文/ 加尼·瑞澤普（Ghani Razaqpur，加拿大） 譯/ 王一帆

加尼·瑞澤普（Ghani Razaqpur，加拿大），加拿大工程院院士、南開大學環(huán)境科學與工程學院講席教授

愛因斯坦說過，物質(zhì)與能量可以互相轉(zhuǎn)化。世界上所有事物都是由物質(zhì)與能量構(gòu)成的。我們觸摸不到能量，但是我們能感受到它們，我們能看到光，聽到聲音，感到熱量。物質(zhì)則是可以碰觸的，它就存在于我們周圍，無時無刻無處不在，但我們卻很少會去思考它。

假如我們能夠用某種物質(zhì)制作一樣東西，那么那種物質(zhì)就叫材料。利用各種各樣的材料，我們可以修筑建筑、大壩、運河、鐵路，也能生產(chǎn)手機、電腦、汽車等。材料與我們的生活息息相關(guān)，離開材料我們將舉步維艱。

材料的分類

世界上有著數(shù)之不盡的材料，我們可以根據(jù)自己的需求用不同的方法將它們分類。其中一種是觀察材料在自然狀態(tài)下如何組合，這種方法叫作化學分類法。據(jù)此，我們將部分材料分類為金屬，如金、銀、銅、鐵、鋅等，它們通常是有光澤和堅硬的物質(zhì)，并且能夠?qū)釋щ姟?/p>

還有一類材料是陶瓷，包含了氧化鋁、二氧化硅、碳化硅、瓷、混凝土、玻璃等材料。它們都有很強的抗壓能力，但是在受拉和彎折時十分易碎。例如，混凝土在抗壓方面性能十分突出，但是，如果我們要用它制作柱子或橫梁，則必須在混凝土里加入鋼筋來增強它的抗拉力和抗扭力。

聚合物這一類中包含了塑料、橡膠、聚乙烯（PE）、聚苯乙烯（PS）、聚氯乙烯（PVC）、聚碳酸酯（PC）等。這類材料應用十分廣泛，塑料制品在我們身邊隨處可見，如在筆、電腦屏幕、汽車、飛機等的生產(chǎn)中都需要塑料。聚合物的應用已經(jīng)涵蓋了我們生活的方方面面。

最后一類是復合材料，這類材料都是由至少兩種上述材料組合而成的。現(xiàn)如今應用最廣泛的兩種復合材料是玻璃復合材料和碳復合材料。例如，復興號動車組、C90飛機、C919飛機、部分火箭等都使用了碳纖維復合材料，因為碳纖維既輕巧又有很高的強度。

我們還有其他的分類方法。根據(jù)材料是天然的還是人造的，可以將其分類為天然材料和人工材料。例如，鋼就是一種人工材料，因為自然界中沒有鋼。天然材料包含了如木頭、泥、石頭等材料。在古代，人們通常使用天然材料，因為天然材料可以隨取隨用。中國的長城就是由石頭修筑的。用泥土很輕松地就可以造出磚塊，改變木頭和石頭的形狀也不需要很復雜的技術(shù)，這就是古人對材料的運用。如今我們創(chuàng)造了更多的新材料，它們也取代天然材料成為主流。

如今，我們在工程建設中使用了大量的人工材料。利用它們，我們建設了橋梁、道路、鐵路、機場、隧道等交通工程，房屋、辦公樓、公寓樓、工廠、發(fā)電站等建筑工程，還建設了水壩、港口、運河、堤堰等水利工程。

無數(shù)的材料被運用于工程建設中，其中最重要的幾種天然材料為泥土、木材和石材，而最重要的幾種人工材料為鋼材、混凝土和瀝青。在面對兩種材料時我們該如何選擇？為什么我們選擇混凝土而不是鋼材來建筑房屋？問題的答案是，我們遵循了重要的材料選擇標準。

