3. Inorganic Functional Materials

3. Inorganic Functional Materials 3.1. Pigments 3.1.1 Definition and Classification 3.1.2 Economic Importance 3.1.3 Production and Synthesis 3.1.4 Catalytic Pigments 3.1.5 Anticorrosive Pigments 3.1.6 Magnetic Pigments 3.1.7 Color Pigments 3.1.8 Effect Pigments 3.1.9 Fillers

Chemical Material Technology Prof. Dr. T. Jüstel

Sheet #1

3.1.1 Definition and Classification Definition (Inorganic) pigments (lat: pigmentum = painter color) consists of micro- or nanoparticles, which are practically insoluble in the application system (suspending medium) Classification Application area 1. Catalytic pigments 2. Anticorrosive pigments 3. Magnetic pigments 4. Color pigments 5. Nacreous pigments 6. Luminescent pigments 7. Fillers 8. Flame Retardants Chemical Material Technology Prof. Dr. T. Jüstel

Example TiO2 Pb3O4 Fe3O4 CoAl2O4 TiO2 on Mica, Pearls ZnS:Ag SiO2 MgO

Chapter 3.1.4 3.1.5 3.1.6 3.1.7 3.1.8 3.2. 3.1.9 Sheet #2

3.1.2 Economic Importance Production of inorganic pigments

World Germany

1995 4.85.106 t ⇒ 13.109 $ 2.0.106 t

Pigment TiO2 Fe2O3 Carbon Black ZnS/BaSO4 Chromates Cr2O3 MeI[FeIIFeIII(CN)6].H2O Ultramarine Chemical Material Technology Prof. Dr. T. Jüstel

Percentage 66% (3.2.106 t) 14% 10% 4% 3% 1% < 0.5% < 0.5%

2006 7.4.106 t ⇒ 18.109 $

Application areas White, UV protection + catalytic pigment Red + magnetic pigment Black pigment White pigment Yellow + corrosion protection pigment Green pigment Blue pigment Blue pigment Sheet #3

3.1.3 Production and Synthesis TiO2 Occurence in nature FeTiO3 Ilmenite TiO2 Rutile, anatase (both contaminated with Fe2O3 and Fe3O4) Mining + Milling process Slurry + Magnetic separation

Sulphate process

Chloride process

Ilmenite

Rutile

+ H2SO4

+ Coke+ Cl2 950 °C

Sulfate cake Leach out

Fe2(SO4)3 + TiOSO4

FeCl3 + TiCl4↑

Reduction

FeSO4 + TiOSO4 - FeSO4.7H2O

Condensation

Evaporation

TiOSO4

Purified TiCl4

+ H2O, Boiling

TiO2.xH2O

+ O2 1000 °C

800 – 900 °C

Enriched ore Chemical Material Technology Prof. Dr. T. Jüstel

TiO2

TiO2 Sheet #4

3.1.3 Production and Synthesis Fe2O3 and Fe3O4 Occur in nature α-Fe2O3 Hematite Fe3O4 Magnetite

Stability of the metastable γ-Fe2O3 -1/6 O2, 200 °C, vacuum

Fe3O4

300 °C, air

γ-Fe2O3

α -Fe2O3

Synthesis of the iron oxide pigments (starting from FeSO4) • α-Fe2O3 (red-brown, antiferromagnetic, corundum structure) 2 FeSO4 + 4 NaOH + ½ O2 → 2 Na2SO4 + H2O + α-FeOOH → α-Fe2O3 (400 – 500 °C) • Fe3O4 (black, ferrimagnetic, inverse spinel: [FeIII]T[FeIIFeIII]OO4) 3 α-Fe2O3 + H2 → 2 Fe3O4+ H2O at 340 - 400 °C 3 α-Fe2O3 → 2 Fe3O4 + ½ O2 above 1200 °C III • γ-Fe2O3 (brown, ferromagnetic, inverse spinel: [FeIII]T[ 0.33Fe 1.67]OO4) 2 Fe3O4 + ½ O2 → 3 γ-Fe2O3 at 200 – 250 °C in air Chemical Material Technology Prof. Dr. T. Jüstel

Sheet #5

3.1.4 Catalytic Pigments Heterogeneous catalysis Autocatalyst Pd/Pt-pigment on ceramic substrate 2 CO + O2 → 2 CO2 C8H18 + 25 O2 → 16 CO2 + 18 H2O 2 NO + 2 CO → N2 + 2 CO2 Oxygen regulation by CeO2 2 CeO2 ⇄ Ce2O3 + ½ O2

