MASE 542/Chem 442 Ceramics and Glasses

stable attachment to connective tissue Medical applications Inorganic / non-metallic Diagnostics, thermometers, tissue culture flasks Fiber optics for biosensors and endoscopy Porous carriers for biomolecules Enzymes, antigens, antibodies Naturally inert and chemically resistant Resistant to microbial attack, pH change, temp Ceramics used in dentistry for dentures, glass filled ionomer cement and crowns Implants Generally used to replace skeletal hard connective tissues Require stable attachment CERAMICS AND GLASSES ARE GENERALLY USED TO REPAIR OR REPLACE SKELETAL HARD CONNECTIVE TISSUES. stable attachment to connective tissue

Ceramic Glass Inorganic compounds contain metallic and non-metallic elements inter-atomic bonding is ionic/covalent generally formed at high temperatures. Glass An inorganic product of fusion that has cooled to a rigid condition without crystallization An amorphous solid. Lacking detectable crystallinity only short-range atomic order glassy or vitreous

Crystal versus Glassy Ceramics Crystalline ceramics have long-range order, with components composed of many individually oriented grains. Glassy materials possess short-range order, and generally do not form individual grains. Most of the structural ceramics are crystalline.

Glass-ceramic: Polycrystalline solids prepared by the controlled crystallization (devitrification) of glasses. Bioactive material: A material that elicits a specific biological response at the interface of the material, resulting in the formation of a bond between the tissues and the material.

Common Ceramics Alumina, Zirconium, Hydroxyapatite, Calcium phosphates, Bioactive glasses are common Porous ceramic materials exhibit much lower strengths extremely useful as coatings for metallic implants.  The coating aids in tissue fixation of the implant by providing a porous surface for the surrounding tissue to grow into and mechanically interlock.  Certain ceramics are considered bioactive ceramics if they establish bonds with bone tissue.

Ceramic Material Properties • Composition • Microstructure • Phase state – Crystal structure – Defect structure – Amorphous structure – Pore structure • Surface – Flatness – Finish – Composition – Porosity • Shape and processing Microstructure is affected by The Thermal processes

Atomic Bonding in Ceramics -- Can be ionic and/or covalent in character. -- % ionic character increases with difference in electronegativity of atoms. • Degree of ionic character may be large or small: CaF2: large SiC: small Adapted from Fig. 2.7, Callister & Rethwisch 8e. (Fig. 2.7 is adapted from Linus Pauling, The Nature of the Chemical Bond, 3rd edition, Copyright 1939 and 1940, 3rd edition. Copyright 1960 by Cornell University.)

Factors that Determine Crystal Structure 1. Relative sizes of ions – Formation of stable structures: --maximize the # of oppositely charged ion neighbors. - + unstable - + - + Adapted from Fig. 12.1, Callister & Rethwisch 8e. stable stable 2. Maintenance of Charge Neutrality : --Net charge in ceramic should be zero. --Reflected in chemical formula: CaF 2 : Ca 2+ cation F - anions + A m X p m, p values to achieve charge neutrality

Coordination # and Ionic Radii cation anion • Coordination # increases with To form a stable structure, how many anions can surround around a cation? Adapted from Fig. 12.2, Callister & Rethwisch 8e. Adapted from Fig. 12.3, Callister & Rethwisch 8e. Adapted from Fig. 12.4, Callister & Rethwisch 8e. ZnS (zinc blende) NaCl (sodium chloride) CsCl (cesium r cation anion Coord # < 0.155 2 linear 0.155 - 0.225 3 triangular 0.225 - 0.414 4 tetrahedral 0.414 - 0.732 6 octahedral 0.732 - 1.0 8 cubic Adapted from Table 12.2, Callister & Rethwisch 8e.

table_12_03 table_12_03

Computation of Minimum Cation-Anion Radius Ratio Determine minimum rcation/ranion for an octahedral site (C.N. = 6) a = 2ranion

Example Problem: Predicting the Crystal Structure of FeO • On the basis of ionic radii, what crystal structure would you predict for FeO? Cation Anion Al 3+ Fe 2 + Ca 2+ O 2- Cl - F Ionic radius (nm) 0.053 0.077 0.069 0.100 0.140 0.181 0.133 • Answer: based on this ratio, -- coord # = 6 because 0.414 < 0.550 < 0.732 -- crystal structure is NaCl Data from Table 12.3, Callister & Rethwisch 8e.

Processing of Ceramics 1. Compounding Mix and homogenize ingredients into a water based suspension = slurry a solid plastic material containing water = clay 2. Forming The clay or slurry is made into parts by pressing into mold (sintering). The fine particulates are often fine grained crystals. 3. Drying The formed object is dried, usually at room temperature to the so-called "green" or leathery state. 4. Firing Heat in furnace to drive off remaining water. Typically produces shrinkage, so producing parts that must have tight mechanical tolerance requires care. Porous parts are formed by adding a second phase that decomposes at high temperatures forming the porous structure.

Metal- Ceramic Comparison

fig_01_05 fig_01_05

Advantages of Ceramics: inert in body (or bioactive in body) Chemically inert in many environments high wear resistance (orthopedic & dental applications) high modulus (stiffness) & compressive strength Comparable to metals esthetic for dental applications

Disadvantages of Ceramics brittle (low fracture resistance, flaw tolerance) low tensile strength (fibers are exception) poor fatigue resistance (relates to flaw tolerance)

TABLE 2 Types of Bioceramic–Tissue Attachment and Their Classification Example Al2O3 (Single crystal and polycrystalline) Al2O3 (Polycrystalline) Hydroxyapatite-coated porous metals Bioactive glasses Bioactive glass-ceramics Hydroxyapatite Calcium sulfate (Plaster of Paris) Tricalcium phosphate Calcium-phosphate salts Type of attachment 1. Dense, nonporous, nearly inert ceramics attach by bone growth into surface irregularities by cementing the device into the tissues or by press-fitting into a defect (“morphological fixation”). 2. For porous inert implants, bone ingrowth occurs that mechanically attaches the bone to the material (termed “biological fixation”). 3. Dense, nonporous surface-reactive ceramics, glasses, and glass-ceramics attach directly by chemical bonding with the bone (termed “bioactive fixation”). 4. Dense, nonporous (or porous) resorbable ceramics are designed to be slowly replaced by bone.