首要的選擇標準是性能。例如，一塊跳板應該具備什么性能？當一個跳水者站在上面時，它應該向下彎，當跳水者跳起時，跳板應該為其提供彈力，并且，它還應該具有足夠的強度而不會折斷。這就是對跳板的性能需求。其余的選擇標準包括材料是否方便獲得，使用該材料需要怎樣的技術(shù)，還有材料的環(huán)保性能及造價——我們應該選用最經(jīng)濟的材料來達成目標。

對材料的選擇取決于我們對材料的需求。沒有最好的材料，也沒有最差的材料。材料的好壞取決于它是否既安全又經(jīng)濟，還能滿足特定的需求。

鋼有許多特征，它的光澤、導電性和導熱性屬于物理學特性；它的硬度和彈性屬于力學特性；它的元素構(gòu)成、易腐蝕性屬于化學特性。任意材料的全部特性都可以根據(jù)物理學、力學和化學來分類。材料的性能往往由它的一種或多種特性決定。特性不分好壞，只在于它能否滿足使用者的需求。擁有萬金油特性的材料是不存在的。工程師會選擇一種材料，一定是因為這種材料的性能比其他材料更能滿足需求，如造價、環(huán)保等。

我們怎么確定材料的特性呢？每制造出一種新材料，工程師們就會取一些樣品，使用特殊的技術(shù)和設備針對它的特性進行測試。在測試了大量的材料之后，工程師們會把這種材料的特性及相關(guān)數(shù)據(jù)記錄在專門的手冊中，方便他們以后為項目選擇最合適的材料。生產(chǎn)材料的公司一般都會自行測試并提供自家材料的性能數(shù)據(jù)。

混凝土

現(xiàn)在我們介紹一些材料運用的實例。瀝青混凝土常用于道路建設。它由瀝青與石?；旌?，可以冷鋪或熱鋪，最后再用壓路機壓實。當工程師選用某種瀝青時，他會考慮它的各種特性，比如黏性、耐久性、延展性。同樣，每當我們選擇一種材料，我們都必須審視它的種種特性，看它是否符合我們的需求。

另一種材料是隨處可見的混凝土，它主要由碎石、水泥和水構(gòu)成。其實，混凝土是世界上除了水以外使用最多的材料。2018年，全世界共使用了370億噸混凝土。中國的混凝土產(chǎn)量和使用量均占全世界的1/3。由于制造混凝土需要水泥，因此生產(chǎn)了大約40億噸水泥。每生產(chǎn)1噸水泥都會產(chǎn)生超過1噸的二氧化碳——這是一種會導致氣候變化的溫室效應氣體。

由于混凝土可以在任意地點制作成任意形狀，所以在建設中我們不必一次就把整個建筑物建成。我們用混凝土建造出了很多偉大的橋梁，如世界上最長的橋梁——港珠澳大橋，世界上最高的橋梁——米洛高架橋。我們還用混凝土建造了很多偉大的建筑，如世界上最美麗的建筑之一多倫多市政廳、世界上最高的大樓迪拜塔。我們還建造出了中國的三峽大壩、美國的胡佛大壩、瑞士的韋爾扎斯大壩，還有無數(shù)的公路與運河。

和其他材料一樣，混凝土有自己的優(yōu)缺點。除了水泥，它的原料碎石、水都是天然易得的，它具有很強的抗壓力，幾乎能做成任意形狀，并且造價低廉，生產(chǎn)和使用混凝土都不需要很高超的技能與技術(shù)，通常還有不錯的耐久性。這些都是它的優(yōu)點。

但是，混凝土需要時間來凝固，所以，使用混凝土的建造時間比鋼材和木材都長；它容易開裂，特別是在干燥的氣候環(huán)境里；它的抗拉力很弱，僅僅很小的拉力就可以讓它斷裂；它的能量吸收能力和延展性都很差。生產(chǎn)水泥能耗較高，同時產(chǎn)生大量二氧化碳。

