Advances on thermal insulation applications of aerogels
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摘要:
气凝胶独特的三维纳米网络结构使其同时具有超低密度和超低热导率的特性，是理想的轻量化隔热保温材料，在微电子、航空航天等对重量要求严格的领域行业具有很大吸引力，激起科研人员广泛的研究兴趣，并对该领域进行了大量的科研工作。本综述首先分析了气凝胶材料的制备方式，重点论述了溶胶-凝胶法、分子路线法、静电纺丝法、3D打印等方法并对气凝胶材料在隔热领域的应用进行了总结；后续又对气凝胶的材料选用等方面进行探讨，最后展望和分析气凝胶的未来研究重点方向。
Abstract:The unique three-dimensional network of aerogel endowed it with characteristics of ultra-low density and ultra-low thermal conductivity, These characteristics made it to an ideal lightweight thermal insulation material. And has highly attractive in fields of microelectronics, aerospace and other fields with strict weight requirements. Therefore, it sparks extensive research interest of researchers work in this field. This review first analyzes the preparation methods of aerogel materials, mianly focus on sol-gel method, molecular route method, electrospinning method, 3D printing and other methods in the thermal insulation application of aerogel. Subsequently, discussions on material selection of aerogel are carried out. Finally, a brief summary of prospect and analysis on the future research directions of aerogels are made.
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Keywords:
- aerogel /
- thermal insulation /
- sol-gel /
- drying method /
- aerogel material
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随着工业化进程的加速推进，汽车工业得到了快速的发展，但同时造成了严峻的环境问题和能源浪费问题。目前我国已成为最大的废弃橡胶产生国，而目前废弃橡胶的主要来源于废弃轮胎、废旧胶带、胶管、垫板以及密封件等[1]。相关统计数据结果表明：我国每年消耗橡胶近200万吨，但其回收利用率却不足30%。传统的处理方式，如集中焚烧、随意堆放或弃于垃圾填埋场，不仅未能实现资源的有效再利用，反而给环境带来了严重的污染[2]。面对这一严峻问题，废旧橡胶的综合处理和循环再利用已成为国内外学者关注的焦点。将废弃橡胶加工成颗粒状，并按一定的比例与水泥基材料相结合，研发出新型橡胶混凝土材料，不仅能够对水泥基材料进行改性，提升混凝土的工作性能，同时解决了废弃橡胶的回收再利用问题[3]。相关研究表明，将橡胶掺入混凝土材料能够显著增强其韧性、抗冲击性能、抗渗性及抗冻性[4-7]。为改善普通混凝土材料拉压比低、韧性差、易开裂以及开裂后裂缝宽度难以控制等缺点，国内外学者通过在混凝土中掺入钢纤维，研究结果表明，纤维的掺入能够显著提高材料的强度和韧性，其优异的性能使得纤维混凝土在工程中得到广泛的应用。朱江等[8-9]通过对不同掺量的钢纤维橡胶高强混凝土进行力学性能试验，结果表明：随着钢纤维体积掺量的增大，钢纤维增强橡胶混凝土的抗压强度、劈裂抗拉强度和抗折强度均有一定程度的提升，钢纤维和橡胶的掺入有效地提升了材料的强度和韧性，高丹盈等[10]研究了纤维类型和掺量对于混凝土剪切性能的影响，结果表明纤维的掺入可以有效改善混凝土的抗剪强度和变形性能。梁兴文等[11]研究结果表明超高性能混凝土抗弯力学性能主要受长纤维的影响，短纤维对其影响较小，并给出了长、短纤维的最优混合掺量。