Oxidation of soot by CeO2 C + 2 CeO2 → CO + Ce2O3 (diesel vehicles, Peugeot + Rhodia) Chemical Material Technology Prof. Dr. T. Jüstel

Oxygen measurement by means of λ-probe Electrochemical chain to measure the O2 partial pressure in the catalyst ⇒ Oxygen ion conductor ZrO2:Y3+

Sheet #6

3.1.4 Catalytic Pigments ⇒ TiO2 pigments Modification Anatase Rutile

Eg [eV] Eg [nm] n 3.5 360 2.55 3.2 390 2.79

100,0

80,0

Reflection [%]

UV-Absorption and photochemistry

60,0

40,0

20,0

Anatas Kronos Rutil Aldrich 0,0 200

300

400

500

600

700

Wavelength [nm]

1. UV-absorption (protective pigment) ⇒ Use of rutile in sun protective creams, window frames, plastic bags..... 2. Photochemistry ⇒ Use of anatase to water and surface cleaning ⇒ TiO2 + hν(UV-A) → TiO2(hVB+ + eCB-) hVB+ + H2O → H+ + OH. eCB- + O2 → O2.- (superoxide) ⇒ Oxidative decomposition of organic compounds Chemical Material Technology Prof. Dr. T. Jüstel

Sheet #7

800

3.1.5 Anticorrosive Pigments As corrosion (lat.: corrodere = gnaw away) denotes the decomposition of economically important materials Countermeasure: Coating valuable materials with protective pigments 1. Cathodic protection (application of a reducing effective pigments) Application of Zn on Fe-sheets Oxidation of base metal according to 2 Zn → 2 Zn2+ + 4 eO2 + 2 H2O + 4 e- → 4 OH2. Passivation (formation of impermeable protective oxide layers) Application of Pb3O4 (PbII2[PbIVO4]), Ca2PbO4, or PbCrO4 Oxidation of metal at the boundary layer according to Fe + Pb3O4 → FeO + 3 PbO Application of silicate layers to slow down diffusion → barrier coatings Chemical Material Technology Prof. Dr. T. Jüstel

Sheet #8

3.1.6 Magnetic Pigments Classification of magnetic materials Magnetic materials

Weakly magnetic

Diamagnetic CaCO3 C6H6 (Bio)Polymers Chemical Material Technology Prof. Dr. T. Jüstel

Paramagnetic O2 NO Fremy‘s Salt

Strongly magnetic

Antiferromagnetic MnO NiO Cr

Ferromagnetic Co Fe Ni

Ferrimagnetic MFe2O4, Fe3O4 Y3Fe5O12 BaFe12O19 Sm5Co, Nd2Fe14B Sheet #9

3.1.6 Magnetic Pigments Magnetic pigments are used for information storage in magnetic or video tapes Which conditions magnetic pigment has to fulfill? 1. Cooperative magnetism (the ability for permanent magnetization M) → ferromagnetic: Fe, CrO2, γ-Fe2O3 → antiferromagnetic: α-Fe2O3 → ferrimagnetic: Fe3O4 → high remanence (residual magnetism after switching off the magnetic field) → needle-shaped particles (orientation in the magnetic field) 2. No loss of the magnetization M due to heating of the magnetic tape → High Curie or Néel temperature 3. Good signal/noise ratio → Pigment with the smallest possible particle size (single domain/particle) 4. Possibility for the complete cancelation of the magnetization → Medium coercive field strength HC (required field strength for demagnetization of the particles) Chemical Material Technology Prof. Dr. T. Jüstel

Sheet #10

3.1.6 Magnetic Pigments Typical properties of magnetic pigments for Pigment

Application Particle size [µm]

γ-Fe2O3

Studio radio tapes

0.40

17 – 20

23 – 27

85 – 92

0.80 – 0.85

γ-Fe2O3

Cassette IEC I

0.35

20 – 25

27 – 30

87 - 92

0.80 – 0.90

γ-Fe2O3 (Co-coated)

Cassette IEC II

0.30

30 – 40

52 – 57

94 - 98

0.85 – 0.92

Fe (metallic nanopart.)