如前所述，混凝土的抗拉性能很弱，所以不能單獨應用于建造建筑或橋梁。而鋼有著優(yōu)秀的抗拉性能，因此，經(jīng)過鋼筋增強的混凝土可以同時擁有抗壓性與抗拉性，這就是鋼筋混凝土。它是一種非常優(yōu)秀的復合材料，能在崩塌前吸收大量的能量，且強度高、造價低。

鋼

鋼在某種意義上也算是復合材料，但其實它是一種主要由鐵和碳構(gòu)成的合金。鋼是現(xiàn)代社會中應用范圍最廣的工業(yè)材料之一。它的原材料是鐵礦石。生產(chǎn)鋼材的過程能耗很高，因此，同樣會排出很多溫室氣體。

鋼的生產(chǎn)過程很復雜，以下是比較重要的幾步：把鐵礦石分成小塊，放入一種特殊的坩堝中，用火爐加熱到1600—1700℃。熔化后的鐵沉于坩堝底部，爐渣上浮。把分離出來的鐵送入另一個爐中與碳和氧混合，最終的成品就是鋼。把鋼送到不同的工廠，就可以加工成各種零件與產(chǎn)品了。

鋼具有很優(yōu)秀的抗拉和抗壓性，它有很強的韌性，在某些環(huán)境中十分耐久。鋼在初次使用后是可以完全回收再利用的。鋼結(jié)構(gòu)的建筑工事的建造時間比混凝土框架要快得多，而且在建造時也不需要模板和臨時主支撐。

但想要生產(chǎn)高質(zhì)量的鋼材、制作鋼構(gòu)件和構(gòu)筑物都需要更高的技能與技術(shù)水平。在如海水、酸和高濕度這樣的特定環(huán)境中，它會被銹蝕并失去耐久性。鋼材造價相比混凝土和木頭要更昂貴。生產(chǎn)鋼材是高能耗的。鋼結(jié)構(gòu)建筑十分需要消防保護。

我們也用鋼材建造過很多偉大的橋梁，如悉尼海港大橋、舊金山的金門大橋。我們建造過很多偉大的建筑，如位于芝加哥的約翰·漢考克中心、位于曼哈頓的世界貿(mào)易中心一號樓、位于倫敦的“小黃瓜”瑞士再保險公司大樓。

木材

最后是木材。木材是一種天然環(huán)保的材料。它能吸收二氧化碳，減少溫室氣體的排放。在不直接暴露在潮濕環(huán)境的情況下，木材可以持續(xù)使用很長時間，它還有合理的強度與硬度。天然木材無法用于建造大跨度構(gòu)筑物，但經(jīng)過人工改良的木材可以增加跨度。如今，木材已經(jīng)不再用于建造行車用的橋梁了。

木材重量很輕，易于砍伐、搬運及組裝。它是環(huán)保材料并且造價低廉。生產(chǎn)木材只需要消耗很少的能量，同時，還能吸收溫室氣體二氧化碳，減緩氣候變化。

當然，木材也有缺點，當它暴露在過度干燥或潮濕的環(huán)境時容易腐朽；它的強度弱于鋼材、混凝土和一部分聚合材料；它的防火性能不強，并且不能隨處取用。

如今，木材主要用于修建房屋與一些特別的建筑物，如位于溫哥華的里士滿冬奧會橢圓運動場館的木質(zhì)屋頂，其設計遵從了“保暖屋頂”的設計理念。

如上所述，木材的防火性能不強。近期，舉世聞名的、美麗的、極具歷史意義的巴黎圣母院幾乎毀于一場大火。雖然它主要是由不可燃的磚石砌筑，但它是由木質(zhì)的屋頂所覆蓋，木材燃燒，整個屋頂都崩塌落下。可見防火性是我們在為建筑挑選材料時需要慎重考慮的重要特性。