随着石油、煤炭等能源的枯竭以及可持续战略的加速推进，增加了社会对于清洁能源的迫切需求，如中国与俄罗斯合作开发的极地LNG-2天然气项目、大型LNG (Liquefied natural gas，LNG)储罐和可燃冰等低温工程。温度低于−50℃的环境通常被称为超低温环境，液化天然气的储存环境往往低于−163℃，因此天然气行业的快速发展促进了混凝土结构在极端低温环境下的研究，混凝土材料在超低温环境下的应用也愈加广泛[12]。Xue等[13]研究指出添加橡胶使混凝土试件在低温(−30℃)下的延性显著提高。Yu等[14]对−20℃下橡胶混凝土的静态力学性能和抗冲击性能的研究结果表明：−20℃下的橡胶混凝土比普通混凝土有着更好的韧性抗冲击性能。在Bu等[15]的研究中发现橡胶的加入可以提高混凝土的耐水性和抗渗性，并且具有更高的保温性能。同时，钢纤维能够改善橡胶混凝土的抗拉、抗弯、抗冲击及抗疲劳性能。因此钢纤维橡胶混凝土极地低温工程中具有独特的应用优势。但目前研究成果主要集中于常温和低温环境下的钢纤维增强橡胶混凝土的性能研究，而对于超低温极端环境下的性能研究却少有文献报道。Dahmani等[16]对超低温作用下混凝土的性能开展了相关研究，发现混凝土低温作用下的性能主要是由其孔隙率决定，由于低温作用材料孔隙中形成了冰，混凝土的强度随着温度的降低而逐渐增加，但混凝土返回室温后其性能却明显变差，研究人员发现超高性能混凝土(UHPC)在超低温环境下的强度较常温状态下有显著的提升[17-19]。Xie等[20]研究表明，当温度降低至−120℃时混凝土的抗压强度达到最大值，当温度继续下降后，混凝土材料的抗压强度增长速度逐渐缓慢。
鉴于目前对于超低温环境下的钢纤维橡胶混凝土(Steel fiber reinforced rubber concrete，SFRRC)的性能研究较少，本文在国内外现有研究成果的基础上，通过四点弯曲性能试验研究超低温作用后钢纤维体积掺量和橡胶掺量对SFRRC弯曲性能的影响，为极端环境下SFRRC材料的性能优化设计和推广应用提供数据支撑和理论参考。
1. 实验材料及方法
试验采用P·O 42.5普通硅酸盐水泥、武汉电厂I级粉煤灰、精细河砂、橡胶颗粒和镀铜微丝钢纤维，镀铜微丝钢纤维性能指标见表1，橡胶颗粒性能指标见表2。试验配合比见表3。试件编号中NC表示为普通混凝土；20%RC表示橡胶颗粒体积掺量为20vol%的橡胶混凝土；0.5%SF-20%RC表示掺入体积分数为0.5vol%钢纤维和20vol%橡胶颗粒的SFRRC，以此类推。
表 1 镀铜微丝钢纤维(SF)性能指标Table 1. Performance index of copper plated microfilament steel fiber (SF)Type Density/(g·cm−3) Diameter/mm Length/mm Elastic modulus/GPa Tensile strength/MPa Copper-plated micro steel fiber 7.8 0.25 13 200 ≥ 2850 表 2 橡胶颗粒性能指标Table 2. Performance index of rubber particlesType Mesh Apparent density/(kg·m−3) Bulk density/(kg·m−3) Average particle size/μm Rubber particles 80 1180 299 175 表 3 钢纤维橡胶混凝土(RC)配合比及试件分组Table 3. Mix proportion of steel fiber rubber concrete (RC) and the grouping of test piecesSpecimen Volume fraction of
steel fiber/vol%Volume fraction of
rubber particle/vol%Steel fiber/
(kg·m−3)Rubber particle/
(kg·m−3)Fly ash/
(kg·m−3)Cement/
(kg·m−3)Sand/
(kg·m−3)Water/
