The Structure of Solids

Transcription

1 生物材料學 BIOMATERIALS The Structure of Solids Sheng-Yang Lee, DDS, MS, PhD, FACD Professor and Dean School of Dentistry, Taipei Medical University 1

2 Purpose of the Class To develop in the students a familiarity with the uses of materials in medicine and with the rational basis for these applications. 2

3 Properties of a material Properties of a material determined by its 1. Chemical composition 2. Structure Internal structural arrangement of the atoms Chemical behavior Levels of scale: atomic or molecular (0.1 1 nm) nanoscale or ultrastructural (1 nm - 1μm) microstructural (1μm 1 mm) macrostructural (> 1 mm) Solid-liquid interface can affect dissolved species in the surrounding fluid: (i) Molecular level (3-15 Å) chemical effect (ii) Macromolecular level ( Å) mechanical nature 3

4 Atomic Bonding 4

5 Atomic Bonding All solids made of atoms held together with the interaction of the outmost (valence) electrons Nature of the patterns: 1. Primary bond (1) Metallic bonding e - loosely held to the ions nondirectional bond plastic deformations 5

6 Atomic Bonding (2) Ionic bonding formed by exchanging electrons between metallic and nonmetallic atoms very directional bonds * Strong repulsive forces of like ions limited atomic arrangement 6

7 Atomic Bonding (3) Covalent bonding formed when atoms share valence e - to satisfy their partially filled electronic orbitals * Overlap of valence orbitals Bonds (Bur limited by strong repulsive forces between nuclei) -- highly directional and strong * diamond the hardest material known 7

8 Atomic Bonding 2. Secondary bonds can be a major factor contributing to material properties Two major secondary bonds: (1) Hydrogen bond arising when H covalently bonded to an electronegative atom (F, O, N) H + electrostatic force formation 8

9 Atomic Bonding (2) van der Waals bond arising through fluctuating dipole-dipole interactions - nondirectional bonds - much weaker than hydrogen bonds * when electrons are not distributed equally among ions dipoles * A fluctuating dipole moment of molecule or atom induce a moment in neighboring atoms weak electrostatic interaction of induced and original moments an attraction force 9

10 Atomic Bonding Bond type van der Waals Hydrogen Metallic Ionic Covalent 1180 Heat of Substance vaporization (kj/mol) N 2 13 Phenol 31 HF 47 Na 180 Fe 652 NaCl 1062 MgO 1880 Diamond SiO

11 Atomic Bonding The real materials may show some combinations of the bonding characteristics. e.g., silicon atoms share electrons covalent bur a fraction of electrons can be freed and permit limited conductivity (semiconductivity) Silicon has covalent & some metallic bonding characteristics 11

12 Crystal Structure 12

13 Crystal Structure 1. Atoms of the Same Size - Arrangement of atoms can be treated as an arrangement of hard spheres in view of their maintenance of characteristic equilibrium distances (bond length) X rays (short wavelengths, ~ 1Å atomic radius) measurement 13

14 Crystal Structure 1. Atoms of the Same Size - Atoms arranged in a regular array represented by a unit cell having a characteristic dimension, the lattice constant, a. * If extended into three dimensions simple cubic space lattice (one of three types of cubic crystals) 14

15 Crystal Structure Another two cubic crystals: (1) Face-centered cubic (fcc) - Close-packed (or actually closest packed) in three dimensions - Each atom touches 12 neighbors coordination number (CN) = 12 (rather than 6 in simple cubic) most efficiently packed structure (packing efficiency 74%) - represented by three layers of planes ABCABC 15

16 Crystal Structure (2) Body-centered cubic (bcc) - An atom located in the center of the cube - lower packing efficiency (68%) than fcc 16

17 Crystal Structure Noncubic crystals Hexagonal close-packed (hcp) - having the most efficient packed planes of atoms (as fcc) with 12 CN (packing fraction 74%) - repeating layers every other plane as ABAB 17

18 Crystal Structure Others: Orthorhombic Unit cell is a rectangular parallelepiped with unequal sides Monoclinic Unit cell is an oblique parallelepiped with one oblique angle and unequal sides Triclinic Unit cell has unequal sides and all oblique angels. 18