最后我要說，沒有最好的材料，也沒有最差的材料。性能符合要求的材料就是好材料。黃金具有很高的價值且價格昂貴，但我們不能用黃金造橋，也不能用黃金生產(chǎn)汽車和飛機，因為它的強度和硬度都不夠。有一些材料之所以昂貴，是因為它不易獲得，或者生產(chǎn)它消耗巨大。造價是在工程建造中一個重要的材料選擇標準，我們都不想付出超過必要的成本。目前的研究主要專注于利用納米技術(shù)和生物技術(shù)使材料更智能、更堅固、更便宜、更耐用。研究同時還致力于使材料可循環(huán)使用，并且更環(huán)保。

當我們將世界交接到你們年輕人的手中時，請讓它變得更好。世界是一個封閉的系統(tǒng)，從外太空獲取資源是不切實際的。被消耗的物質(zhì)不會再次憑空出現(xiàn)。我們所擁有的都來自地球，如水和空氣，這些都是無法被替代的資源。因此，你們應當學會像大自然一樣進行循環(huán)利用與回收。大自然從不拋棄任何事物，一切都會被回收再利用。這是一個完整的循環(huán)，不停回到原點并再次向前。我們應當學習自然，努力地模仿它，回收利用所有的材料，讓地球成為一個更好的地方。

Einstein said: you can change energy to matter or matter to energy.In this world, everything we know is made from matter or energy.We can’t touch energy, but we can feel it. For example, we can see light, we can hear sound, we can feel heat, etc. However, we can touch matter as it’s around us everywhere and every day. We don’t think about it too much!

If we can make something from some matter, we call the matter “Material”. From materials we build buildings, dams, canals, railways and produce phones, computers, cars, etc. Materials are very important, and we can’t live without them.

Materials Can Be Classified in Different Ways

There are millions of materials in this world that can be classified in different ways, depending on what you want and what the purpose of each is. One way to look at materials is to see how they are put together by nature. We call this the chemical structure of materials.In this way, we have materials that are metals like iron, steel, copper,zinc, gold, silver—generally shiny and hard, through which electricity and heat can travel.

There are also ceramics. These materials are inclusive to aluminum oxide, silicon dioxide, silicon carbide, porcelain, concrete, as well as glass. They are very strong when pushed upon but easy to break if pulled or twisted. For example, from concrete we build a lot of buildings. Concrete is very strong in compression, but we have to reinforce it by steel bars to give it resistance to tension.

Then we have polymers, like plastic, rubber, polyethylene (PE), polystyrene (PS), polyvinyl chloride (PVC), polycarbonate (PC). They are very important because almost everything we see around us is made from plastic. For example, it takes plastic to produce pens, computer screens, cars, airplanes, etc. Polymer is used in practically everything now.

Finally, composites. This means they are materials that are made from combinations of the above materials. The most important composites today are glass composites and carbon composites. Fuxing trains, C90 airplane, C919 airplane, many rockets and many other things are creations of carbon fiber composites as carbon fiber is light and strong.

We also classify materials in other ways. One of which is based on whether they are found naturally or under artificial conditions. We call them, under this guideline, non-engineered and engineered materials. For example, steel is an engineered material because we can’t get steel naturally. Non-engineered materials would be like wood, soil, stone, etc. In the old days, people preferred to use nonengineered materials because they are naturally available. The Great Wall of China is made from stone. People made bricks from soil easily, and shaped wood and stone without complicated technology. That was what people did before. Today, we create many more new materials, and we use more engineered materials than non-engineered ones.

Nowadays, we use a large amount of engineered materials in construction. We build transportation structures like bridges, roads,railways, airports, tunnels. We build building structures like houses,office buildings, apartment buildings, factories and power plants. We build hydraulic structures like dams, harbours, canals and dykes.

There are many materials used in construction, but the most important materials are soil, wood and stone as non-engineered materials, and steel, concrete, asphalt as engineered materials. How do we choose one material versus another? Why do we choose concrete to build some buildings but not steel? There are important material selection criteria as follows.

