🧪 GEB 2602 材料学 小测笔记

Lithos

GEB 2602 Material Science

Overview

Processing Decides Structure Decides Property Decides Performance

A material is some substance/mixture of it used to make something else.

Material 核心特点 常见应用
Metals（金属） Conductive, Ductile, Tough but not hard Bridges, wires
Ceramics（陶瓷） Hard but not tough, Brittle, Heat-resistant Glass, porcelain
Polymers（聚合物） Lightweight, Flexible Plastic, rubber
Composites（复合材料） Combine different properties Concrete, carbon fiber
Semiconductors（半导体） Controllable conductivity Chips, sensors

Hardness (硬度)：抗压入/划伤/塑性变形

Strength (强度)：抗非弹性弯折变形 Plastic Deformation

Toughness (韧性)：抗断裂 (不一定抗弯折)，反义: brittleness

Stiffness (刚度)：抗弹性变形 Elastic Deformation

Fatigue: Repeated Loading



Paper

Paper is not a uniform solid film. It is a three-dimensional porous fibre network.

Wood fibers contains:

  • Cellulose: long chained polymer, stable, colorless, reflects light well

    Basis of white paper: Rich in –OH groups, forms Hydorgen Bonds, which provides strength of paper.

    Wet paper are weaker: H2O molecules enter fibre network and interfere with fibre-fibre hydrogen bonding.

    Cotton paper is mainly cellulose so it’s strong (e.g. bank notes)

  • Lignin: brown, reactive polymer, gives wood rigidity, photo-oxidize to turn yellow. Acid and light ages paper. Gives the smell of paper.

A resource for paper is GOOD if contains much Cellulose


Use pulping to extract fibers:

  • Mechanical Pulping: Preserves 95% of Lignin, Weak and prone to photo-oxidization. Newspaper
  • Chemical Pulpiing: Preserves long fibres of Cellulose, de-lignin, strong and stable. Kraft Paper

Beating & Refining: More contact area → More hydrogen bonds → Stronger paper. Overtreating can hurt the fibre structure and lower Porosity


Anisotropy: different properties in differed directions (due to fiber orientation)

  • Machine Direction 平行于纤维 (造纸机器): Aligned Fibres, higher tensil strength 抗拉强度, straighter tearing 因为裂纹可能沿纤维之间传播
  • Cross Direction 垂直于纤维: Less Aligned, lower tensil strength, irreguler tearing
  • Fibres tend to align along the machine direction. 抗拉强度与抗撕裂强度无关

Porous: multi-holed (pores). Porosity = Fraction of empty volumn inside a material.

  • Higher porosity → Lower density, better air permeability/water absorption, lower strength, higher drying speed.
  • Tissue paper 吸水性强: porous network on its surface for water to absorp + Capillary Action 毛细作用；while Coated Paper (涂布纸) 表面较致密

Corrugated Board (瓦楞纸板): Structure can improve performance without changing the material itself.

  • Sandwich Structure increases the second moment of area/cross-section and therefore bending stiffness.

3R: reduce, reuse, recycle


Insights

Q1. Why does wet paper tear more easily?

Water weakens hydrogen bonding between cellulose fibres, reducing the strength of the fibre network.

Q2. Why is newspaper more likely to turn yellow than kraft paper?

Mechanical pulping retains more lignin, which undergoes photo-oxidation and contributes to yellowing.

Q3. Why does paper have different strengths in different directions?

Fibres tend to align along the machine direction, creating anisotropic mechanical properties.

Q4. Why is corrugated board stiffer than flat paper?

Its sandwich-like structure increases the second moment of area and therefore bending stiffness.



Ceramics

Keywords: Inorganic, Non-metallic and Solid. Inert, hard, high m.p., high stiffness.

⚠️ Strong bonding → High hardness, but not necessarily high toughness. High Elastic Modulus.

Ceramics are either ionic or covalent. Unlike a sea of elctrons in that of metals, forming current within their free electrons, thus conducting electricity.

  • High bond energy, therefore hard, high m.t.

  • Ionic: nondirectional bonding, but cleavage on crystal planes. Transfered electron, like-charge repulsion

  • Covalent: directional bonding, shared electron, fixed bond angles

  • Ionicity is a continuum/proportion instead of an acurate classification

    Al₂O₃ 约 67% ionic；Si₃N₄ 约 28% ionic；SiC 约 10% ionic

Coordination Number: how many anions a given cation attracts around it


Clay sintering (烧结) is irreversible, putting it in water does not soften it. Higher bond energy requires higher sintering temp. Sintering is not necessarily at m.p., to diffuse atoms, not to melt.