19 Crystal Structure Cr Co Material Fe Ferrite(α) Austenite(γ) Delta iron(δ) Mo Ni Ti Rock salt (NaCl) Alumina (Al 2 O 3 ) Polyethylene Polyisoprene Crystal structure bcc hcp (below 417 ) fcc (above 417 ) bcc (below 912 ) fcc ( ) bcc (above 1394 ) bcc fcc hcp (below 900 ) bcc (above 900 ) fcc hcp orthorhombic orthorhombic 19

20 Crystal Structure 2. Atoms of Different Size Pure materials are seldom used for implants Most of the materials used for implants made of more than two elements. Two or more different sizes of atoms mixed in a solid two factors must be considered: (1) Type of site (2) Number of sites occupied 20

21 Crystal Structure At a certain radius ratio of the host and interstitial atoms the arrangement will be most stable (i.e., the maximum interaction between atoms) 21

22 Imperfections In Crystalline Structures 22

23 Imperfections In Crystalline Structures Imperfections in crystalline solids called defects play a major role in determining physical properties 1. Point defects lattice vacancies substitutional or interstitial atoms called alloying elements if put in intentionally called impurities if they are unintentional 23

24 Imperfections In Crystalline Structures 2. Line defects (Dislocations) created when an extra plane of atoms is displaced or dislocated out of its regular lattice space registry * L t Screw dislocation (Line parallel to shear direction); R t Edge dislocation (Line shear direction) 24

25 Imperfections In Crystalline Structures the strength of a solid crystal enormously ( it takes much less energy to move or deform a whole plane of atoms one atomic distance at a time rather than all at once) e.g., moving a carpet on the floor or a heavy refrigerator 25

26 Imperfections In Crystalline Structures If a lot of dislocations introduced in a solid strength considerably ( the dislocations become entangled with each other impeding their movement) e.g., Blacksmith heats a horseshoe red-hot and hammers it repeatedly number of dislocations without breaking the horseshoe 26

27 Imperfections In Crystalline Structures 3. Planar defects exist at grain boundary * Two or more crystals mismatched (with different orientation) at the boundaries (occurs during crystallization) grain boundaries (Grain: All of the atoms are in a lattice of one specific orientation) * Grain boundary is less dense than the bulk Most diffusion of gas or liquid takes place along the grain boundary 27

28 Imperfections In Crystalline Structures * Grain boundary atoms possess higher energy than the bulk a more chemically reactive site at the boundary (can be seen by polishing & etching of a polycrystalline material) * Grain size affect physical properties e.g., A fine-grained structure stronger than a coarse one ( the former contains more grain boundaries interfere with the movement of atoms during deforming a stronger material) 28

29 Long Chain Molecular Compounds (Polymers) 29

30 Long Chain Molecular Compounds (Polymers) Polymers have very long-chain molecules formed by covalent bonding along the balckbone chain The long chains held together by (1) secondary bonding forces (e.g., van der Waals & hydrogen bonds) or (2) primary covalent bonding forces through cross-links between chains The long chains are very flexible & tangled easily 30

31 Long Chain Molecular Compounds (Polymers) Each chain can have side groups, branches, and copolymeric chains or blocks interfere with the long-range ordering of chains Steric hindrance a more noncrystalline structure Semicrystalline more commonly occurring structure for linear polymers: Disordered noncrystalline regions Ordered crystalline regions 31

32 Long Chain Molecular Compounds (Polymers) Degree of Polymerization (DP) Defined as average number of mers or repeating units per molecule (i.e., chain) Each chain may have a small or large number of mers depending on the condition of polymerization Average molecular weight = DP x molecular weight of mer 32

33 Long Chain Molecular Compounds (Polymers) The longer molecular chains (by progress of polymerization) relative mobility physical properties of final polymer The higher the molecular weight, the less the mobility of chains the higher strength & greater thermal stability 33

34 Long Chain Molecular Compounds (Polymers) The polymer chains can be arranged in three ways: (1) linear Polyvinyls, polyamides, polyesters, etc. Much easier to crystallized than (2) & (3) However, they cannot be crystallized 100% as metals (2) branched (3) cross-linked or three-dimensional network e.g., (poly)phenolformaldehyde Cannot be crystallized at all noncrystalline, amorphous polymers 34