(kg·m−3)NC 0 0 0 0 533.33 120 133.3 248 20%RC 0 20 0 0.24 0.5%SF-20%RC 0.5 20 0.16 0.24 1%SF-10%RC 1.0 10 0.31 0.12 1%SF-20%RC 1.0 20 0.31 0.24 1%SF-30%RC 1.0 30 0.31 0.36 1.5%SF-20%RC 1.5 20 0.47 0.24 Note: NC—Normal concrete. 将胶凝材料、河砂以及橡胶颗粒置于搅拌机中干拌，使其充分混合后，加入水和高效减水剂，为防止纤维成团，待拌合物呈现团状状态后均匀投入钢纤维，搅拌后得到最终拌合物。将拌合物倒入模具中，放置常温环境下1 d后脱模，得到成型试件后放入标准养护室养护28 d，取出后置于自然环境下晾干。每组试件包括3个尺寸为100 mm×100 mm×400 mm的梁式试件，将每组试件贴上热电偶以监测试件的实时温度，放入无锡腾川仪器设备有限公司生产的CDWX-3000超低温深冷试验箱中，采用通入液氮的方式对试件进行降温，降温速率为2℃/min。试验温度梯度设置为20、0、−50、−100、−150和−196℃。试验观测到热电偶恒温80 min后示数达到基本稳定，为了保证温度控制的准确性，确保试件整体达到目标温度值，当试件达到目标温度后恒温100 min[21]，降温完成后将试件放入保温箱中等待进行下一步试验。降温过程如图1所示。本文根据《纤维混凝土试验方法标准》设计四点弯曲试验[22]，加载试验机采用美特斯工业系统(中国)有限公司生产的MTS微机控制抗折试验机，试验机支座布置在梁试件标距的三分点处，跨中挠度采用红点激光位移计进行测量，降温设备和试验装置见图2。
2. 试验结果分析
2.1 SFRRC的破坏形态与荷载-挠度曲线
图3为SFRRC试件裂缝开展与破坏照片。从图3(a)可以看出，经过超低温作用后，SFRRC试件表面已出现细微裂缝，且随着降温幅度的增加，细微裂缝的数量更加密集，但SFRRC试件仍然具有较好的完整性，无明显外鼓以及剥落现象。超低温下SFRRC试件在受弯破坏时，主要表现为主裂缝扩展，出现呈撕裂状的延性破坏断口，同时试件表面附着一层“白霜”，如图3(b)、图3(c)所示。
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图 3 钢纤维增强橡胶混凝土(SFRRC)试件裂缝开展与破坏Figure 3. Crack development and failure of steel fiber reinforced rubber concrete (SFRRC) specimens本次试验测得各组试件在四点弯曲作用下的荷载-挠度曲线如图4所示。从图4(a)、图4(b)可以看出，随着荷载的增加，普通混凝土和单掺橡胶颗粒试验组在弯曲过程中迅速出现明显裂缝，表现出“一裂就断”的脆性特性；而掺入钢纤维的试验组在整个受弯破坏过程中表现出良好的韧性，破坏形态较为完整。掺入纤维后，SFRRC试件开裂前，此时荷载较小，荷载挠度曲线呈线性上升阶段；随着荷载增大，试件底部开始出现细微裂缝，低温作用后的部分试件荷载-挠度曲线出现陡降趋势，此时试件处于弹塑性阶段。继续增加荷载，裂缝将沿一条主裂缝继续扩展，在裂缝扩展的尖端位置出现明显的应力集中现象，截面上的应力发生重分布，钢纤维通过桥联作用将应力传递给基体，显著起到了增韧和阻裂的效果。当荷载达到峰值荷载的90%以上时，裂缝宽度逐渐增加，试件挠度迅速增大，并伴随着钢纤维的拔断和拔出，最终导致试件破坏。
2.2 试验结果分析
为评价超低温作用后SFRRC的抗弯性能，本文从强度和韧性两个方面对其进行分析，表4给出了各试验组在不同温度下的弯拉强度。
表 4 SFRRC平均弯拉强度试验值(MPa)Table 4. Average flexural strength test value of SFRRC specimens (MPa)Specimen 20℃ 0℃ −50℃ −100℃ −150℃ −196℃ NC 2.70 3.31 6.14 5.73 2.77 0.35 20%RC 2.24 1.95 4.91 10.39 5.82 5.13 0.5%SF-20%RC 2.28 4.84 10.39 14.02 11.87 12.76 1%SF-10%RC 2.96 2.46 6.35 4.98 7.22 6.38 1%SF-20%RC 4.36 4.57 8.00 10.98 8.67 10.18 1%SF -30%RC 4.51 4.87 8.35 14.95 9.76 9.13 1.5%SF -20%RC 7.06 3.68 8.81 18.91 14.82 17.43 3. 影响因素分析
3.1 超低温作用对SFRRC强度的影响
各试验组初裂荷载、峰值荷载与温度关系如图5所示。未掺入钢纤维的试验组(NC、20%RC)在加载过程中表现出明显的脆性，试件开裂后迅速丧失承载力，因此NC和20%RC的峰值强度记为其初裂强度。试验结果表明，各试验组的初裂荷载和峰值荷载均表现出同一趋势，在降温初期，普通混凝土和SFRRC从常温状态降低至0℃时，弯拉强度略有下降，部分试件略有波动；继续降低温度至−50℃时，各试验组的弯拉强度均有明显的增大，当温度降低至−100℃时，弯拉强度达到最大，相对于常温状态下，峰值荷载最大可提升约514%；进一步降低温度，弯拉强度略有下降，当温度继续降低至−196℃时，峰值荷载提升幅度略有降低，最大为460%，抗弯强度最大可提升151.6%。混凝土强度随着温度变化可分为3个阶段：损伤阶段、快速增长阶段和平稳波动阶段，相应的温度区间分别为：−20~20℃、−100~−20℃和−196~−100℃[23]。
