Production of NTCR Thermistor Devices based on NiMn2O4+d Electroceramics VIII 2002, Rome Rainer Schmidt Production of NTCR Thermistor Devices based on NiMn2O4+d Rainer Schmidt University of Durham Semiconductors and Electroceramic Research Group

Experimental designs for film production Electroceramics 2002, Rome Rainer Schmidt Introduction Motivation for film production Basic properties of NiMn2O4+d Experimental designs for film production Powder production Electron-Beam-Evaporation Screen-printing RF magnetron sputtering Film Properties X-Ray diffraction patterns AFM/ SEM/ EDAX Electrical Properties Conclusions Comparison of all 3 methods

Applications of NiMn2O4+d Introduction Motivation for film production Experimental designs Film Properties Conclusions Electroceramics 2002, Rome Rainer Schmidt Applications of NiMn2O4+d Temperature sensing devices in engines, coolants etc. (electrical properties) Infrared detectors (optical properties) Problems with bulk material applications High porosity due to incomplete inter-granular contact Poor run-to-run reproducibility of the device  Production of dense and even polycrystalline films

T t Spinel structure: General formula A2+ B3+2 O4 Introduction Basic Properties of NiMn2O4+d Experimental designs Film Properties Conclusions Electroceramics 2002, Rome Rainer Schmidt Spinel structure: General formula A2+ B3+2 O4 Oxygen atoms form FCC sub-lattice, A and B cations on tetrahedral and octahedral lattice interstices Regular spinel: all A2+ cations are on tetrahedral and B3+ on octahedral sites in NiMn2O4 some Ni2+ ions move to [octahedral sites] Electrical Conduction : thermally activated electron hopping between Mn3+ and Mn4+ Resistance decreases logarithmically with increasing temperature : NTCR effect T t Tetrahedral Interstice Octahedral Interstice

  Production of NiMn2O4+d powder Introduction Experimental designs Powder production Film Properties Conclusions Electroceramics 2002, Rome Rainer Schmidt Production of NiMn2O4+d powder - solid reaction between NiO + Mn2O3 requires high temperature - phase impurities in NiMn2O4+d occur over 1100 °C - annealing process necessary - high average grain size (~ 5 mm) Mixed oxide route NiO + Mn2O3  NiMn2O4+d sintering at 1200 °C  24 h annealing at 800 °C  50 h  Oxalate route Production of Ni2+, Mn2+ and (C2O3)2- solutions Co-precipitation of NiMn2(C2O3)3 thermal decomposition of NiMn2(C2O3)3 sintering at 850 °C  30 min  NiMn2O4+d - less heat exposure - homogeneous grain size distribution without agglomerates - small average grain size (< 1 mm) 

Electron-Beam Evaporation Introduction Experimental designs Electron-Beam Evaporation Film Properties Conclusions Electroceramics 2002, Rome Rainer Schmidt Electron-Beam Evaporation Process parameter : - Potential drop (H.V.) between filament and source - Filament current - Distance source - filament - Distance source - substrate - Substrate temperature - Exposure time Variation of the process parameters enables control on: - source temperature - film thickness - film shape

Screen-printing Process parameter : Introduction Experimental designs Screen-printing Film Properties Conclusions Electroceramics 2002, Rome Rainer Schmidt Screen-printing Process parameter : - Size of screen meshes - Paste (ink) composition ( solid loading, solvents, binder, dispersing agent, glass) - Distance screen - substrate (gap) - Sintering process of printed films Variation of the process parameters enables control on: - film thickness - film density - porosity

RF magnetron sputtering Introduction Experimental designs RF magnetron sputtering Film Properties Conclusions Electroceramics 2002, Rome Rainer Schmidt RF magnetron sputtering Target Cathode Substrate Viewport Substrate heater Substrate holder Shutter 2 e Process parameter : - target preparation - substrate temperature - argon/ oxygen gas ratio during deposition can be changed - Distance target - substrate - Sintering process of sputtered films Variation of the process parameters enables control on: - crystal orientation in the films - crystallinity of the films - oxygen content in NiMn2O4+d - film thickness

XRD-characteristics of source powder from oxide and oxalate route Introduction Experimental designs Film Properties X-Ray diffraction Conclusions Electroceramics 2002, Rome Rainer Schmidt XRD-characteristics of source powder from oxide and oxalate route XRD-characteristics of E-beam evaporated, screen printed and sputtered films R E-beam evaporated Oxide route Screen-printed Oxalate route Sputtered  = NiMn2O4+d JCPDS71-852  = NiMn2O4+d , 71-852;  = Al2O3 , 46-1212;

L L L SEM pictures of E_beam evaporated films Introduction Experimental designs Film Properties AFM/ SEM/ EDAX Conclusions Electroceramics 2002, Rome Rainer Schmidt SEM pictures of E_beam evaporated films AFM pictures of sputtered films SEM picture of a screen-printed film L L Non-annealed Non-annealed L Screen-printed film: including a glass phase Film sintered at 850 °C for 30 min Annealed in air at 850 °C for 30 min Annealed in air at 800 °C for 30 min, substrate temp. 200 °C

EDAX spectra of E-beam evaporated films and source powder Introduction Experimental designs Film Properties AFM/ SEM/ EDAX Conclusions Electroceramics 2002, Rome Rainer Schmidt EDAX spectra of E-beam evaporated films and source powder K K Mn Mn Ni Ni Mn Mn Ni Ni NiMn2O4+d source powder from mixed oxide route NiMn2O4+d electron-beam evaporated film  Stoichiometry in electron-beam evaporated films can not be controlled easily

Electrical Properties Introduction Experimental designs Film Properties Electrical Properties Conclusions Electroceramics 2002, Rome Rainer Schmidt Electrical Properties Electrical conduction in NiMn2O4+d follows a variable-range hopping model : F g Electron-beam evaporated Screen-printed RF magnetron sputtered D I K T0 = 2.23 105 K T0 = 1.92 105 K T0 = 2.73 105 K

Conclusions Advantages Disadvantages Electron-Beam Evaporation Introduction Experimental designs Film Properties Conclusions Comparison of all 3 methods Electroceramics 2002, Rome Rainer Schmidt Conclusions Advantages Disadvantages Loss of stoichiometry in the films, annealing process necessary for crystallisation, high temperatures during deposition, poor reproducibility Electron-Beam Evaporation Screen printing RF magnetron sputtering Small grain size, dense and even films, easy preparation of source powder Stoichiometry is maintained, deposition at room temperature, film thickness controllable, screen-printing kit easy to realise, good reproducibility Problems with porosity, time intensive preparation of source powder, many parameters have to be adjusted (paste composition & viscosity, gap, sintering process) Small grain size, dense and even films, all film properties can be related to process parameters easily (crystallinity, film thickness), good reproducibility Time intensive preparation of the target, low deposition rate, annealing process necessary for crystallisation