35 Long Chain Molecular Compounds (Polymers) 35

36 Long Chain Molecular Compounds (Polymers) Linear polymers usually become semicrystalline: Arrangement of chains in crystalline regions a combination of folded and extended chains Chain folds more difficult to form 36

37 Long Chain Molecular Compounds (Polymers) Vinyl polymers The most important linear polymer Repeating unit: -CH2-CHR- R is some monovalent side group R = H PE (HDPE, LDPE, LLDPE) R = CH3 PP R = C6H5 PS R = Cl PVC 37

38 Long Chain Molecular Compounds (Polymers) Three possible arrangements of side groups (R): (1) Atactic Side groups randomly distributed (2) Syndiotactic Side groups in alternating positions (3) Isotactic Side groups in one side of the main chain 38

39 Long Chain Molecular Compounds (Polymers) The isotactic and syndiotactic polymers usually crystallize even when the side groups are large 39

40 Long Chain Molecular Compounds (Polymers) Copolymerization Two or more homopolymers (one type of repeating unit throughout its structure) chemically combined always disrupting the regularity promoting the formation of noncrystalline structure 40

41 Long Chain Molecular Compounds (Polymers) Addition of plasticizers preventing crystallization by keeping chains separated from one another a noncrystalline version of a polymer (that normally crystallizes) e.g., (i) Celluloid normally crystalline nitrocellulose plasticized with camphor (ii) Rigid noncrystalline polymer (like polyvinyl chloride, PVC) + plasticizers more flexible solid 41

42 Long Chain Molecular Compounds (Polymers) Elastomer or Rubber Large stretchability at room T and can snap back to their original dimensions when load released Recoverability Noncrystalline (Amorphous) polymers that have an intermediate structure consisting of long-chain molecules in three-dimensional network kink or bend in chains straighten when a load applied 42

43 Long Chain Molecular Compounds (Polymers) Below glass transition temperature (Tg) natural rubber loses its compliance a glasslike material Tg Transition temperature between a supercooled liquid and its rigid glassy solid To be flexible, all elastomers should have Tg well below room temperature 43

44 Supercooled And Network Solids 44

45 Supercooled And Network Solids Glass An amorphous solid below its transition temperature Lacks long-range crystalline order, but normally has short-range order Usually supercooled from the liquid state and thus retain a liquidlike molecular structure less density than that of the crystalline state of the same material indicating inclusion of some voids (free volume) 45

46 Supercooled And Network Solids When a liquid cooled contracts rapidly and continuously ( decreased thermal agitation atoms develop more efficient packing arrangements) 46

47 Supercooled And Network Solids In the absence of crystallization, the contraction continues below Tm to the Tg material becomes a rigid glass Below Tg, no further rearrangements occur. The only further contraction is caused by reduced thermal vibrations of the atoms in their established locations 47

48 Supercooled And Network Solids Due to quasi-equilibrium state of the structure amorphous material tends to crystallize More brittle and less strong than crystalline counterpart Very difficult to make metals amorphous, metal atoms are extremely mobile The ceramics and polymers can be made amorphous because of the sluggish mobility of their molecules 48

49 Supercooled And Network Solids Network structure of a solid a three-dimensional, amorphous structure since the restrictions on the bonds and rigidity of subunits prevent them from crystallizing not flow at high temperature 49

50 Composite Material Structure 50

51 Composite Material Structure Consisting of two or more distinct parts Distinct phases separated on a scale larger than the atomic, and properties (e.g., elastic modulus) significantly altered in comparison with those of a homogeneous material Bone and fiberglass Composites Brass, or metals (e.g., steel with carbide particles) not composites Although many engineering materials (including biomaterials) are not composites virtually all natural biological materials are composites 51

52 Composite Material Structure The properties of a composite material depend upon 1) Shape of the inhomogeneities (second phase material) e.g., particle, fiber, platelet or lamina) 2) Volume fraction occupied by them 3) Stiffness and integrity of the interface between the constituents 52

53 Reference 自行編纂 53

54 Summary Biomaterials Biocompatibility Biological Environment Swelling and Leaching Interfacial-Dependent Phenomena in Biomaterials The Structure of Solids Characterization of Materials 54