Rostásy等[24]根据不同大小的孔隙溶液的冰点和混凝土的热膨胀系数的变化，总结了20~−170℃温度范围内混凝土的热应变行为：(1) 20~0℃时，孔隙溶液未冻结，材料变形主要由热胀冷缩引起；(2) 0~−20℃时，大孔隙溶液开始冻结，冰体积膨胀挤压溶液至小孔，引起损伤和裂缝；(3) −20~−60℃时，大孔隙被冰填充，中等孔隙溶液冻结，吸附水层使更多溶液进入中等孔，导致微裂缝；(4) −60~−90℃时，中等孔中的冰体停止生长，胶凝孔中的孔隙溶液开始结冰；(5) −90~−170℃时，部分溶液迁移到胶凝孔隙中，大量的孔隙被冰填充。
超低温环境对SFRRC内部作用机制如图6所示。在降温初期，由于大孔隙中的水结冰膨胀产生的拉应力以及橡胶颗粒收缩导致孔壁损伤和基体细微裂缝扩展，造成混凝土内出现一定的损伤，但由于钢纤维的桥联作用，一定程度上缓解了材料内部的微裂缝出现。进一步降低环境温度，存在于SFRRC中较小孔隙中的孔隙水随温度降低开始逐渐结冰，这在一定程度上增大了SFRRC的有效受力面积，0℃后弯拉强度开始由降转为升。这是由于经超低温作用后，试件孔隙中的水过渡成冰的状态，填补了试件中的细微孔隙以及缺陷，使得试件内部更加密实，其强度随着温度的降低明显提升。随着温度的进一步降低，一方面更小孔隙中的孔隙水也开始冻结，另一方面由于冰体、骨料与基体间的弹性模量差异，其收缩程度不同，随着温度的降低，冰体、骨料产生收缩，导致其传递应力作用减弱，这两个因素相互耦合影响，导致−100℃后继续降低温度使得SFRRC的强度有所降低[25-26]。
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图 6 超低温环境对SFRRC作用机制示意图Figure 6. Schematic diagram of the mechanism of ultra-low temperature effects on SFRRC图7(a)为钢纤维体积掺量1.0vol%时不同橡胶掺量的初裂荷载和峰值荷载，图7(b)为橡胶颗粒掺量20vol%时不同体积掺量钢纤维的初裂荷载和峰值荷载。可以看出，钢纤维体积掺量不变时，橡胶的掺入对材料强度有一定的提升，当橡胶掺量从10vol%增大至30vol%，其峰值荷载增大约27%，弯曲强度增大约52%。当橡胶掺量达到30vol%时，其峰值荷载达到最大。但橡胶掺量的增加对SFRRC材料初裂强度未见明显提升。同时随着橡胶掺量的增大，在−100~−196℃区间范围内强度下降趋势更为显著，这是由于橡胶的热膨胀系数较大，在低温作用下其收缩更为显著，与基体间产生了更为显著的不均匀热变形，导致弯拉强度显著降低。而钢纤维的掺入明显提升了材料抗弯强度，当掺量从0.5vol%增加至1.5vol%，其初裂荷载提升并不明显，而峰值荷载提升了约209%，弯曲强度增大约215%。且随着温度的降低，其提升幅度更加明显，这归因于孔隙水结冰对钢纤维与基体界面过渡区的改善作用和钢纤维的桥接作用的增强[27]。
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图 7 橡胶和钢纤维掺量对SFRRC强度影响Figure 7. Effects of rubber and steel fiber contents on the strength of SFRRC3.2 超低温作用对SFRRC变形能力的影响
各试验组初裂荷载、峰值荷载对应的挠度如图8所示。初裂变形主要与材料弹性模量大小相关，而峰值变形主要包括初始阶段弹性变形和塑性变形两个部分构成[10]，在经历超低温作用后，一方面SFRRC的弹性模量发生了较大变化，导致其弹性变形随之发生变化，其变形记为ε1；另一方面，超低温作用下材料内部细观结构受损、微裂缝也不断扩展，导致其塑性变形也有相应变化，记为ε2，两种因素均对材料变形能力造成影响，应综合考虑分析两方面因素的影响。
试验结果表明，各试验组的初裂挠度、峰值挠度变形趋势均表现出先增大后降低的规律。在降温初期，由于冰体膨胀产生的挤压作用导致材料内部出现损伤，试件弹性模量降低，变形能力略有提升，但进一步降低温度后，由于冰体的填充效应，增加了材料的密实度，部分裂缝和孔洞被冰体填充，材料的弹性模量会有所增大[28]，变形能力有一定的降低。