8 mm Video

0.25

50 - 60

115 - 127

130 - 160

0.85 – 0.90


Specific Coercive field surface area [m2/g] strength [kA/m]

Saturation magnetization MS/δ [µTm3/kg]

Sheet #11

MR/MS

3.1.7 Color Pigments Cause for chromaticity: Selective absorption in the visible spectral range ⇒

subtractive color blending, i.e. by a color filter

UV 350

IRIR

VIS 400

450

500

550

600

λ [nm]

650

700

750

800

⇒ Yellow Green

⇒ Magenta ⇒ Cyan

Yellow White

⇒ Red ⇒ Green ⇒ Blue Chemical Material Technology Prof. Dr. T. Jüstel

Cyan

Red

Blue Magenta

paintings, color printer Sheet #12

3.1.7 Color Pigments General requirements Technically desired • High saturation: High absorption intensity • High opacity: High refractive index • High light fastness: (Photo) chemical stability • Ecological harmlessness: No toxic elements Consequence • Allowed optical transitions – VB-CB transitions: CdS – CT-transitions: CrO42-, MnO4– Intervalence transitions (MMCT): Fe2+/Fe3+ – 3d-3d-transitions: Co2+ – 4f-5d-transitions: Ce3+ • High density • Inorganic materials Chemical Material Technology Prof. Dr. T. Jüstel

Sheet #13

3.1.7 Color Pigments Chemical composition of modern color pigments White pigments PbCO3-Pb(OH)2 ZnO TiO2 BaSO4 Sb2O3

White lead Zinc white Titanium white Barium sulfate Antimony oxide

Blue pigments Na8Al6Si6O24S2 CoAl2O4 KFe[Fe(CN)6] CaCuSi4O10

Ultramarine Thenard‘s blue Prussian blue Egyptian blue

Green pigments Cr2O3 ZnCo2O4

Chrome green Rinmann‘s green

Yellow pigments CdS PbCrO4 FeO(OH) Pb3(SbO4)2 BiVO4 K[Co(NO2)6]

Cadmium yellow Chrome yellow Lepidocrocite Antimony yellow Bismuth vanadate Cobalt yellow

Red pigments HgS Fe2O3 Pb3O4 PbCrO4.PbO CdS-HgS LaTaON2

Cinnabar Oxide red Minium Chrome orange Cadmium cinnabar La. tantalum oxynitride

Applications: paint, artists paint, porcelain paint, plastic coloring Chemical Material Technology Prof. Dr. T. Jüstel

Sheet #14

3.1.7 Color Pigments Technical applications •

Colors ⇒ Micro scale pigments (scattering) Paintings Coatings Colored plastics (tires, plastic)

•

Color filter ⇒ Nano scale pigments (no scattering) Incandescent lamps Fluorescent lamps Cathode ray tubes Plasma displays LCDs FEDs Size dependance of color of CdSe nanoparticles Chemical Material Technology Prof. Dr. T. Jüstel

Sheet #15

3.1.7 Color Pigments Technical applications Color filters on light sources: decorative lamps, IR-A emitter, brake and tail lights

Schematic build-up of the color filter Glass SiO2-adhesion layer Fe2O3-nanopigment Polymer varnish


Sheet #16

3.1.7 Color Pigments Technical applications Contrast enhancement in cathode ray tubes without color filter


LCP =

L R

with CoAl2O4 color filter

Sheet #17

3.1.7 Color Pigments Technical applications Emission and reflection spectrum of the pigments in cathode ray tubes

1,0

80

0,8

60

0,6

40

0,4

20

0 400

500

600

Wavelength [nm]


700

1,0

80

0,8

60

0,6

40

0,4

0,2

20

0,2

0,0

0

Transmission [%]

100

Emission intensity [a. u.]

Transmission [%]

100

0,0 400

500

600

700

Wavelength [nm]

Sheet #18

Emission intensity [a. u.]

