科学家们创造出钻石的几个潜在竞争对手

发布时间：2008-03-08 00:00:00 点击：466

[导读] 现代科学家们已经花了数十年时间来寻找更加廉价、坚硬和更实用的钻石替代品，每隔几年就会有新闻爆出：创造了一种新的“世界上最坚硬的物质”。但是这些挑战者真的成功了吗？

中国科技网1月25日报道（张微 编译）提到地球上最坚硬的材料，大多数人的回答可能都是钻石。钻石的名字来来源于希腊语，ἀδάμας意思是“牢不可破”或“无敌”，英文里这个词的意思是“坚硬的东西”。钻石的硬度赋予它无以伦比的切割能力，而且它还具有高“颜值”，这使得钻石在千百年的时间里一直备受推崇。

现代科学家们已经花了数十年时间来寻找更加廉价、坚硬和更实用的替代品，每隔几年就会有新闻爆出：创造了一种新的“世界上最坚硬的物质”。但是这些挑战者真的成功了吗?

虽然说钻石光彩夺目，但本质上来讲，它仅仅是一种特殊形态或称为碳的“同素异形体”。碳家族有几种同素异形体，包括碳纳米管、无定形碳、钻石和石墨。所有这些物质都是由碳原子构成的，但是他们之间的原子键类型不同，从而导致不同的材料结构和特性。

每一个碳原子的最外层都有四个电子。在钻石中，这些电子与其它四个碳原子共享，形成非常强大的化学键，造就了钻石非常坚固的四面体晶体结构。就是这样一种简单的、致密的结构使得钻石成为地球上最坚硬的物质。

到底有多坚硬?

硬度是材料的一个重要特性，这往往决定了它们的用武之地在何处，但是硬度又很难定义。对于物质来说，划痕硬度用于衡量物质硬度，用另一种矿物划过物质表面，根据划痕确定该物质的软硬。

还有几种测量硬度的方法，通常使用仪器来将材料表面局部塑性变形，塑性变形的面积与压力载荷的比值用来比较被测材料的软硬。材料越硬，比值越大。维氏硬度也是根据压痕单位面积上的载荷来计算硬度值。所不同的是维氏硬度试验的压头是金刚石的正四棱锥体。

低碳钢具有的维氏硬度值约为9GPa，钻石的维氏硬度约为70–100GPa。钻石的耐磨性是世界上绝无仅有的，今天世界上70%的天然钻石都用来制造用于切削，钻井和磨削，或作为磨料磨具添加剂等工具的耐磨涂层。

钻石也不是完美的，虽然它很坚硬，但也非常不稳定。当钻石在空气中加热到800摄氏度以上时，其化学性质发生改变，这会影响其强度而且能让它与铁反发生应，使得它不适合于加工钢。

这些方面限制了它的应用，因此科学家们一直在开发新的、化学稳定性强的、超硬材料作为替代品。更好的耐磨涂层能够让工业工具减少更换磨损零件，延长其使用时间，以及降低对污染环境的冷却剂的需求。迄今为止，科学家们已经开发出钻石的几个潜在竞争对手。

氮化硼

合成材料氮化硼，诞生于1957年，与碳类似，它也有几种同素异形体。立方氮化硼(c-BN)与钻石具有相同的晶体结构，但它不包含碳原子，而是由硼和氮原子构成的晶体结构。立方氮化硼具有化学和热稳定性，目前，作为超硬机械工具涂层，主要用于汽车和航空工业领域。

但是，目前立方氮化硼充其量只是世界上第二坚硬的材料，它的维氏硬度约为50GPa。纤锌矿氮化硼(w-BN)最初被认为比钻石更坚硬，这个结论是基于理论模拟的结果(预测它的压痕强度比钻石高18%)。不幸的是，w-BN在自然界中非常稀有，而且很难开发出足够的量来进行实验测试这种主张。

人造钻石

有关人造钻石的报道，自20世纪50年代以来就不绝于耳，因为晶体结构不同，所以它比天然钻石要硬。人造钻石的制造方法是，将石墨置于高温高压环境下，其碳原子会重新排布从而形成新的钻石，但这个过程非常耗时而且价格昂贵。另一种制造方法是，从加热的烃类气体中提取碳原子，但是这种基材的用途有限。

人造钻石是多晶结构，是一种由几微米到几纳米大小的微晶体和“晶粒”结晶聚合而成的多结晶钻石。与大多数用于加工首饰的大的单晶体结构不同。晶粒尺寸越小，晶界越大，材料也就越坚硬。对于人造钻石的最新研究表明，它的维氏硬度高达200GPa。

Q-carbon

最近，北卡罗来纳州立大学的研究人员创造了一种新形态的碳，不同于其它的同素异形体，这种材料比钻石还硬。这种新材料是用高能快速激光脉冲将非晶碳加热到3700°C，然后迅速冷却或“淬火”，形成微米大小的钻石，由此得名“Q-carbon”。

科学家们发现Q-carbon比类金刚石膜(一种无定形碳，与钻石的特性相似)的硬度高60%。这让科学家们很期待Q-carbon和钻石的硬度较量结果，虽然这仍需实验证明。Q-carbon有些独特的性质，它具有磁性，哪怕暴露在较低的能量下，它也会闪闪发光。但是，到目前为止，它只是用于常温常压下生产微小人造钻石的一个中间步骤。这些纳米钻石尺寸太小没法做成珠宝，但作为切割和抛光工具的廉价涂层材料却是理想之选。

Have scientists really found something tougher than nature's invincible material?