初裂变形主要与材料的弹性模量有关，其中1%SF-20%RC试验组的初裂变形明显大于其他试验组，但继续增加掺量发现材料的变形能力略微下降。这主要是由于橡胶作为一种超弹性不可压缩材料，弹性模量要远小于其他类型的矿物材料，因此混凝土中的橡胶颗粒起到了弹性孔的作用，改善了材料内部的物理结构。现有的研究成果也表明混凝土梁的抗弯刚度和开裂荷载会随着混凝土中的橡胶含量的增加而降低，但其变形能力会体现相反的趋势[5]，其主要原因是由于随着橡胶掺量的增加，过多的橡胶颗粒与基体间缺少“强”化学键因素，降低了颗粒-基体界面效应，颗粒间浆体包裹层均匀程度降低，在颗粒-基体界面引入大量微观缺陷，降低了SFRRC的初始弹性模量[29]。
图9(a)为1.0vol%钢纤维掺量下不同橡胶掺量的初裂变形和峰值变形，图9(b)为20vol%橡胶掺量下不同体积掺量钢纤维的初裂变形和峰值变形。结果表明在钢纤维掺量不变时，随着橡胶颗粒掺量的增加，材料的变形能力明显提升，当达到30vol%掺量时，变形能力达到最大，但当温度降低至−196℃时，其变形能力要略低于20vol%橡胶掺量试验组；而橡胶颗粒掺量不变时，随着钢纤维的掺入，材料的变形能力略有提升，但超过1.0vol%体积掺量后，材料的变形能力急剧下降，当达到1.5vol%体积掺量时，变形能力已低于0.5vol%掺量。
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图 9 橡胶和钢纤维掺量对SFRRC变形影响Figure 9. Effects of rubber and steel fiber contents on the deformation of SFRRC这是由于橡胶的存在增加了材料的能量吸收能力，当裂缝开始出现并扩展时，橡胶颗粒能够有效地分散应力，降低裂缝尖端的应力集中，延缓裂纹发展。同时，掺入的钢纤维通过桥接作用分担了部分荷载并且对裂缝的快速扩展起到了抑制作用[30]。但过量的钢纤维降低了纤维在浆体中的比表面积，造成了纤维结团现象，降低材料的性能。
3.3 超低温作用后SFRRC的韧性评价
3.3.1 等效弯曲强度和弯曲韧性比
参考《纤维混凝土试验方法标准》[22]，计算了不同温度作用后SFRRC的等效弯曲强度，其计算表达式为
fe=ΩkLbh2δ (1) 式中：fe为等效弯曲强度(MPa)；Ωk为跨中挠度为L/150的荷载-挠度曲线下的面积(N·mm)；L为试验梁支座间的跨距(mm)；b为试件截面宽度(mm)；h为试件截面高度(mm)；δ为跨中挠度为L/150时的挠度值(mm)。
为进一步评价极端温度作用后的SFRRC的韧性性能，规范中进一步提出了弯曲韧性比Re，其计算表达式为
Re=fefcr (2) 式中，fcr为SFRRC的四点弯曲初裂强度(MPa)。
图10为不同影响因素下的等效弯曲强度和弯韧比，由于NC和20%RC组在试验过程中表现出显著的脆性，其韧性性能较差，因此在本节中未对此两组试件进行参数分析，计算结果表明：等效弯曲强度和弯韧比的趋势表现出与弯拉强度相同的规律，随着温度的降低，等效弯曲强度有所降低，但继续降低温度，等效弯曲强度明显提升，温度在−100℃时，各试验组的等效弯曲强度达到峰值，随着温度的进一步降低，等效弯曲强度进入波动阶段，但总体呈现出下降趋势。但弯韧比表现出不同的变化趋势，在20~−100℃温度区间范围内为试件损伤阶段，SFRRC的弯韧比表现出降低的趋势，但进一步降低温度后，SFRRC弯韧比有一定程度的提升，其主要原因是由于更小孔隙中的孔隙水结冰，填充了SFRRC中的孔隙和裂缝，提升了材料的密实度。
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图 10 不同影响因素下SFRRC的等效弯曲强度和弯韧比Figure 10. Equivalent bending strength and bending toughness ratio of SFRRC under different influencing factors3.3.2 弯曲韧性系数计算
韧性指标体现了材料在吸收能量和发生大变形后残余强度的能力[31]。《纤维混凝土试验方法标准》(CECS 13—2009)[22]中提出通过计算初裂挠度δ的3.0、5.5和10.5倍挠度点与荷载-挠度曲线围成的面积来确定弯曲韧性，但由于极端温度作用，规范建议的弯曲韧性计算方法难以计算超低温作用后的SFRRC的韧性，因此本文采用文献[21]所采用的韧性评估方法来计算超低温作用后SFRRC的韧性系数。
图11为不同影响因素下的SFRRC的韧性系数，计算结果表明：除1%SF-10%RC和1.5%SF-20%RC试验组试验结果出现波动， SFRRC韧性主要在降温初期下降，主要原因是材料内部毛细孔隙中的水冰点受孔隙大小和孔隙中盐溶液浓度的共同影响。Skapski等[32]提出的孔隙水冰点方程认为当温度降低至−2℃时，直径为50 nm的毛细管孔隙中的水会结冰，而当温度降至−7℃时，直径为10 nm的毛细孔隙水才会冻结。较大孔隙中的水结冰引起的膨胀效应更加显著，对SFRRC的力学性能影响较大。随着温度的进一步降低，较小孔隙中的孔隙水逐渐冻结，但对SFRRC力学性能造成的影响较小，SFRRC的韧性系数进入波动阶段，总体趋势趋于平缓。而随着钢纤维和橡胶颗粒的掺入，材料韧性出现明显改善，常温状态下材料韧性系数最高提升138.81%，但当纤维掺量达到1.5vol%时，材料的韧性系数较小，约2.5左右。这是由于纤维的掺入对材料性能具有正负效应，一方面，纤维的加入能够有效提升基体性能，起到增韧和阻裂的效果；另一方面，纤维的加入也会在纤维-基体界面产生一定的缺陷。随着纤维掺量的增加，纤维在浆体中的比表面积将显著降低，从而减弱纤维-基体界面的效应。