Fe2O3 for red (Y2O2S:Eu)

CoAl2O4 for blue (ZnS:Ag)

3.1.7 Color Pigments Technical applications: Pigmentation of ZnS:Ag with CoAl2O4 Procedure • Co-precipitation of Co2+ and Al3+ by hydrolysis of Co(CH3COO)2 and Al(CH3COO)3 in aqueous solution • Calcination: hydroxide mixture → CoAl2O4 (Thenard’s blue – a spinel) Nanoscale pigment particles are formed on the microscale phosphor particles


Sheet #19

3.1.8 Nacreous Pigments In Nature Pearls = CaCO3 + Protein i.e. alternating layers of high and low refracting materials


Sheet #20

3.1.8 Nacreous Pigments In Technology Pb(OH)2.2PbCO3 BiOCl Mica = KAl2(AlSi3O10)(OH)2


Sheet #21

3. Inorganic Functional Materials 3.2 Phosphors (luminescent pigments, luminophores) 3.2.1 Operation 3.2.2 Composition 3.2.3 Synthesis of Phosphors 3.2.4 Areas of Application 3.2.5 Processing of Phosphors 3.2.6 Degradation of Phosphors 3.2.7 Particle Coatings 3.2.8 Recycling 3.2.9 Nanoscale Phosphors 3.2.10 Nitride Phosphors


Sheet #22

3.2.1 Operation Fundamental steps Excitation: 1. Energy transfer: 2. Relaxation :

Excitation source

Absorption of energy from an external source To activators or defects (storage) Radiative: Emission (luminescence) → Phosphors Non-radiative: Heat (phonons) → Pigments

Emission Heat

Heat

Heat

S

A

D

ET

ET

ET

A

Emission

ET A

Heat

Heat

SEM image of BaMgAl10O17:Eu

Average particle size ~ 1 - 10 µm

Energy transfer (ET) often occur prior to emission process Chemical Material Technology Prof. Dr. T. Jüstel

Sheet #23

3.2.1 Operation The overall picture

S1

S0

Fluorescence Relaxation ~10-9s

Eg

S2

Excitation process

Energy [eV]

Conduction band (empty metal orbitals) Defect

Storage A2

T1 ET

A1 A0

- Sensitizer energy levels - Activator energy levels Valence band (anion orbitals filled by electrons) Type Fluorescence Phosphorescence Afterglow Chemical Material Technology Prof. Dr. T. Jüstel

Physical process Spin-allowed transition Spin-forbidden transition Thermal activation of charge carriers

time scale ns - µs ms s – min/h Sheet #24

3.2.1 Operation Excitation by UV or visible radiation Excitation energy < EG of the host compound ⇒ Excitation of optical centres

CB

activator excitation A** A* kr knr

CB

sensitizer excitation S* ET ηTransfer

ηEscape

A** A* ηEscape

hν

hν S

A VB

A

VB IQE = ηAct = kr/(kr + knr) = τ/τr

EQE = ηAct * ηTransfer* ηEscape

with 1/(kr + knr) = τ and kr = 1/τr Chemical Material Technology Prof. Dr. T. Jüstel

Sheet #25

3.2.1 Operation Excitation by VUV, EUV, or ionising radiation (x-ray, high energy particles) Excitation energy > or ~ EG of the host lattice ⇒ Band-to-band excitation CB

CB Electron trap Eg

ηtransfer

Electron trap Eg

A* ηEscape

hν VB

Hole trap

A+

ηPI

Hole trap

A

ηtransfer

hν A+

A

VB

EQE = ηAct* ηTransfer * ηEscape * (1- ηPI) Probability of photoionisation (PI) depends on energy distance of the excited state to the conduction band edge Chemical Material Technology Prof. Dr. T. Jüstel

A* ηEscape

Sheet #26

3.2.2 Composition An (inorganic) luminescent material is a material which converts energy absorbed from an external source into electromagnetic radiation beyond thermal equilibrium ….. Host matrix Number of sites, coordination number and symmetry for cations suitable to host activator Optical band gap Phonon spectrum Eu2+

Dopants, impurities, and defects Concentration Form solid solution → change of Tm Energy level diagram

Eu2+ Eu2+ Mn2+ VO

Particle surface Surface potential and morphology Coatings → Light in- and out-coupling ⇒ Impact on PL spectra, thermal and photo stability, quantum efficiency, linearity, decay time, and thermal quenching Chemical Material Technology Prof. Dr. T. Jüstel

Sheet #27

3.2.2 Composition Phosphor = host matrix + dopants + impurities + defects 1 1

Zn H 3

Host matrix = halides, oxides, sulfides, nitrides, phosphides, ... Dopants = activators/sensitizers = RE-ions, transition metal ions, s2-ions

