1. 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

2. 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

3. 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

4. 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

5.   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) 

6. 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

7. 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

8. 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

9. 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 , ;  = Al2O3 , ;

10. 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

11. 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

12. 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 = K
T0 = K
T0 = K

13. 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