Performance is the number one criterion. For example, what is the performance we want from a diving board? It should go down when a diver stands on it, then a bounce should be given to him/her when he/she jumps from it. It should be strong enough to do this repeatedly without breaking. These are its performance requirements. The second criterion is material availability. The third criterion is the ability to properly use the material. The fourth criterion is not being harmful to the environment. The final criterion is the cost, as one should use the most economical material that can do the job.

So, choosing materials depends on what we want the material for.There is no such things as “the best material”, just like there is no such thing as “the worst material”. It just depends on whether it can safely and economically meet the specified need.

When we talk about steel, we often say that it’s shiny, it conducts heat and electricity. These are physical properties of steel. We also say that steel is strong and elastic. These are mechanical properties of steel. We say that steel is made from iron and carbon, steel rusts easily. These are chemical properties of steel. Properties of any material can be classified based on their physical, mechanical or chemical properties. Performance depends on one or more properties of a material. There is no good or bad property, this depends on what is required by the user. No single material can meet all performance requirements in every situation. Engineers choose a material because when compared to other materials, it may better satisfy the criteria.

How does one determine these properties? Every time a new material is produced, engineers take samples of it to the laboratory and test them for the property they are interested in, using special techniques and equipment. By testing many materials, engineers determine their properties and collect the data in special handbooks that they can later use to select the best material for a given job. Usually,companies that produce a material will test them and will provide information about the relevant properties of that material.

Concrete

Here are some examples of materials used in real construction.Asphalt concrete is used in road construction. It is made of asphalt mixed with stone particles, laid cold or hot, and compacted by a roller. When a certain asphalt type is selected by engineers, they consider several properties, such as: How well does it stick to the stone particles? (adhesion) Can it last a long time? (durability) Can it stretch without cracking or rupturing? (ductility) In a similar way,every time we engineers choose a material, we have to look at many properties of it to see if it will perform as we expect.

The other material is concrete, which is everywhere around us.Concrete is mainly made from stone, cement and water. In fact, no other material is used more than concrete in the world except water.In 2018, 37 billion metric tons of concrete was used globally. China produces and consumes one-third of all the concrete produced in the world. Since concrete is made from cement, approximately 4 billion tons of cement is needed to make that much concrete. Each ton of cement produces more than one ton of CO2, a greenhouse gas, contributing to the climate change.

Concrete can be cast in any shape and anywhere, so we don’t have to build the whole structure at once. From concrete we have built great bridges, like The Hong Kong-Zhuhai-Macao Bridge, the longest bridge in the world, and the Millau Viaduct, the tallest bridge.We build great buildings like the Toronto City Hall, one of the most beautiful buildings, and the Burj Khalifa in Dubai, the tallest tower in the world. And we build dams like the Three Gorges Dam in China, the Hoover Dam in USA, the Verzasca Dam in Switzerland. We build highways and canals.

Like all other materials, concrete has advantages and disadvantages.Raw materials of it, except cement, are natural and readily available.Concrete has high compressive strength. It can be made into practically any shape. The cost of concrete is competitive. It can be produced and placed with a relatively low level of skill and technology,and it’s usually durable. These are its advantages.

But it needs time to gain strength. It requires a longer construction time than steel or wood. It shrinks and cracks, especially in dry climates. It has low tensile strength, and cracks under low load. Cement production produces a lot of carbon dioxide, which is a greenhouse gas and contributes to climate change.

As is mentioned above, concrete is too weak when subjected to pulling (tension), so it cannot be used by itself to make buildings or bridges. But steel is very strong in resisting tension. So, concrete can be reinforced with steel bars or rods, then it can resist both push(compression) and pull (tension) forces. This type of concrete is called reinforced concrete. It is an excellent composite material and it can absorb a lot of energy before failure. It is also very strong and cost effective.