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图 11 SFRRC韧性系数Figure 11. SFRRC toughness coefficientT2—Toughness coefficient at 0.85 peak load; T3—Toughness coefficient at 0.5 peak load; T4—Toughness coefficient at 0.2 peak load同时钢纤维体积掺量为1.0vol%时SFRRC表现出良好的韧性性能，其韧性并未随温度的降低而下降；橡胶的掺入表现出与掺入钢纤维类似的趋势，由于橡胶颗粒不能和周围水泥砂浆牢固结合，橡胶与基体材料之间缺乏“强”化学键的联系，随着橡胶掺量的增加，韧性性能明显下降，综合评价表明当钢纤维体积掺量为1.0vol%，橡胶掺量为10vol%时，SFRRC材料韧性性能达到最优。
4. SFRRC荷载-挠度曲线本构模型
钢纤维、橡胶和超低温共同作用于SFRRC，使其在受弯荷载-挠度曲线的下降段表现出显著的增韧和阻裂效果。因此，本研究基于过镇海[33]提出的单轴压缩本构模型以及Wu等[34]提出的钢纤维混凝土受弯本构模型，在曲线下降段引入材料特征参数[35]，并对温度进行修正，提出了符合钢纤维橡胶混凝土弯曲荷载-挠度曲线特征的本构模型如下：
y=x，0⩽ (3) y = \frac{x}{{\alpha {{(x - 1)}^2} + x}}\text{，}x > 1 (4) \begin{split} \alpha =& (0.404f_{\text{f}}^{0.785} - 0.705) \\ & [2 - {(T + 273.15)^{0.2211}}({\lambda _{\text{f}}}^{ - 0.0148} - 0.974{\lambda _{\text{r}}})] \\ \end{split} (5) 其中：y=F/Fpeak，F为荷载，Fpeak为峰值荷载；x=δ/δpeak，δ为挠度，δpeak为峰值挠度；ff为SFRRC抗弯强度(MPa)；T为温度(℃)；λf=Vflf/df，其中Vf为钢纤维体积掺量(vol%)，lf为钢纤维长度(mm)，df为钢纤维直径(mm)；λr=ρrdr，ρr为橡胶体积掺量(vol%)，dr为橡胶平均粒径(mm)。对不同低温环境下的SFRRC荷载-挠度曲线进行拟合，拟合结果如图12所示。
通过对比分析本构模型和实验曲线的拟合程度，发现引入钢纤维和橡胶特征参数并对温度修正后的受弯本构模型在大多数情况下与实验数据呈现出高度的相关性。除纤维掺量1.5vol%、橡胶掺量20vol%、温度0℃特定条件下下降段拟合效果较差外，其余各种温度和掺量组合下拟合曲线与数据点吻合度较好。这一结果说明，经过钢纤维和橡胶特征系数和温度的修正后的模型能够较好地反映SFRRC的受弯挠度-荷载曲线，能够有效地表征不同温度下SFRRC的受弯力学行为。本试验提出的本构模型适用于长径比约52的镀铜微丝钢纤维、纤维体积掺量不超过1.5vol%、橡胶平均粒径为0.175 mm、橡胶体积掺量不超过30vol%、温度在−196~20℃的SFRRC荷载-挠度曲线的拟合。若用于其他掺量和温度条件，需要对模型下降段进行进一步的调整和优化。
5. 结 论
通过对超低温作用后不同钢纤维和橡胶掺量的钢纤维增强橡胶混凝土(SFRRC)试件进行四点弯曲性能试验，计算出SFRRC的修正弯曲韧性系数，分析了钢纤维体积掺量和橡胶颗粒体积掺量对SFRRC抗弯性能的影响，主要结论如下：
(1)常温下随着钢纤维和橡胶体积掺量的增加，SFRRC抗弯强度均会明显提升。当纤维掺量从0.5vol%增加至1.5vol%，其初裂荷载提升并不明显，而峰值荷载提升了约209%，弯曲强度增大约215%。橡胶的掺入能够提升材料的变形能力，当掺量从10vol%增大至30vol%，其峰值荷载增大约27%，弯曲强度增大约52%；
(2) SFRRC在超低温环境下抗弯强度会进一步提升。在降温过程中，SFRRC从常温状态降低至0℃时，弯拉强度均略有下降，随着温度进一步降低至−100℃时，SFRRC的弯拉强度达到最大值，与常温状态相比，弯拉强度最大可提升约514%。当温度继续降低至−196℃时，提升幅度略有降低，最大为460%；