Production of ceramics: Powder → Forming → Drying → Firing → Finishing

Ceramics cannot bend because unlike metals, its atoms cannot shuffle position without breaking, Dislocation motion is difficult:

  • charge repulsion when trying to slide and shuffle atoms

  • directed bond (covalent) WILL break

  • Few slip systems


⚠️ 陶瓷 Pottery and Porcelain: the latter is smoother, translucent. The former has lower sintering/firing temp. and porous.

⚠️ 钢铁 Steel and Iron: the former is mixture (alloy).


Insights

Q1. Why are ceramics hard but brittle?

Strong ionic and covalent bonds provide high hardness, but restrict dislocation motion and plastic deformation. As a result, cracks can propagate without significant plastic energy dissipation.

Q2. Why does increasing porosity reduce ceramic strength?

Pores reduce the effective load-bearing area and act as stress concentrators, promoting crack initiation and propagation.

Q3. Why is sintering irreversible?

Sintering involves atomic diffusion and the formation of strong bonds between particles, producing a consolidated structure that cannot simply be reversed by adding water.

Q4. If crack size increases four times, what happens to fracture strength?

According to Griffith’s relationship [\sigma_f\propto 1/\sqrt c]

Therefore, fracture strength decreases to half its original value.

Q5. What is the difference between hardness and fracture toughness?

Hardness measures resistance to local indentation or scratching, while fracture toughness measures resistance to crack propagation.



Metal

Iron: Soft (scratched by nail) and Malleable, relatively week

delocalized electron sea——conductive, malleable, non-directional bonding

plastic deformation: under stress atoms rearranged, while ceramic breaks

BCC 体心立方：顶点+正方体心

FCC 面心立方：顶点+六面面心

HCP 六方最密：正四面体，各向异性

铁在升温时 (910C) 从 BCC 到 FCC，可逆

Carbon is at interstitial (离最近铁原子最远空隙处)，提高硬度

Chromium：>11% 保护性氧化膜——不锈钢

镍：stabilizes austenite, improves toughness

锰、钼、钒：形成碳化物提高硬度

FCC 可以溶解更多的碳原子 (2.1%) 而 BCC 更少 0.02%

Steel 的碳原子含量低于 ⚠️ 2.14%，更高就是铸铁 cast iron (便宜、脆 brittle)

铸铁常含有 graphite 石墨/cementite 黑色区域，熔点低易铸造脆性高难锻造 (forge)

​ Grey Cast iron：flake graphite 对拉力/压力脆弱，可以减震

​ Ductile Cast iron：spherical graphite

钢铁碳含量更低 Ductile。


Q1. Why is pure iron soft but steel much harder?

Carbon atoms occupy interstitial sites and distort the iron lattice, hindering dislocation motion and increasing hardness.

Q2. Why can Austenite dissolve more carbon than Ferrite?

Austenite has an FCC structure with interstitial sites that can accommodate more carbon than the BCC structure of ferrite.

Q3. What is Pearlite?

Pearlite is a lamellar microstructure consisting of alternating ferrite and cementite layers.

Q4. Why does Quenching produce hard steel?

Rapid cooling suppresses carbon diffusion and forms martensite, where trapped carbon distorts the lattice and restricts dislocation motion.

Q5. Why is Tempering necessary after Quenching?

Tempering relieves internal stresses, reduces brittleness and improves toughness, usually at the cost of some hardness.

Q6. Why is Ductile Cast Iron tougher than Grey Cast Iron?

Spherical graphite causes less stress concentration than flake graphite, improving ductility and tensile strength.

Q7. Why does stainless steel resist corrosion?

Chromium forms a thin, protective Cr₂O₃ passive layer on the steel surface, limiting further corrosion.



Glass

Prism：disperse light (white - rainbow/spectrum)

Glass is an amorphous supercool liquid

石英 (晶体) 有一个固定 sharp 熔点 Tm，melts abruptly；

玻璃 (非晶体) 只有 gradual transition 开始点 Tg；Tg 以下是 high viscosity (粘度)/坚硬的，以上是流动的过冷却液体；在一个 broad range 上随着升温逐渐软化 (can be blown or shaped)，Tg 不是相比，不会有体积突变。

晶体是宏观上重复规则的 (regular lattice)，非晶体只有局部规则 (amorphous)

Zachariasen’s Network Trio：SiO₂、B₂O₃、P₂O₅ 等氧化物由于其 flexibility 更容易作为玻璃 Formers；Na₂O, K₂O, CaO 作为 Modifiers (Break network bonds 以降低前者熔点)；Al₂O₃ 作为 Intermediates (提升 durability)。

SiO4 Tetrahedron Network Flexibility：可以以等键角组成晶体，也可以与其他氧化物共熔为不规则氧化物。


Why is glass transparant?

