Cellulose is a crystalline polymer with intriguing, amorphous-like, temperature dependence of thermal conductivity κ. To determine its origin, we have studied κ of cellulose nanocrystals (CNCs) derived from cotton by sulfuric acid hydrolysis, in both porous and nonporous states by pressure densification; κ increases weakly with increasing temperature and density, like in a fully amorphous material, and it is remarkably similar to that of cellulose fibers (CFs) and cellulose nanofibers (CNFs). For a powder derived from a natural material, like cellulose, amorphous-like κ may originate from poor thermal contact between particles or a high amorphous content, but the latter is not the case for CNCs. Moreover, the amorphous-like behavior is unaffected by densification and, therefore, improved thermal contacts. Instead, we attribute the behavior to CNCs’ nanometer-sized fibrils, which limit the phonon mean free path to a few nanometers in a network of randomly oriented CNCs. This explains why κ is essentially the same in networks of CNCs, CFs, and CNFs, which are materials with the same structural unit─elementary fibrils of 3–5 nm in diameter. We obtain κ = (0.60 ± 0.01) W m^–1 K^–1 for a nonporous network of randomly oriented CNCs at 295 K and atmospheric pressure, and κ increases by only 14% GPa^–1, which is unusually weak for a polymer. By using a model for such a network, we find κ = 1.9 W m^–1 K^–1 along a CNC and argue that this is a good estimate also along a CNF and a CF at room temperature.

中文翻译：

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纤维素纳米晶体的多孔和致密网络的热导率

纤维素是一种结晶聚合物，具有有趣的、无定形的、热导率 κ 的温度依赖性。为了确定其来源，我们研究了通过硫酸水解从棉花中提取的纤维素纳米晶体 (CNC) 的 κ，通过压力致密化在多孔和无孔状态下；κ 随温度和密度的增加而微弱增加，就像在完全无定形的材料中一样，它与纤维素纤维 (CFs) 和纤维素纳米纤维 (CNFs) 非常相似。对于源自天然材料（如纤维素）的粉末，类似无定形的 κ 可能源于颗粒之间的不良热接触或高无定形含量，但后者不是 CNCs 的情况。此外，类似无定形的行为不受致密化的影响，因此改善了热接触。反而，我们将这种行为归因于 CNCs 的纳米级原纤维，这将声子平均自由路径限制在随机定向的 CNCs 网络中的几纳米。这就解释了为什么 κ 在 CNC、CF 和 CNF 网络中基本相同，它们是具有相同结构单元的材料——直径为 3-5 nm 的基本原纤维。我们得到 κ = (0.60 ± 0.01) W m在 295 K 和大气压下，随机取向 CNCs 的无孔网络为^–1 K ^{–1 ，κ 仅增加 14% GPa –1}，这对于聚合物来说是异常弱的。通过使用此类网络的模型，我们发现沿 CNC 的 κ = 1.9 W m ^–1 K ^–1并认为这也是在室温下沿 CNF 和 CF 的良好估计。