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在英语论文写作中，时态正确是最基本的要求，但也是最容易出问题的地方。特别是在写Literatu Rereview，由于涉及到的资料类型较多，表述不同资料的时态又不一致，很容易混淆，甚至出现不合适或者错误的地方。不过Literatu Rereview是最常见的论文类型之一，即使时态较为麻烦，也需要牢牢掌握。下面详细介绍Literatu Rereview中最常用的三种时态，并通过实例分析时态在Literatu Rereview中的灵活运用。 Literatu Rereview中最常用的时态是一般现在时、一般过去时和现在完成时。使用不同的时态会给句子的意义带来变化。 1. 介绍一般性资料、现象或普遍事实时，句子的主要动词多使用一般现在时。如： A Cryptocurrency is a peer-to-peer digital exchange system in which cryptography is used to generate and distribute currency units. 2. 引述其他学者过去某个时间所做过的某一项具体的研究时，用一般过去时。如： More recently, Yatskar et al. (2014) proposed to generate descriptions from densely-labeled images, which incorporate object, attribute, action, and scene annotations. （请注意：定语从句中的资料为普遍事实，所以谓语动词用了一般现在时。） 3. 描述特定研究领域中最近的某种趋势，或者强调表示某些“最近”发生的事件对现在的影响时，常采用现在完成时。如： In the last decade graphene has emerged as an exciting new material, with potential to impact many areas of science and technology. 我们在撰写综述类文章时，不仅要对所涉及领域的研究现状进行概括和评述，还要对未来发展趋势进行合理的预测，所采用的时态种类尤为丰富。 下面我们以一篇时态使用非常有代表性的综述类文章Multiprocess 3D printing for increasing component functionality为例，看看作者选用具体时态的语境和用意。该综述发表在2016年的《科学》杂志上，时态的交替使用非常有代表性。除了以上三种最为常用的时态之外，还使用了现在进行时、现在完成进行时和一般将来时。 摘要 Layer-by-layer deposition of materials to manufacture parts—better known as three-dimensional (3D) printing or additive manufacturing—has been flourishing as a fabrication process in the past several years（前半句谓语动词选用现在完成进行时，意在指出3D打印的蓬勃发展不仅从几年前持续到现在，而且这种趋势还将持续下去） and now can create complex geometries for use as models, assembly fixtures, and production molds（后半句谓语为现在时情态动词+原形动词，介绍3D打印现在可以实现的功能，是众所周知的事实）. Increasing interest has focused on the use of this technology for direct manufacturing of production parts（现在完成时介绍到目前为止该领域已经取得的成果）; however, it remains generally limited to single-material fabrication, which can limit the end-use functionality of the fabricated structures（此处用一般现在时指出当前存在的问题）. The next generation of 3D printing will entail not only the integration of dissimilar materials but the embedding of active components（用一般将来时预测未来发展方向） in order to deliver functionality that was not possible previously（用一般过去时指出过去的状况）. Examples could include arbitrarily shaped electronics with integrated microfluidic thermal management and intelligent prostheses custom-fit to the anatomy of a specific patient（此处情态动词could属于模糊限制语，要表达的不是过去发生的事，而是对未来应用前景的预测）. We review the state of the art in multiprocess (or hybrid) 3D printing, in which complementary processes, both novel and traditional, are combined to advance the future of manufacturing（本句使用一般现在时，主句指出综述的重点内容，从句进行补充说明）. 本篇综述文章摘要首句没有像大多数论文摘要那样在概述研究背景时使用现在完成时，而是选用了强调持续动作的现在完成进行时，非常契合3D打印领域的研究现状，不仅表明该技术方兴未艾，有很大的研究空间，还意味着相关科研成果会如雨后春笋般不断更新发展。此句的现在完成进行时与正文第一段中多处出现的现在进行时形成呼应，表明该领域的研究每时每刻都在不断地变化发展。之后用一般现在时指出该研究领域目前存在的问题，再用一般将来时预测未来发展方向。 正文第一段 Traditional manufacturing is well optimized for mass production of identical parts but can involve complex assembly steps, result in material waste, and incur substantial expense for low-volume production. Three-dimensional (3D) printing, a technology receiving considerable attention recently for direct part production, allows the creation of complex geometric shapes that can be mass-customized without a need for part-specific tooling such as dies or molds. Originally referred to by terms such as rapid prototyping, solid freeform fabrication, and others, this layer-by-layer fabrication methodology converts a precursor material in a spatially controlled manner to create a complex shape.（在上面几句中，作者选用了一般现在时，意在指出该领域众所周知的事实和研究现状） As the technology has developed over the past three decades, both the spatial resolution and the diversity of usable materials have improved. These advancements have led to an increase in end-use production of consumer, aerospace, and biomedical devices as industry has recognized the potential opportunities offered by 3D printing for improving designs, reducing assembly requirements through part consolidation, and optimizing the manufacturing supply chain through point-of-use manufacturing.