Sumithra, S

etd@IISc Electronic Theses and Dissertations at Indian Institute of Science

The phenomenon of negative thermal expansion along all the crystallographc dvec- t~ons has been identified recently The discovery of negat~ve thermal expansion in ZrW20s led to some basic rules for ldentlficatlon of materxals wlth the unusual phenomenon of neg- atlve thermal expanslon These rules have further led to the discovery of many new class of oxldes which exhibit negative thermal expanslon like M02, AM2O7, A2M3Ol2, AMOs etc, where A is an octahedral cat~on and M IS a tetrahedral cat~on The corner sharing of polyhedra's in these systems allows for transverse thermal vibrations of oxygen perpen- dlcular to A-0-M linkages The objective of the them is study of thermal expansion In A2M3Ol2 family of oxldes by hgh  temperature X-ray diffraction and dilatometry The first chapter brxefly summarizes the hterature on negative thermal expansion ma- terials and the proposed phenomenon for thermal contraction The change in structural features In A2W3012 as a function of A cation size will be described This chapter ad- dresses the objective of our research on A2M3Ol2 farmly of oxldes The second chapter describes the synthesis of these materials and the characterizat~on techniques adopted Some experimental setups hke hot presslng unlt and hornontal dllatometer, that were fabricated for the study are described The thlrd chapter describes the thermal expanslon of Y2W3Ol2 The hygroscop~city m Y2W3012, and its effect on different storage conditions has been studled by thermo- gravlmetry The material gets hydrated and dehydrated wlthout destroying the basic framework of the structure But, the presence of water molecules seem to i n h ~ b ~ t  the phenomenon of negatwe thermal expansion The hygroscopic nature of these materials and W03 evaporation at high temperatures llrmts the sinterablllty Therefore, Y2 W3012 was hot pressed to achleve better slnterability In the fourth chaptex, the effect of varlous cation substitut~ons at A slte in A2W& omdes 1s investigated Subst~tution of light rare earth trivalent cations like La, Nd, Dy crystallizes in an edge shared monoclinmc (C2/c) and dense structure On substituting heavy rare earth trivalent cations lmke Y, Er, Yb, and Lu, they crystalhze in an or- thorhornbic (Pnca), corner shared and more open structure As the network becomes more open, they tend to hydrate and the unlt cell volume of the hydrated phases in all the lsostructural corner shared tungstates is smaller than the correspondmg unhydrated pattern The light rare earth A cation m an edge shared structure is eight coordinated compared to suc coordinated A catlon in a corner shared structure The light rare earth tungstates are non-hygroscopic in nature and show posltive thermal expansion The fifth chapter describes the effect of Mo substltutlon at W site for the same A cation The orthorhombic corner shared molybdates are also highly hygroscopic and isostructural wlth the rare earth tungstates The thermal expansion behav~our of rare earth orthorhombic corner shared molybdates is simlar to the tungstates The swth chapter describes various attempts to improve the properties of A2W3012 oxldes The content of thls chapter alms at achieving low thermal expansion and ehm- inatlng hygroscopicity, thermal expansion hysteresis Chemlcal modifications have been adopted hke substitution sohd solution study and combustion synthesis route to achieve fine particles Microstructural modifications were achleved by adoptmg a compos~te a p  proach, addition of addrtives and physical blockage to moisture absorption by glazmg Composites were made between an expandmg phase and contractmng phase, further heat treated above the melting point of one of the phases This reduces thermal expansion hysteresis and hygroscopmty The seventh chapter describes the slmulatlon studles carried out to understand the phenomenon of negatlve thermal expansion, using GULP computer code based on free energy m~n~mzation(FEM) The two body buckinghnm and three body potential parame- ters were fitted The results obtained show volume cont~actlon from 10-300K The phonon modes associated m t h  negative thermal expansion has been Identified Low energy trans- verse optic modes are responsible for negatlve thermal expansion In these materials The work reported In thls thesis has been carrled out as a part of the PhD tramng x-ogramrne It 1s hoped that the thesls wlll contribute towards the basic understandmg ~f the unusual phenomena of negative thermal expansion in framework morgamc oxldes 

Study Of Negative Thermal Expansion In Inorganic Framework Structure Oxides

http://etd.iisc.ac.in/static/etd/abstracts/1842/G19117-Abs.pdf

Study Of Negative Thermal Expansion In Inorganic Framework Structure Oxides

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etd@IISc Electronic Theses and Dissertations at Indian Institute of Science