2

2 4

Li Be 11

K 37

14

15

16

17

He Zn

5

6

7

8

9

10

13

Na Mg 3 19

13

B

12 24

Ca Sc

Ti

V

Cr Mn Fe Co Ni

Cu Zn Ga Ge As Se

38

40

41

42

47

Rb Sr

39

Y 57

43

44

45

28 46

11 29

12 30 48

Al 31 49

Si 32 50

P 33 51

S 34 52

Zr Nb Mo Tc

Ru Rh Pd

Ag Cd In

Sn Sb Te

72

76

79

82

55

56

Cs

Ba La Hf Ta W

Re Os

87

88

107

Fr

Ra Ac Rf

89

27

10

16

23

26

9

15

O

22

25

8

14

N

5

21

7

C

4

20

6

104

73 105

74 106

75

108

77

Ir 109

78

80

81

Pt Au Hg Tl Pb 110

18

F 17

Ne 18

Cl Ar 35

36

Br Kr 53

I

83

84

85

Bi

Po At

54

Xe 86

Rn

111

58

59

Ce

Pr Nd Pm Sm Eu Gd Tb Dy Ho Er

Tm Yb Lu

90

91

101

Th Pa Chemical Material Technology Prof. Dr. T. Jüstel

92

61 93

U

62 94

63 95

64 96

2 3 4 5 6 7

Db Sg Bh Hs Mt Ds Rg 60

1

65 97

66 98

Np Pu Am Cm Bk Cf

67 99

68 100

69

70 102

71 103

Es Fm Md No Lr Sheet #28

6 7

3.2.2 Composition Dopants •

•

Activators – Lanthanide ions

Examples Eu2+, Eu3+, Tb3+

– Transition metal ions – s2-ions Sensitizers – Lanthanide ions – Complex anions

Cr3+, Mn2+, Fe3+ Sn2+, Pb2+, Bi3+

Optical transitions [Xe]4fn – [Xe]5d14fn-1 [Xe]4fn – [Xe]4fn nd - nd nsx – nsx-1np1

Ce3+ VO43-

[Xe]4f1 – [Xe]5d1 Charge-Transfer

Cr3+, Co2+, Cu2+ Eu3+ in Tb3+ phosphors Cl-, F-, BO33-, ....

→ competitive absorption → energy transfer → defect formation

Impurities • • •

Transition metal ions Lanthanide ions Fluxing agent residuals Chemical Material Technology Prof. Dr. T. Jüstel

Sheet #29

3.2.2 Composition Defects Types • Surface defects • Anion or cation vacancies VK, VA • Ions on interstitial sites Effect on the physical properties • Afterglow • Quenching of the luminescence – Competitive absorption – Energy transfer to defects + non-radiative relaxation – Re-absorption of emission • Color point shift • Stability reduction – Formation of color centers by electron capture Chemical Material Technology Prof. Dr. T. Jüstel

Sheet #30

3.2.2 Composition Deviation from the ideal composition: Loss mechanisms 1.

The absorbed energy does not reach the activator ion a) Competitive absorption b) ET to defects or non-luminescent impurity ions c) Excited state absorption (ESA) d) Auger processes

(ηTransfer)

2.

The absorbed energy reaches the activator ion, but non-radiative channels exists at the cost of radiative return to the ground state a) Crossing of excited and ground state parabola b) Multi-phonon relaxation (MPR) c) Cross-relaxation (CR) d) Photoionisation (PI) e) Energy transfer to quenching sites ηtransfer = f(spectral overlap, p, T, …)

(ηAct)

3.

Emitted radiation is re-absorbed by the luminescent material a) Self-absorption due to spectral overlap between excitation and emission band b) Additional absorption bands due to degradation of the material, e.g. by colour centre formation

(ηEsc)


Sheet #31

3.2.3 Synthesis of Phosphors Ceramic method: General procedure (see also chapter 2.1) 1. Preparation and purification of the reactants 2. Homogenization of the reactants 3. Pre-sintering (Decomposition of precursors) 4. Sintering: Conversion into the product (phase formation) 5. Washing 6. Grinding 7. (Thermal post-treatment) 8. Fractioning (Binning) Chemical Material Technology Prof. Dr. T. Jüstel

Sheet #32

3.2.3 Synthesis of Phosphors Ceramic method: Synthesis of Zn2SiO4:Mn2+ SiO2 + 1.9 ZnO + 0.1 MnCO3 → (Zn0.95Mn0.05)2SiO4 + 0.1 CO2↑

530 nm

100

Relative intensity

Emission spectrum Excitation spectrum 80 Reflection spectrum 60

40

20

200

300

400

500

Wavelength [nm]

