Solid state reactions. Often defined as reactions between two solids. Here the definition is extended to any reaction involving a solid:

Transcription

1 Sold state reactons Often defned as reactons between two solds Here the defnton s extended to any reacton nvolvng a sold: Sold/sold Sold/gas (Reacton, decomposton) (Sold/lqud) Intercalaton 1 2 Sold-State Reactons 2.1 Reactons Between Sold Compounds Ceramc Method Carbothermal Reducton Combuston Synthess Snterng 2.2 Sold Gas Reactons 2.3 Decomposton and Dehydraton Reactons 2.4 Intercalaton Reactons General Aspects Preparatve Methods Pllarng of Layered Compounds 2

2 Ceramc method Drect reacton of solds to form the fnal product In prncple no decomposton s nvolved. Solds do not react wth solds at room temperature even of thermodynamcs s favorable. Hgh temperature must be used. Sold-sold reactons are smple to perform, startng materals are often readly avalable at low cost and reactons are clean.e. do not nvolve other chemcal elements. Dsadvantages nclude the need for hgh temperatures, the possblty of non-homogenety, contamnaton from contaners etc. etc. 3 Synthess of YBCO, YBa 2 Cu 3 O 7-x Drect reacton between Y 2 O 3, BaO 2, CuO (Reacton between three sold components) Grnd to obtan large surface area Press nto pellets (contact) Heat n alumna boat, temperature profle: Oxdaton step 4

3 Other precursors may be used, e.g. BaCO 3, whch may be decomposed to fne graned BaO durng the reacton. ( + ntrates etc.) Decomposton must be performed n a controlled manner n order to avod volent decomposton (.e. choosng an approprate temperature) Many other ways of syntheszng YBCO: Precursor methods Sol/gel Flux Many melt-texturng methods ncludng partally meltng/fluxng 5 6

4 Sold state reactons, general aspects Nucleaton at the nterface (depends on the degree of structural rearrangement nvolved) Formaton of product layer, two reacton nterfaces Dffuson/counter dffuson through the product layer Longer dffuson paths, slower reacton rate. Smple case: lattce dffuson through a planar layer: dx/dt = k * x -1 7 Reactons often nvolve other reactons and transport mechansms. Electrons, gaseous speces, oxygen ons, defects 8

5 Dffuson s a thermally actvated process 9 Transport: Dffuson n a gradent (.e. chemcal potental gradent, Δμ) 10

6 Mass transport n an onc sold s often acheved by transport of a charged speces and counter transport of a dfferently charged on or defect n the opposte drecton. Then the electrochemcal potental s the drvng force, and not the chemcal potental or the concentraton gradent. The electrochemcal potental (η ) of speces s the sum of the chemcal potental and electrostatc potental φ: η = μ + Z Fφ Where Z s the effectve charge and F s the Faraday constant. The flux s gven by: j = c v = c B η x = c B μ x φ + ZF x 11 Implcatons of the expresson for flux: The effect of the terms may pull n opposte drectons (.e. a concentraton gradent may be countered by an electrcal feld gradent) A dfferent type of effect may occur due to the local electrcal feld between oppostely charged speces. Ths may affect the transport of an on relatve to other(s). Ths couplng results n e.g. that both Al 3+ and O 2- move n Al 2 O 3 (n contrast one would normally expect O 2- to by farly mmoble due to the large onc sze) 12

7 Example of parabolc growth: NO + Al 2 O 3 NAl 2 O 4 13 Ions n solds are not moble at low temperatures. Hgh temperatures, as a rule of thumb, at 2/3 of the meltng temperature (of one component) the dffuson s suffcent to acheve sold state reactons. Formaton of BaTO 3 by reactng BaCO 3 and TO 2 s an example of a seemngly smple reacton s more complex than expected. BaTO 3 s an mportant electroceramc, (Thermstors, capactors, optoelectronc devces, DRAMs) BaCO 3 s decomposed to reactve BaO: (Rock salt, ccp of the oxde anons, Ba 2+ n octahedral stes), TO 2 (Rutle, hcp of oxde ons, T 4+ n half of the octahedral stes) At least three stages are nvolved n formaton of BaTO 3 from BaO and TO 2. BaO react wth the surface of TO 2, formng nucle and a surface layer of BaTO 3. Reacton between BaO and BaTO 3 to form Ba 2 TO 4. NB! Ths s a necessary phase for ncreasng mgraton of Ba 2+ ons. Ba 2+ ons from the Ba-rch phase mgrate nto the TO 2 phase and form BaTO 3. 14

8 Reactons between two solds: Reacton rates depends on: Area of contact between the reactng solds,.e. surface area and densty The rate of nucleaton Rates of dffuson of ons (and other speces) Dsadvantages, e.g.: Nucleaton and dffuson related problems (hgh temperature) Formaton of undesred phases (reacton paths) (e.g. BaT 2 O 5 ) Homogeneous dstrbuton, especally for dopants, s dffcult Dffcult to montor the reacton drectly, n-stu?? Separaton of phases after synthess s dffcult Reacton wth contaners/crucbles Volatlty of one or more of the components 15 16

9 Surface area: Surface area s ncreased by crushng/mllng 1 cm3 cube: surface area 6 cm 2 1 mm cubes: surface area 60 cm 2 10μm cubes: surface area 6000 cm 2 10 nm cubes: surface area cm 2 (600m 2 ) 17 Increase dffuson rates (decrease dffuson lengths) Small partcles by grndng, ball mllng, spray dryng, freeze dryng, spray pyrolyss etc. Coolng and regrndng may be necessary Intmate mxture of reactants by coprecptaton, sol-gel processes, spray pyrolyss etc. Reducton of dffuson dstances by ncorporatng the catons n the same sold precursor. Sold state reactons nvolvng melts, molten fluxes or hgh temperature solvents. 18

10 Nucleaton Nucleaton lmted reactons may be descrbed by the Avram-Erofeyev equaton: x(t) = 1-exp(-kt n ) {or x(t) = 1-exp(-(kt) n )} N s a real number, usually between 1 and 3 (n>4 s often nterpreted as autocatalytc nucleaton) For n > 1 the curve s sgmodal Note: Ths s more emprcal for most sold state reactons. It s dffcult to talk about reacton order and actvaton energy for these reactons. 19 Nucleaton may be facltated by structural smlarty between the reactng solds, or products. In the reacton between MgO and Al 2 O 3, MgO and spnel have smlar oxde on arrangements. Spnel nucle may then easly form at the MgO surfaces. There s often a structural orentatonal relatonshp between the reactant, nucle and product. The lattce spacng (e.g. dstance between oxde ons) should not be too dfferent for orented nucleaton. 20

11 Eptactc and topotactc reactons Eptactc: structural smlarty restrcted to the surface / nterface between two crystals layer Topotactc: Structural smlarty through the crystal The ease of nucleaton depends also on the actual surface structure of the reactants. 21