（这几句使用现在完成时来介绍该领域科研人员已经取得的研究成果，强调其对现在的影响和作用） General Electric’s chief executive officer, Jeff Immelt, has recently stated that by 2020, the corporation plans to produce over 100,000 3D-printed parts for jet engines, and to meet these goals, General Electric plans a $3.5 billion investment in 3D printing (1). （此处尽管通用电气CEO发出声明的动作已经结束，主句动词state仍使用了现在完成时，而不是一般过去时，意在表明其声明内容与现在和未来相关；从句两个动词plan则用了一般现在时，表明该公司的计划已经提上日程）Simultaneously, as 3D printing patents are expiring, the costs of these systems are decreasing dramatically, and they now are becoming accessible to the general public. Collectively, these trends are leading to a democratization of manufacturing (2). 3D printing, now often referred to as additive manufacturing to emphasize production rather than prototyping (3), is evolving. （上面三句话的谓语动词连续使用了五处现在进行时，旨在强调3D打印技术的更新换代是一个连续不间断的过程，与摘要首句的完成进行时形成呼应）Figure 1 illustrates （展示和解读图表用一般现在时）the potential for fabricating end-use structures with embedded fluidic control printed from a traditional single-material 3D printer (powder bed fusion of electron beam–melted titanium) (4). 在上面的正文第一段中，作者交替使用了一般现在时、现在完成时和现在进行时，主要目的是概述该领域的研究背景、介绍研究现状，意图表达清晰，时态的选择与3D技术领域的发展现状非常契合。 正文第二段 One potentially disruptive step in the evolution of 3D printing will be to increase the functionality of the manufactured components.（此句为本段中心句，谓语动词选用一般将来时，旨在预测3D打印技术的未来发展方向） The 3D printing process can be started and stopped to incorporate complementary fabrication processes or embed subcomponents manufactured using traditional methods（此处使用情态动词can表明目前可以实现的功能）; however, until recently, 3D printing has been generally relegated to the prototyping of single-material structures for form and fit evaluation (5).（现在完成时对前期研究进行概述点评，指出不足） The next generation of 3D printers will enable products with multifunctional capabilities, including combinations of features not possible with a single print material, produced within a unified, tooling-free, multiprocess printing environment.（预测未来的3D打印技术可以弥补这些不足） In this context, multiprocess (or hybrid) 3D printing is defined as additive manufacturing enhanced with complementary processes（一般现在时，用于解读专业术语）. These complementary processes can include traditional manufacturing such as machining, cutting, dispensing, robotic placement, and more（情态动词can表明该技术当前可以覆盖的领域）. This new approach represents a paradigm shift in which the goal is to fabricate, in a nonassembly process, multifunctional end-use devices, potentially combining electronic, electromagnetic, optical, fluidic, actuation, chemical, and thermal features simultaneously—all with the inherent geometric benefits of 3D printing（一般现在时，用于评述新技术的意义和贡献）. Moreover, without tooling requirements, these advanced 3D printing technologies continue to leverage mass customization (6), through which, for instance, sophisticated biomedical devices can now be fabricated for the specific anatomy of a patient（一般现在时，指出当前发展趋势及可实现的功能）. Figure 2 illustrates two examples of multifunctional devices that highlight the possibilities of using multiprocess 3D printing systems: (i) a gaming die that includes a processor, an accelerometer, and light-emitting diodes (LEDs) to enhance the outcome of a roll by illuminating the top surface (7) (Fig. 2A) and (ii) a periodic spiral antenna that allows for physical patterns of conductors and dielectrics with unprecedented geometries, which will inevitably provide new levels of performance in next-generation antennas (Fig. 2B).（本段最后一句解读图表，主要使用了一般现在时，最后在预测对未来可做出的贡献时选用了一般将来时）#p#分页标题#e# 由于文章较长,以下只选择典型段落继进行介绍： 作者介绍及评述3D打印的多功能成型过程 The cornerstone of 3D printing is the layer-bylayer fabrication methodology, which allows manufacturing flexibility with the provision of interrupting the process to leverage complementary processes.（一般现在时介绍3D打印的概念和原理） The ASTM (American Society for Testing and Materials) F42 subcommittee was established in 2009 (8) to provide standards for common terminology, testing methods, and file formats, among other concepts for additive manufacturing, and has identified a taxonomy of seven basic 3D printing processes（前半句用一般过去时指出分委会是2009年建立的，而后半句则强调分委会所确立的分类法现在仍然沿用）. Each of these processes stands to benefit from enhancements enabling the printing of structures with increased functionality.