Applications Flashcards
Ceramic Fabrication Methods: Milling and Screening
Raw materials are ground to the desired particle size to ensure uniformity.
Ceramic Fabrication Methods: Hydroplastic Forming
Clay mixed with water forms a pliable mass suitable for shaping.
Extrusion: A stiff mass is forced through a die, creating products like bricks and tiles; a vacuum chamber enhances density.
Ceramic Fabrication Methods: Slip Casting
A clay suspension (slip) is poured into porous molds made of plaster of Paris.
Process: The mold absorbs water, forming a solid layer; excess slip can be drained for drain casting. The mold is reusable and economical.
Ceramic Fabrication Methods: Drying and Firing
Green Body: The unfired piece retains porosity and lacks sufficient strength.
Drying Process: Controlled evaporation is crucial to avoid defects like warpage; factors such as body thickness and water content influence drying rates. Microwave drying can maintain temperatures below 50°C.
Firing Process: Typically occurs between 900°C and 1400°C, where density increases and porosity decreases.
Vitrification: The formation of liquid glass, which enhances strength and durability.
Ceramic Fabrication Methods: Sintering and Powder Pressing
Sintering: Powder particles coalesce during firing, reducing porosity and increasing strength.
Powder Pressing Techniques:
Uniaxial Pressing: Compaction occurs in one direction, suitable for simple shapes.
Isostatic Pressing: Uniform pressure applied from all directions allows for complex shapes but requires more time.
Hot Pressing: Combines compaction and heat treatment at elevated temperatures.
Ceramic Fabrication Methods: Tape Casting
Thin ceramic sheets are created from slips containing ceramic particles, binders, and plasticizers, with de-aired slips to prevent bubbles. These sheets are commonly used as substrates for integrated circuits and multilayer capacitors, with a thickness of 0.1 to 2 mm.
Ceramic Fabrication Methods: Cementation Process
Cement mixed with water forms a paste that hardens through chemical reactions, allowing for the creation of solid structures.
Glasses
Amorphous glasses are noncrystalline silicates designed for high mechanical strength and low thermal expansion, making them ideal for applications requiring resistance to thermal shock and excellent dielectric properties. They are easily fabricated using conventional glass-forming techniques, resulting in nearly pore-free ware. Common commercial brands include Pyroceram, CorningWare, Cercor, and Vision. These glasses find widespread use in ovenware, tableware, oven windows, and electrical insulators, as well as substrates for printed circuit boards and architectural cladding.
Glass Properties
Temperature Sensitivity: Glassy materials become more viscous with decreasing temperature and lack a definite solidification point.
Volume vs. Temperature: Crystalline materials show a discontinuous volume decrease at the melting temperature (Tm). Glassy materials have a continuous volume decrease; the glass transition temperature (Tg) indicates a change in behavior.
Viscosity-Temperature Characteristics
Melting Point: Viscosity = 10 Pa·s; behaves like a liquid.
Working Point: Viscosity = 10³ Pa·s; easily deformed.
Softening Point: Viscosity = 4 × 10⁶ Pa·s; maximum temperature for handling.
Annealing Point: Viscosity = 10¹² Pa·s; allows removal of residual stresses.
Strain Point: Viscosity = 3 × 10¹³ Pa·s; below this temperature, fracture occurs before deformation.
Glass Forming
Raw Material Composition: Typically includes silica (SiO₂) with Na₂O (soda) and CaO (lime) for optical transparency and homogeneity.
Porosity Management: Gas bubbles must be absorbed or eliminated by adjusting viscosity.
Forming Methods
Pressing: For thick pieces (plates, dishes).
Blowing: Used for jars, bottles, and light bulbs; involves creating a parison. Drawing: For long pieces (sheets, rods, tubing, fibers). Sheet Glass Production: The float process allows for uniform thickness and smooth finishes by floating molten glass on liquid tin. Fiber Formation: Continuous fibers are drawn through orifices, with viscosity controlled by temperature.
Heat Treating Glasses
Annealing: Reduces thermal stresses during cooling; involves heating and slow cooling to room temperature.
Glass Tempering: Enhances strength by inducing compressive residual surface stresses through rapid cooling after heating.
Glass Ceramics
Glass-ceramics are produced through the crystallization of glasses under high-temperature heat treatment, resulting in fine-grained polycrystalline materials. The process includes nucleation and growth stages, akin to metallic phase transformations.
Properties and Applications
Mechanical Strength: High strength, low thermal expansion, good thermal shock resistance, and favorable high-temperature performance.