玻璃中电子有两个可以处于的能级 energy levels，有一定能量带隙：valence band-conduction band gap。光子 photon 的能量不足以激发跃迁 transition/elevation，因此光子直接穿过所以是透明的；但玻璃对高能量 UV 可以吸收 (短波紫外线 UVb 可以被吸收，长波 UVa 不行)

金属-电子海，任意能量的光子都可以触发 (no gap)，所以光子被absorbed or reflected：opaque and shiny

Why Does a Straw Look Bent in Water?

玻璃中光传播速度低于空气，导致折射 refraction：lenses 的原理。Lead Glass is sparkler

What Gives Glass Its Color?

绿色：沙子中常见杂质 Fe2+，便宜。玻璃窗/瓶

红色：gold nanoparticles，吸收蓝绿段

蓝色：Co(II)，吸收红橙段

琥珀色：Fe3+ sulfur，阻挡 UV，啤酒瓶/药瓶防止光解


自然玻璃：Fulgurites (petrified lightning)，Libyan Desert Glass

玻璃窗：70% Quarz、15% Soda (Sodium Carbonate) 作为 flux 降低熔点 (Quarz-1700 to 1000 C)、10% Limestone (Calcium Carbonate) 没有碳酸钙的钠玻璃会在水中溶解

Float Process: Pour molten glass onto liquid tin, spreads flat. Flat glass costs 90% less then before.


相比便宜受热易碎的 soda-lime glass (Heats unevenly)，Pyrex (Borosillicate Glass) 含 B₂O₃ 对 thermal shock 更 resistent (Less heat expansion)。

Why Does Car Side Window Glass Shatter into Cubes?

Tempered glass 被快速加热冷却 (heated and air-cooled/quenched rapidly)，表面先收缩并变得致密；内部保持受压状态 (Surface compressed, inside in tension) —— 5x stronger

Rupert’s Drop：被冷水快速冷却，头强尾弱


Griffith Theory：tiny scratches under tension, stress concentrates at the crack tip，Strong in compression, weak in tension

Laminated：polymer holds cracks，Fiber：无缺陷 Drawn Glass

玻璃稳定 (Si-O 键很强) 但 hydrofluoric acid attacks the Si-O network directly



Carbon

Allotropes 同素异形体：Same element, Different structure

6Carbon: 4 valence electrons, 4 covalent bonds per atom.

  • Diamond: sp3-Tetrahedral 3D Isotropic Network

    4 strong sigma bonds of 109.5° angle. 无自由电子-insulator 绝缘体

    No weak planes (通常 defectless)

    钻石在 STP 下亚稳定 (石墨稳定) metastable，热力学 thermodynamics 上有转向石墨的倾向，但动力学 kinetics 不允许 (activation barrier too high)。

  • Graphite: sp2-Trigonal Planar, 3D Stacked Planes (sheets), Anisotropic

    石墨烯层内 hexagonal: 3 strong sigma bonds of 120° angle + 离域大派键 delocalized pi electrons = conductive: For makng Electrodes 电极

    层间只有微弱范德华力 Van der Waals Forces，Easy to slide and slip: For making pencil lead (More clay Harder-H, less clay Softer-B), using pencil is peeling graphene sheets onto paper.

    Anisotropy: within sheet 金属导电性，高导热性，高强度；across sheets 弱导电导热强度。

  • Graphene 石墨烯: 2D Single atom layer of Graphite

    Stronger than steel by weight, Flexible, Transparent. 最强导电导热体

  • Carbon Nanotubes (CNT): 1D rolled Graphene into seamless cylinder

    High tensile strength

  • Fullerenes 富勒烯: 0D C60, C70 … sp2 Nanotube caps

    C60: 12 Pentagons + 20 Hexagons

  • Amorphous carbon: sp2+sp3 mix, E.g. Soot 烟灰, Charcoal.

  • Activated carbon: sp2+sp3 mix

    Porous: 1g have surface area of teenis court.

    Absorb odor, pollutant & color: water filter, mask air filters, CO2 capture

Utilization: supercapacitors, fuel cells, hydrogen storage, high strength carbon fibers for wind turbines, airplanes etc, graphene flexible screens, CNT sensors, tire rubber (strengthened by carbon), lithium-ion batteries…



comments powered by Disqus