(3) SFRRC在超低温环境下韧性会随着温度的降低而下降。同时韧性系数在降温初期的降低程度最为显著，继续降低温度，韧性系数进入平缓波动阶段，综合弯拉强度以及材料的韧性性能，1.0vol%钢纤维掺量和10vol%橡胶掺量的SFRRC在常温和超低温环境下具有最优的力学性能；
(4)基于传统本构模型提出了在不同温度下SFRRC的荷载-挠度曲线本构模型，该模型能够较好地拟合SFRRC荷载-挠度曲线的试验数据。
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图 10 (a) 直接书写法制备SiC纳米线气凝胶及热学性测试[75]; (b) 冷冻法制备碳纳米管/纤维素纳米纤维水凝胶[76]; (c) 交联增稠法打印SiO2气凝胶及隔热应用[77]
Figure 10. (a) process and thermal characterization of SiC nanowire aerogels by direct writing[75]; (b) process of carbon nanotube/cellulose nanofiber mixed hydrogel by freezing method[76]; (c) process and thermal insulation application of SiO2 aerogels by Crosslinking thickening method[77]
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表 2 不同纤维增强二氧化硅气凝胶的热-力性能
Table 2 Thermo-mechanical properties of SiO2 aerogel after enhancement by different fiber.
Aerogel composition Mechanical
property/MPaThermal
conductivity/
(mW·m−1·K−1)Ref. SiO2 0.01 0.030 [16] SiO2/Carbon Nanotubes 0.20 0.031 [24] SiO2/Graphene oxide 0.65 0.018 [25] SiO2/cellulose nanofibril 0.12 0.023 [26] SiO2/Quartz fiber 1.24 0.033 [27] SiO2/Glass fiber 1.34 0.021 [28] SiO2/Zr2O2 fiber 0.17 0.032 [29] SiO2/Aramid fiber 0.14 0.022 [30] SiO2/Blown foam 0.31 0.012 [31] 
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表 3 不同打印方式对比
Table 3 Comparison of 3D printing methods
Printing methods Description Characteristics Extrusion-based printing Preparing the ink with appropriate viscosity firstly, and form the required 3D structure layer by layer by means of nozzle extrusion. 1.Various shapes printable.
2.Low equipment cost.Light-based printing Using photo-curable resin to cure layer by layer to form a 3D structure by ultraviolet or visible light in the presence of a photoinitiator. 1.Usually short curing time and relatively high efficiency.
2.High printing accuracy.3D printed templating Using 3D printed resin as a template or mould. After injecting sol,
the template is removed by dissolution or pyrolysis to form a 3D structure1.No special requirements for sol control.
2.Suitable for traditional molecule-derived sols.
