane thermal conductivity of 15.2 W/m·K. Along with additional favorable water resistance and bending tolerance, this tactfully engineered film ensures promised applications for heat dissipation in powerful electronic devices.
Highlights
With the rapid development of the highly integrated electronic devices toward high power and increasing miniaturization, research on the flexible thermo-conductive material becomes urgent to satisfy the thermal management requirements in portable and wearable applications (Li et al, 2015; Chen et al, 2018a; Wu et al, 2020)
The anisotropic and dual-crosslinked regenerated cellulose (RC)/boron nitride nanosheets (BNNSs) films were welldesigned through a pre-orientation assisted with dual crosslinking strategy (Figure 1a)
The cellulose/BNNS suspension could present a very uniform dispersion according to the optical microscope result (Figure 1e, no obvious BNNS aggregations), and it was verified that the BNNS sheets could remain stable and dispersive in the suspension after long standing according to the TGA test of the supernatant (Figure 1f and Figure S2)
Summary
With the rapid development of the highly integrated electronic devices toward high power and increasing miniaturization, research on the flexible thermo-conductive material becomes urgent to satisfy the thermal management requirements in portable and wearable applications (Li et al, 2015; Chen et al, 2018a; Wu et al, 2020). A green pathway that is using NaOH/urea aqueous solution as the dissolving agent was reported (Cai and Zhang, 2005), in which BNNSs were intensely mixed with RC by direct mechanical stirring, resulting in an in-plane TC of 2.97 W/m K, mechanical strength of 70 MPa, and elongation of 14% at a BNNS addition of 30 wt% (Lao et al, 2018) These limited in-plane TC, mechanical strength, and toughness are mainly attributed to the following reasons: (i) Low BNNS addition, derived from the challenging dispersion of BNNSs in aqueous solution of cellulose, leads to insufficient BNNS–BNNS interconnections and discontinuous phonon pathways within the RC matrix (Cai et al, 2017; Ma et al, 2020). These limited in-plane TC, mechanical strength, and toughness are mainly attributed to the following reasons: (i) Low BNNS addition, derived from the challenging dispersion of BNNSs in aqueous solution of cellulose, leads to insufficient BNNS–BNNS interconnections and discontinuous phonon pathways within the RC matrix (Cai et al, 2017; Ma et al, 2020). (ii) Isotropic alignment of BNNS without adequate inplane orientation could not take advantage of the highly thermoconductive trait of BNNSs to the fullest (Wang et al, 2019; Liu et al, 2020). (iii) Weak bonding interactions merely by intermolecular hydrogen bonds of cellulose and their random molecular configuration is easy to give rise to mechanical fracture during uniaxial stretching, without enough interfiber slippage to dissipate the stress energy (Osorio-Madrazo et al, 2012; Tu et al, 2020)
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