Steel

Steel is also in some sense a composite material, but it is actually an alloy of mainly iron and carbon. Steel is one of the most widely used industrial materials in the modern world. It is made from iron ore, which is extracted from the ground. A lot of energy is needed to make steel. Therefore, it is contributing in a significant way to greenhouse gas emissions.

Producing steel is a very time-consuming process, but here are some steps. First, we take iron ore from the mine, break it into small pieces, then put it in a furnace. Next, we heat it to around 1600-1700 degrees Celsius to melt it in a special crucible, with iron settling near its bottom and slag sitting on top of the iron. The two are separated and the iron is sent to another furnace where it is mixed with carbon and oxygen, and the final product is steel. Steel is then sent to mills to produce different steel shapes and products.

Steel has high tensile and compressive strength. It is very ductile,and very durable when not exposed to severe environments. Steel is completely recyclable after its initial use. Construction of steel frames requires much less time than similar concrete frames, and no forms or major temporary supports are needed during construction.

But it needs a higher level of skill and technology to produce good quality steel and fabricate steel components and structures. It corrodes and lacks durability when exposed to certain environments like seawater, acids and high humidity environments. It is generally more expensive than concrete and wood. Steel production is highly energy intensive and steel buildings need fire protection.

From steel we have built a lot of great bridges like the Sydney Harbour Bridge, and the Golden Gate Bridge in San Francisco. We have also built great buildings like the John Hancock Center in Chicago,the No.1 World Trade Center in Manhattan, and the Swiss Re Building (the Gherkin) in London.

Wood

And there is wood. Wood is a natural and environmental-friendly material. It stores carbon dioxide, so it reduces greenhouse gas emissions. When protected from humid conditions, it can last a very long time, and it has reasonable strength and stiffness. Natural woodcannot be used for very long span, but for longer spans engineered wood can be used. Wood is not currently used to build bridges for vehicular usage.

Wood is light, easy to handle and assemble, and easy to cut. It is environmentally friendly, and the cost is competitive. It requires low energy to produce, and as it stores the greenhouse gas CO2, it mitigates the climate change.

Wood has disadvantages too. It rots when exposed to excessive wetting and drying and high humidity. It is not as strong as steel, concrete and some polymeric materials, it is not very fire-resistant, and it is not readily available everywhere.

Wood is currently used to mainly build houses and some special buildings, like the roof of the Richmond Olympic Oval in Vancouver that is designed on the “warm roof ” principle.

As mentioned above, wood can’t resist fire. Recently, the famous,beautiful, and historic Notre Dame Cathedral in Paris was almost destroyed by fire. Although it was mainly made from bricks and stones, which did not get hurt by fire, it was covered by a wood structure. Once the wood burned, the whole roof fell apart and was destroyed. So, fire resistance is an important property that we should consider in selecting materials for buildings.

To finish, I can say that there is nothing as the best or worst material. Any material that can perform as required is good. You may know that gold is very valuable and expensive, but you cannot build a house with gold, you cannot make a car with gold, or make an airplane with gold, because gold is not strong and stiff enough. Some materials are expensive because they are not readily available or cost a lot to produce. Costs constitute a very important criterion for selection of materials in construction, as we don’t want to pay more than we have to. Research currently focuses on making materials smart,stronger, cheaper, and more durable by using nanotechnology and biotechnology. Research is also going on to make materials reusable and environment friendly.

After we leave this world to you, please keep it in a good shape. We have not kept it in as good of a shape as you should. You should reuse and recycle materials because the world is a close-circuit system. You cannot bring materials from space into earth. Once you use something, it cannot be replenished. Practically, there is nothing you can replace on the earth by something from another planet. Everything is just from this planet. You cannot bring water or air from another planet. So, you need to reuse and recycle as nature does.You will never see nature throwing something away, and everything gets recycled. It’s a full cycle, always goes back and forth. We should learn from nature and that’s what we should strive to imitate, make all materials recyclable and make this world a good place for everyone to live in.