（本句起到承上启下的作用，谓语动词采用了一般现在时，既概括了上文中介绍的几种3D打印过程，又指出它们得益于前期研究成果，旨在引出本句后面的具体研究）The earliest reported research in the 1990s leveraged the layered technique of 3D printing by interrupting the process to integrate functional components and conductive traces within a structure (9). Kataria and Rosen enhanced vat photopolymerization with inserts (10), and Lopes et al. dispensed conductive inks to manufacture electronics with antennas built directly into solid polymer structures (11). （上面两句分别介绍了具体的研究成果，谓语动词均采用了一般过去时）Ultrasonic consolidation, a sheet lamination process, creates metal components by ultrasonically bonding a sheet of metal foil to a previously deposited sheet. A machining process using an end mill renders the required geometry and follows the bonding. Thus, 2D sheets are essentially stacked together to provide a 3D part fabricated with two processes—an early example of multiprocess 3D printing.（上面两句插入对专业术语的解读，属于普遍接受的事实，所以选用了一般现在时） Robinson et al. interrupted the ultrasonic consolidation process to insert simple circuits through the combination of material extrusion with both thermoplastic and conductive ink (12). Using the material extrusion 3D printing technique, Malone et al. demonstrated a circuit and clever electromechanical applications by using an open-source fabrication system that dispensed multiple materials, including conductors and dielectrics (13). Navarrete et al. described enhancements to a vat photopolymerization process that introduced microchannels into the substrate to guide and contain curable conductive fluid, which was microdispensed to leverage the minimum feature size of the laser-curing process (14). Conductive trace spacing was thus controlled by the precision of the laser beam, rather than the secondary dispensing process, illustrating the symbiotic benefit of a compound process over either process acting alone.（本段后面几句继续介绍及转述具体的研究成果，谓语动词仍然使用一般过去时） 上面这一段中分别对已有研究成果进行了具体的介绍，除了首句和中间插入两句对专业术语解读时采用了一般现在时之外，其他句子谓语动词均采用了一般过去时。我们再来看一段： The fabrication of most (but not all) multifunctional structures requires processing with multiple integrated technologies, including the combination of 3D printing with other complementary processes to provide or improve spatial control of material, geometry, and functionality.（此段首句的谓语动词使用了一般现在时，介绍该领域普遍接受的事实） These additional manufacturing capabilities can embed components, wires, batteries, antennas, and other subcomponents（情态动词can的一般现在时，表明当前可实现的功能）. The introduction of electrical and thermal interconnects allows for subsystem communication or the delivery of energy or heat across a structure（一般现在时，指出技术价值）. Conductive inks and pastes have been used in conjunction with 2D printing, given the manufacturing flexibility of direct writing (e.g., the ability to print conformally, no tooling required), and the combination has been investigated for over a decade with microdispensing (22–25), ink jetting (26–30), and aerosol jetting (31). Generally, the inks have been dispensed onto external surfaces of finished structures, but only a limited number of examples have involved interrupting and re-initiating a 3D printing process to fully embed interconnect capability.（上面几句连续使用现在完成时，概述前期研究成果） Conductive inks have improved over the past two decades but still suffer from high resistance relative to traditional printed circuit boards created with a bulk copper plating process（本句交替使用现在完成时和一般现在时，前者用于概述成果，后者用于指出面临困难）. Higher resistance conductors result in performance reductions with voltage drops and power loss, and reliability remains an additional concern (32)（本句谓语动词使用一般现在时，继续指出当前研究面临的问题）. Low-temperature metal alloys have been printed with modified thermoplastic extrusion heads (33) and through injection into polymer structures (34) to provide interconnection with improved conductivity relative to inks. Although these alloys tend to have higher conductivity relative to conductive particle-loaded inks, interconnection continues to disappoint in comparison with bulk plated copper used in traditional electronics. The use of laser direct structuring to allow for selective plating on 3D printed substrates has been demonstrated, provides bulk properties, and is an exciting development, but it does require an additional chemical bath process (35)（最后三句中交替选用了现在完成时和一般现在时，前者用于概括研究成果，后者则用于指出当前面临的问题并加以评述）. 由以上几个从该综述文章中摘选出的段落我们可以看出，对文献进行综述时除了遵照前面指出的基本原则之外，作者可以根据具体的语境和话题选用多种时态，向审稿人和读者恰当有力地传递自己的意图。 |
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