Transparency: Some glass-ceramics are optically transparent, while others are opaque. Fabrication: Easily produced using standard glass-forming techniques, yielding nearly pore-free products. Applications: Similar to amorphous glasses, including ovenware, tableware, electrical insulators, substrates for circuit boards, and architectural cladding.
Characteristics of Clay
Hydroplasticity: The addition of water to clay renders it plastic and pliable, essential for shaping.
Fusion Temperature Range: Clays melt over a range of temperatures, allowing the creation of dense ceramics without complete melting. Composition: Primarily aluminosilicates (Al₂O₃ and SiO₂), clays also contain bound water and various impurities (e.g., oxides of barium, calcium, sodium, potassium, iron). Structure: Typically layered, with kaolinite (Al₂(Si₂O₅)(OH)₄) being common; water forms thin films around particles, enhancing movement and plasticity.
Composition of Clay Products
Ingredients: Whitewares are composed of clay, nonplastic fillers (like quartz), and fluxes (such as feldspar).
Role of Quartz: Acts as a filler, providing hardness and stability; it melts to form glass.
Flux Definition: A substance that promotes glass formation during firing; feldspar (containing K⁺, Na⁺, Ca²⁺ ions) is a typical flux.
Typical Porcelain Composition: Approximately 50% clay, 25% quartz, and 25% feldspar.
Refractory Cements
Refractory ceramics are designed to withstand high temperatures without melting or decomposing, providing thermal insulation and remaining unreactive in severe environments. Commonly formed as bricks, they are used in furnace linings for metal refining, glass manufacturing, and power generation. Performance depends on composition, classified into fireclay, silica, basic, and special refractories. Fireclay refractories, made from alumina-silica mixtures, can endure temperatures up to 1587°C (2890°F). Silica refractories, primarily silica-based, support temperatures up to 1650°C (3000°F) and are found in steel and glass furnace roofs. Basic refractories, rich in periclase (MgO), resist basic slags, while special refractories include materials like alumina and silicon carbide for specialized applications. Overall, the selection of refractory ceramics is crucial for ensuring the durability and efficiency of high-temperature industrial processes.
Abrasive Ceramics
Abrasive ceramics are essential for wearing, grinding, or cutting softer materials, requiring properties such as hardness, wear resistance, toughness, and refractoriness to withstand friction-induced heat. Common abrasives include diamonds, silicon carbide, tungsten carbide, aluminum oxide, and silica sand. They come in various forms: bonded abrasives feature particles attached to grinding wheels with glassy ceramics or resins, allowing for cooling via surface porosity; coated abrasives have abrasive powders applied to paper or cloth for polishing; and loose abrasives are suspended in oil or water for grinding and polishing applications.
Cements and Binding Agents
Cements are inorganic ceramic materials, including cement, plaster of Paris, and lime, that harden when mixed with water to form solid structures. Portland cement, the most common type, is produced by calcining a mix of clay and lime-bearing minerals at about 1400°C, resulting in clinker that is ground with gypsum to control setting. The hydration process initiates immediately upon adding water, with setting occurring within hours and hardening lasting years. Unlike nonhydraulic cements, hydraulic cement like Portland cement gains strength through chemical reactions with water, binding aggregates into a cohesive mass.
Carbon Materials: Diamond
Diamonds are renowned for their extraordinary physical properties, including chemical inertness, high thermal conductivity, and resistance to corrosion. They are the hardest known bulk material due to their strong interatomic sp³ bonds and possess the lowest sliding coefficient of friction among solids. Diamonds can be synthetic, produced through high-pressure, high-temperature techniques, with applications ranging from industrial tools, such as diamond-tipped drill bits and abrasives, to gem-quality stones.
Carbon Materials: Graphite
Graphite exhibits a highly anisotropic structure, with electrical properties varying significantly based on crystallographic direction. It has low resistivity along the graphene plane due to delocalized electrons, allowing for excellent electrical conductivity and lubricative properties. Unlike diamond, graphite is soft, opaque, and suitable for high-temperature applications, making it ideal for use in lubricants, battery electrodes, and heating elements.
Carbon Materials: Carbon Fibers
Carbon fibers are high-strength, small-diameter fibers used as reinforcements in polymer-matrix composites. Composed of graphene layers, their structure can be graphitic or turbostratic, influencing their mechanical properties. Carbon fibers are highly anisotropic, displaying greater strength and modulus along the fiber axis. They are lighter yet stronger than many other reinforcing fibers, making them valuable in aerospace and automotive industries.