600

700

0 800

Sample: Philips LightingG210

Problem: Evaporation of ZnO resulting in Zn-deficient phosphor (Zn0.95-xMnII/III0.05)2SiO4 Chemical Material Technology Prof. Dr. T. Jüstel

Sheet #33

Reflection [%]

Synthesis procedure • Determination of the (metal) content of the reactants • Starting materials water or ethanol suspension 1,0 • 10 minutes ultrasonic bath treatment 0,8 • Concentrate on the rotary evaporator 0,6 • Drying residue at 100 °C • 2 h sintering under nitrogen /hydrogen (5%) 0,4 atmosphere at 1200 °C (fluxing agent NH4Cl) 0,2 • Grinding 0,0 100 • Sieving

3.2.3 Synthesis of Phosphors Precursor method: Synthesis procedure 1.

Preparation of a precursor solution

2.

Precipitation as hydroxides, carbonates, sulfates, phosphates, …

3.

Regenerating the precipitate

4.

Sintering: Conversion into the product (phase formation)

5.

Sintering: Crystallization (particle growth)

6.

Grinding

7.

Fractionation (Binning)


Sheet #34

3.2.3 Synthesis of Phosphors Precursor method: Synthesis of LaPO4:Ce3+Tb3+ 0.8 La(NO3)3(H2O)6 + 0.1 Ce(NO3)3(H2O)6 + 0.1 Tb(NO3)3(H2O)6 + H3PO4 → (La0.8Ce0.1Tb0.1)PO4 + 7.5 H2O↑ + 3 NO2↑ + O2↑

100

Emission spectrum Excitation spectrum Reflection spectrum 80

Relative intensity

545 nm


60

40

20

200

300

400

500

Wavelength [nm]

600

700

0 800

Sample Nichia NP220

Sheet #35

Reflection (%)

Synthesis procedure • Dissolving nitrates in H2O • Addition of phosphoric acid + overnight stirring • Concentrate on the rotary evaporator 1,0 • 2 h sintering at 800 °C under CO 0,8 • Addition of LiF and grinding • 2 h sintering at 1000 °C under CO 0,6 • Cooling to room T in 4h 0,4 • Washing of phosphor in diluted HNO3 • Extraction by suction, acid-free washing 0,2 • Drying at 100 °C 0,0 100 • Grinding and sieving

3.2.3 Synthesis of Phosphors Precursor method: Synthesis of CaS:Eu2+ Ca(NO3)2 + Eu(NO3)3(H2O)6 + SO42- → 2 NO3- + CaSO4 + Eu2(SO4)3 → (Ca,Eu)S

Relative intensity

1,0

Emission spectrum Excitation spectrum Reflection spectrum

0,8

80

0,6

60

0,4

40

0,2

20

0,0

100

200

300

400

500

Wavelength [nm]


100

655 nm

600

700

Sample PTL FL63/S-X

Sheet #36

0

800

Reflection [%]

Synthesis procedure • Dissolving nitrates in H2O • Precipitation as sulfates • Washing • 2 h sintering at 500 °C in air • 24 h sintering at 1000 °C under N2/H2/H2S • Addition of NH4I and grinding • 4 h sintering at 1100 °C under N2 • Milling in cyclohexane • Extraction by suction • Drying • Grinding and sieving • Packaging (under inert gas)

3.2.3 Synthesis of Phosphors Influence of a halide fluxing agent • •

Increase of the Eu2+ ions (activators) mobility in the host material No clustering of Eu2+ ions ⇒ Reduction of concentration quenching

Sr

S

Sr

S

Sr

S

Sr

S

X

Sr

S

Eu

X

Sr

2

S

2

Sr

X

Eu S

Sr

X

S

Sr

S

S

Sr

Sr

S S

Sr

Defect equation: 4 HX + 2 SSx + SrSrx ⇔ 2 H2S + 2 XS• + VSr’’ + SrX2


Sheet #37

3.2.4 Areas of Application Technical area

Applications

Lighting Low-pressure mercury lamps High-pressure mercury lamps Ne-discharge lamps Xe-excimer lamps Inorganic LEDs OLED light sources

Fluorescent tubes, energy saving lamps Street lighting, Shop lighting Indicator signal lighting Backlighting, UV irradiation White and colored LEDs Flat and flexible light sources

Imaging Cathode ray tubes Plasma panels Electroluminescent screens (Emissive) LCDs OLED screens

