exam 2019 Flashcards
(18 cards)
what is gas phase synthesis
means making ceramic powders from chemical reactions in hot gases. these reactions happen in furnaces, flames, lasers, or plasma torches. they create very small, pure ceramic particles like tiny specks of dust. this method is often used when we need super clean very fine ceramic powders like for electronics or optical fibers.
first method of gas phase synthesis
flame hydrolysis - used for making high purity silica or alumina. starts with silicon tetrachloride that is burned in a hot flame with water vapor. this makes solid silica powder and HCl. think of it like spraying mist into a fire and getting a powder and gas out.
pros - pure tiny particles good for optical fibers
cons - powder is fluffy so hard to shape and sinter. produces HCl which needs to be removed. acid gas
second method of gas phase synthesis
chemical vapor condensation (CVC) - chemicals in gas form are heated until they react and form solid powder. the solid ceramic powder forms in the air and is collected. think of it like cooking steam that cools into powder.
pros - pure tiny particles, can change the set up to control size
cons - can’t control everything, particles can stick together
third method of gas phase synthesis
laser pyrolysis - a strong laser heats gases to a very high temperature. causes a fast reaction that makes powder. think of it like a laser beam shooting through gas and making dust
pros - super small, even particles, clean and controlled process
cons - expensive, so used in labs or for small amounts
fourth method of gas phase synthesis
plasma synthesis - a plasma torch (10000c) is used to break down gases and make ceramic powder. think of it like lightning in a tube making ceramic dust.
pros - you can make ceramics that need extreme high heat, works for non-oxide ceramics not just oxygen-based ones.
cons - uses a lot of energy, expensive machines, hard to keep powder size even
fifth method of gas phase synthesis
gas phase combustion/self-propagating high-temperature synthesis - two gases are mixed and set on fire. the reaction spreads by itself like a burning log and creates ceramic powder.
pros - simple and cheap, one started doesn’t need much more energy
cons - powder may be rough and uneven, not small or clean powder
mechanism of electrical conduction in zirconia
in pure form, it is highly resistant to heat, a bad conductor of electricity at room temperature. at high temps, zirconia can conduct oxygen ions, which makes it useful for oxygen sensors, fuel cells, oxygen concentration, and thermal barrier coatings. zirconia is an ionic conductor, not an electronic conductor. in stabilized zirconia, oxygen ions move through the crystal
temperature dependance of electrical conductivity in zirconia
zirconia only becomes a good conductor at high temperatures (600+). at low temps, there is not enough energy for oxygen ions to move. at high temps, ions gain energy and jump between vacancies.
why zirconia must be stabilized if it is to be used as an electrical conductor
pure zirconia has a crystal structure that does not allow easy movement of oxygen ions. so add yttrium, which creates oxygen vacancies (empty spots where and oxygen ion should be). these vacancies allow oxygen ions to hop around creating ionic conductivity. pure zirconia changes structure as it heats up so it is unstable and cracks or falls apart. adding yttria keeps structure cubic.
what is the principal mechanism of failure in glass
brittle fracture from surface cracks. glass breaks easily from surface cracks or flaws. these flaws are stress concentrators even under moderate force. cracks grow and the glass snaps without warning. glass is strong in theory because its bonds are strong but in real life it fails at much lower stress
first method of overcoming mechanical failure in glass
fire polishing/flame polishing - surface of the glass is heated with flame until it softens slightly, causes sharp cracks and scratches to smooth out and flow. creates a smooth surface with fewer stress concentrators.
cons - works only on surface, doesn’t add long-term strength like other methods
second method of overcoming mechanical failure in glass
surface dissolution (hydroflouric acid etching) - glass is dipped in hydroflouric acid which dissolves the top surface layer. it removes crack tips and flaws, rounds out sharp flaws that could grow under stress
cons - hydroflouric acid is dangerous, highly toxic and corrosive. temporary solution, mainly for labware or precision optics
third method to overcome mechanical failure in glass
thermal toughening (tempering) - heat glass to high temperature, then cool the surface quickly with air. the outside cools and hardens in compression and the inside is in tension. the cracks have to be 4-5 times stronger to break through the compressive outside layer. used for windows, phone screens, car side windows.
cons - can’t be drilled or cut after tempering, shatters into small pieces
fourth method of overcoming mechanical failure in glass
chemical strengthening - glass is placed in molten salt glass like potassium oxide at high temperature. bigger potassium ions squeeze into the sodium spots in the surface which creates compressive strength. like tempering but done at lower temperature and gives deeper compression layer.
pros - can be done after shaping/cutting, high surface strength
what is a glass ceramic, structure, properties, how it is achieved.
glass that is partially crystallized. tiny crystals dispersed in glass matrix. melt then cool then reheat to grow crystals. high strength, low expansion, durable. used for cooktops, telescopes, dental materials
what is solvent-catalyst technique for diamonds
high-pressure, high temp method used to grow synthetic single crystal diamonds. mimics natural conditions deep inside earth where diamonds form but faster and more controlled
process of solvent-catalyst technique
carbon is dissolved in a metal catalyst (solvent) at high pressure and temperature. carbon atoms move from the graphite and dissolve in the molten metal solvent. as the solution cools slightly, carbon crystalizes into diamond on a seed crystal. as temperature is lowered slightly at the seed location, carbon atoms come out of solution and deposit onto the seed as diamond. the growth is epitaxial, follows the crystal structure of the seed, creating a single crystal
advantages of solvent-catalyst technique
produces high-quality single crystals, used for cutting tools, optics, electronics. cheaper than CVD for bulk crystals