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表 4 不同类别的阻燃气凝胶性能对比
Table 4 Comparison of the performance of different types of insulating aerogel
Category types Advantages Disadvantages Inorganic aerogel Oxidation aerogels, Nitride aerogels, Carbide aerogels. Low density, low thermal conductivity, high porosity Insufficient mechanical properties, deformation at high temperatures organic aerogel Cellulose aerogel, polyimide aerogel Good mechanics and abundant raw materials Poor high temperature resistance, easy aging, poor durability organic - inorganic composite aerogel Binary and multi-component composite aerogels, composite aerogels with substrates Balancing mechanical properties and flame retardancy, Integrated versatility Low porosity and high density
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目的
由于气凝胶独特的三维纳米网络结构使其同时具有超低密度和超低热导率的特性，因而关于气凝胶的研究一直是热点领域，大量科研人员对此进行了很多研究工作，形成丰硕的研究成果。本工作通过对近些年气凝胶研究成果进行梳理总结，为其他想了解、学习气凝胶隔热前沿进展的科研人员提供借鉴，考虑到当前围绕气凝胶进展研究主要集中在功能、方法的区分，具体到隔热这一具体领域的研究较少，因此，本综述立足气凝胶的阻燃隔热应用，分析了气凝胶材料的制备方式，重点论述了溶胶-凝胶法、分子路线法、静电纺丝法、3D打印等方法并对气凝胶材料在隔热领域的应用进行了总结；后续又对气凝胶的材料选用等方面进行探讨，最后展望和分析气凝胶的未来研究重点方向。
方法本研究围绕气凝胶的阻燃隔热主题，采取定量分析和定性论述相结合的办法。由于气凝胶的隔热阻燃性能与气凝胶的结构和组成密切相关，而结构与制备方法存在直接联系，因而，从气凝胶的制备方法和材料2个方面论述了气凝胶在隔热阻燃方面的应用，并通过Citespace文献计量工具，利用分析关键词整理了气凝胶的隔热阻燃用的发展趋势和动向，进而总结了不同方法和材料的优缺点，最后展望了气凝胶未来研究重点。
结果纵然阻燃隔热用气凝胶制备方法和制备材料多种多样，但均存在缺点不足，达到商业化的成熟应用仍需进一步努力。在制备方法方面，溶胶-凝胶制备路线虽然技术简单且路径成熟，但气凝胶制备材料受限，且制备的气凝胶骨架结构仅通过纳米粒子相互连接，脆性大且易塌陷，需要增强工艺提升力学性能；纳米构筑单元组装法扩大了凝胶种类，优化了凝胶性能，实现对凝胶功能精准调控，但也是需要增强气凝胶力学性能；其他制备方法，诸如3D打印、静电纺丝等方式，存在加工效率低、制备成本高等缺点，需进一步优化制备设备、改进接收装置。在制备材料方面，包括无机气凝胶、有机气凝胶和通过多组分材料制备的无机/有机复合气凝胶3类，无机气凝胶虽然阻燃性好，但力学性需要改进提升；有机气凝胶与之相反，力学性能优异但高温易燃分解；进而提出无机/有机复合气凝胶制备路线，有效兼顾了力学-耐高温性能，但性能也需要进一步提升。
结论纵然气凝胶在阻燃隔热领域的研究取得了突破和发展，但距离产业化和商业化的应用仍存在差距，未来还需要开展更多工作。在此，本文提出关于阻燃隔热用气凝胶建设建议：一是进一步加强机理研究。探讨气凝胶的合成方法、孔隙形成机制、内部结构与其热-机械能之间的联系，为更好制备高性能气凝胶提供理论指导；二是开发新型纳米结构。当前，通过纳米构筑气凝胶的基础单元多为均匀结构，关于多孔结构或其它新型结构研究较少；且构筑单元的形貌、大小对气凝胶多孔结构的影响进而如何作用于阻燃隔热的研究较少；三是建立气凝胶阻燃评价体系。气凝胶阻燃隔热性能评价主要通过灼烧样品后观察结构完整性判断其性能。然而对其他性能，诸如力学性能、体积变化等无过多测试；四是研制集成多功能气凝胶。现代工业高速发展对材料的综合性能提出越来越高的要求，单一卓越性能的材料已难以满足市场需要，要充分利用气凝胶隔热阻燃性能的天然属性，开展交叉研究，研制多功能气凝胶，扩展在不同领域的应用性。诸如在建筑行业，开发透光隔热气凝胶，能够较普通玻璃减少50％以上的供暖能耗，降低光照强度；在纺织行业，针对高原寒区官兵对冬服御寒、抗菌、轻便的特殊需求，气凝胶纤维能够屏蔽人体热辐射、降低服装30％重量，另外赋予其抗菌功能，提升卫生性，更好保障高原官兵。五是如何精准控制气凝胶的孔洞结构、开发气凝胶双网络体系、研究快捷高效的干燥方法等将成为未来研究重点，对此大量的研究者也在积极探索，在制备方法、干燥方式以及材料研制上不断有新进展，以此提升气凝胶的综合性能和实现低成本简单的制备。我们相信，随着研究的不断深入，气凝胶将会推动现有领域的发展并可能创造新功能。
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