RGB + B/W TV or monitors RGB + B/W TV Radar screens, EL foils Monitors Mobile phones, digital cameras, razors


Sheet #38

3.2.4 Areas of Application Technical area Optical brighteners Product anticounter feiting Security Advertising Medicine Dentistry Astronomy Biochemistry Analysis Lithography Cosmetics Water purification Chemical Material Technology Prof. Dr. T. Jüstel

Sample applications Paint, paper, clothes, detergents Banknotes, stamps, credit cards, certificates, tickets, documents, Emergency lighting Ne discharge lamps Computer and positron emission tomographs x-ray films, psoriasis lamps, bilirubin lamps Dental ceramics EUV/VUV-amplifier Fluorescence markers for DNA, RNA, proteins Immunoassays Photocopier Tanning lamps Xe excimer lamps, LEDs Sheet #39

3.2.5 Processing of Phosphors Powder → Suspension, paste, ceramics → Phosphor layer (“luminescent screen”) 1.

Phosphor powder → phosphor suspension, printing paste, luminescent ceramic

Solvent Binder Adhesion medium Dispersing agent 2.

butyl acetate nitrocellulose Alon-C (Al2O3) 2-Methoxy-1-propanol

Phosphor suspension → Phosphor layer

Screens: Light sources:

distilled water polyethylene oxide Ca2P2O7 poly acrylic acid [- CH2-CH(COOH)-]n

Flow-coating Printing process (screen printing, flexi printing, etc.) Sedimentation Up-Flushing Electrophoretic deposition (EPD)


Sheet #40

3.2.5 Processing of Phosphors Flow-coating: Coating of RGB cathode ray tubes

PVA + Cr2O7

2-

hν → Polymerization/cross-linking

Process cycle • Applying green phosphor suspension • Exposure to UV radiation • Rinsing • Applying blue phosphor suspension • Exposure to UV radiation • Rinsing • Applying red phosphor suspension • Exposure to UV radiation • Rinsing

PVA = Polyvinyl alcohol [- CH2-CHOH-]n Chemical Material Technology Prof. Dr. T. Jüstel

Sheet #41

3.2.6 Degradation of Phosphors Degradation mechanisms

Example

•

Thermal oxidation or reduction of the activator

BaSi2O5:Pb2+

•

Photo-oxidation or reduction of the activator

BaMgAl10O17:Eu2+

•

Dissolution/decomposition in suspension

BaSi2O5:Pb2+

•

Reactions with the glass wall

(Ce,Gd)MgB5O10:Tb3+

•

Hg-take up in fluorescent lamps

Zn2SiO4:Mn2+

•

Hydrolysis by moisture

(Mg,Ca,Sr,Ba)S:Eu2+ (Ca,Sr,Ba)SiN2:Eu2+

⇒ Protection by particle coatings, also useful for color pigments Chemical Material Technology Prof. Dr. T. Jüstel

Sheet #42

3.2.7 Particle Coatings Technologies •

Encapsulation with polymers

•

Precipitation methods – Homogeneous – Inhomogeneous

•

Pigmentation with nanoscale particles – By adhesion in suspension – By addition to the dry phosphor powder

•

Fluidised Bed Chemical Vapour Deposition (FB-CVD) – Oxidation of metal organic compounds, e.g. Al(CH3)3 – Deposition of elements, e.g. diamond-like carbon (DLC) Chemical Material Technology Prof. Dr. T. Jüstel

Sheet #43

3.2.7 Particle Coatings Process of a homogeneous precipitation for particle coatings •

Preparation of the phosphor suspension and fixing the pH value (buffer)

•

Dissolution of the coating material precursor (nitrates), possibly addition of a complexing agent

•

Precipitation – by homogeneous pH value increase, e.g. hydrolysis of urea (H2N)2CO + H2O → 2 NH3 + CO2

•

Separation – Filtration – Centrifugation – Sedimentation

•

Densification of the particle coating - Calcination (hydroxides → oxides) Chemical Material Technology Prof. Dr. T. Jüstel

Sheet #44

3.2.7 Particle Coatings Materials for particle coatings Requirements • Chemical and thermal stability • Optical transparency (→ wide band gap) • Appropriate isoelectric point