Ask most people what the hardest material on Earth is and they will probably answer "diamond". Its name comes from the Greek word ἀδάμας (adámas) meaning "unbreakable" or "invincible" and is from where we get the word "adamant". Diamond's hardness gives it incredible cutting abilities that – along with its beauty – have kept it in high demand for thousands of years.

Modern scientists have spent decades looking for cheaper, harder and more practical alternatives and every few years the news heralds the creation of a new "world's hardest material". But are any of these challengers really up to scratch?

Despite its unique allure, diamond is simply a special form, or "allotrope", of carbon. There are several allotropes in the carbon family including carbon nanotubes, amorphous carbon, diamond and graphite. All are made up of carbon atoms, but the types of atomic bonds between them differ which gives rise to different material structures and properties.

The outermost shell of each carbon atom has four electrons. In diamond, these electrons are shared with four other carbon atoms to form very strong chemical bonds resulting in an extremely rigid tetrahedral crystal. It is this simple, tightly-bonded arrangement that makes diamond one of the hardest substances on Earth.

How hard?

Hardness is an important property of materials and often determines what they can be used for, but it is also quite difficult to define. For minerals, scratch hardness is a measure of how resistant it is to being scratched by another mineral.

There are several ways of measuring hardness but typically an instrument is used to make a dent in the material's surface. The ratio between the surface area of the indentation and the force used to make it produces a hardness value. The harder the material, the larger the value. The Vickers hardness test uses a square-based pyramid diamond tip to make the indent.

Mild steel has a Vickers hardness value of around 9 GPa while diamond has a Vickers hardness value of around 70 – 100 GPa. Diamond's resistance against wear is legendary and today 70% of the world's natural diamonds are found in wear-resistant coatings for tools used in cutting, drilling and grinding, or as additives to abrasives.

The problem with diamond is that, while it may be very hard, it is also surprisingly unstable. When diamond is heated above 800℃ in air its chemical properties change, affecting its strength and enabling it to react with iron, which makes it unsuitable for machining steel.

These limits on its use have led to a growing focus on developing new, chemically-stable, superhard materials as a replacement. Better wear-resistant coatings allow industrial tools to last longer between replacing worn parts and reduce the need for potentially environmentally-hazardous coolants. Scientists have so far managed to come up with several potential rivals to diamond.

Boron nitride

The synthetic material boron nitride, first produced in 1957, is similar to carbon in that it has several allotropes. In its cubic form (c-BN) it shares the same crystalline structure as diamond, but instead of carbon atoms is made up of alternately-bonded atoms of boron and nitrogen. c-BN is chemically and thermally stable, and is commonly used today as a superhard machine tool coating in the automotive and aerospace industries.

But cubic boron nitride is still, at best, just the world's second hardest material with a Vickers hardness of around 50 GPa. Its hexagonal form (w-BN) was initially reported to be even harder but these results were based upon theoretical simulations that predicted an indentation strength 18% higher than diamond. Unfortunately w-BN is extremely rare in nature and difficult to produce in sufficient quantities to properly test this claim by experiment.

Synthetic diamond

Synthetic diamond has also been around since the 1950s and is often reported to be harder than natural diamond because of its different crystal structure. It can be produced by applying high pressure and temperature to graphite to force its structure to rearrange into the tetrahedral diamond, but this is slow and expensive. Another method is to effectively build it up with carbon atoms taken from heated hydrocarbon gases but the types of substrate material you can use are limited.

Producing diamonds synthetically creates stones that are polycrystalline and made up of aggregates of much smaller crystallites or "grains" ranging from a few microns down to several nanometers in size. This contrasts with the large monocrystals of most natural diamonds used for jewellery. The smaller the grain size, the more grain boundaries and the harder the material. Recent research on some synthetic diamond has shown it to have a Vickers hardness of up to 200 GPa.

Q-carbon

More recently, researchers at North Carolina State University create what they described as a new form of carbon, distinct from other allotropes, and reported to be harder than diamond. This new form was made by heating non-crystalline carbon with a high-powered fast laser pulse to 3,700 °C then quickly cooling or "quenching" it – hence the name "Q-carbon" – to form micron-sized diamonds.

The scientists found Q-carbon to be 60% harder than diamond-like carbon (a type of amorphous carbon with similar properties to diamond). This has led them to expect Q-carbon to be harder than diamond itself, although this still remains to be proven experimentally. Q-carbon also has the unusual properties of being magnetic and glowing when exposed to light. But so far it's main use has been as an intermediate step in producing tiny synthetic diamond particles at room temperature and pressure. These nanodiamonds are too small for jewellery but ideal as a cheap coating material for cutting and polishing tools.

来源：中国科技网 2016年01月25日　13:45

地址：http://www.wokeji.com/guojipindao/dujiabianyi/201601/t20160125_2180057.shtml

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