Color filter • Fe2O3 • CoAl2O4

Band gap [eV] 8.7 8.6 8.4 8.3 8.0 7.3 5.6 5.5 5.4

Transmission of Y2O3 at 185 and 254 nm 1,0

254 nm

0,8

Transmission

Protective-coating • α-Al2O3 • LaPO4 • SiO2 • Ca2P2O7 • MgO • γ-Al2O3 • Y2O3 • La2O3 • C (diamond)

0,6

0,4

185 nm

0,2

red blue


0,0

0

100

200

300

400

500

600

700

800

Layer thickness [nm] Sheet #45

900

1000

3.2.7 Particle Coatings Example of use: Coating of BaMgAl10O17:Eu with MgO Coating process • Dissolving Mg(NO3)2 in water • pH-value increase: Mg2+ + 2 NH3 + 2 H2O → Mg(OH)2 + 2 NH4+ • Calcination at 600 °C: Mg(OH)2 → MgO + H2O ⇒ MgO nanoparticle onto the phosphor particles SEM image of BaMgAl10O17:Eu


SEM image of BaMgAl10O17:Eu (MgO)

Sheet #46

3.2.7 Particle Coatings Example of use: Coating of BaSi2O5:Pb (BSP) with La2O3 Problem: Hydrolysis in water to the hydroxide • BaSi2O5 + H2O → Ba(OH)2 + 2 SiO2 ⇒ pH 9 - 10 • Hydrolysis of Ln3+ at pH > 4-5 → La(OH)3

Emission intensity [a.u.]

1,0

Consequences • Surface becomes porous • Activator Pb2+ is washed out • Coating at low pH is not possible

0,6

0,4

0,2

0,0 200

300

400

1,0

BSP from production

0,8

BSP from suspension 0,6

0,4

0,2

0,0 300

400

500

600

Wavelength [nm]


500

Wavelength [nm]

Reflection [%]

The coating process • Neutralization to alkaline suspension • Precipitation at pH 8 - 10 • Masking of La3+ is required

0,8

Sheet #47

700

800

3.2.7 Particle Coatings Example of use: Coating of BaSi2O5:Pb with La2O3 Masking of La3+ with EDTA and precipitation in the alkaline solution

La3+

+

HEDTA3-

pH 6-7

TEM image (magnification: 125000x)

[La(HEDTA)(OH2)3] Precipitation at pH 9

La(OH)3 + HEDTA31) Washing 2) ∆T

La2O3 TEM image (magnification: 260000x) Surface is coated with La2O3 „nanostructured“ Chemical Material Technology Prof. Dr. T. Jüstel

Sheet #48

3.2.7 Particle Coatings Example of use: Coating of SrS:Eu with SiO2 CO SrS is very hydrolysis sensitive: SrS + 2 H2O → H2S↑ + Sr(OH)2 → 2 SrCO3↓ + H2O ⇒ Coating can not be carried out in an aqueous suspension ⇒ Coating process in ethanol or propanol Si(OEt)4 + 2 H2O → SiO2 + 4 EtOH “TEOS” The coating is impermeable and thus increases the resistance towards hydrolysis The coating is nanostructured and increases the light output of the phosphor by about 5% by refractive index fitting (antireflection coating)


Sheet #49

3.2.8 Recycling Fluorescent lamps Due to the relatively high price of rare earths their recycling is worthwhile (Y2O3: 150 €/kg, Lu2O3: 900 €/kg, Eu2O3, Tb4O7: ~ 1200 €/kg “Status 2010”) ⇒ Recycling of fluorescent lamps (linear and compact lamps) Procedure 1. Removal of Al- or plastic caps including electrodes 2. Washing out of the phosphor 3. Removal of Hg by sublimation 4. Regeneration of the phosphor Halophosphates ⇒ Disposal Trichromatic phosphor mixtures ⇒ Direct reuse or recovery of rare earths using digestion methods


Sheet #50

3.2.8 Recycling Fluorescent lamps At present in Germany approximately 100 million discharge lamps are used. Therefore the recycling have a high environmental relevance regarding the mercury content: Mercury content in used discharge lamps • Standard fluorescent tubes • Three bands fluorescent tubes • Compact fluorescent lamps • High pressure discharge lamps • Special emitter

Typical content < 15.0 mg < 7.5 mg < 7.0 mg < 30.0 mg 1.5 g

Hg diffuses mainly into the lamp glass (in exchange for Na)

⇒ The mercury content in recycled glass from used fluorescent lamps usually lies between 4 and 6 mg/kg of glass


Sheet #51

3.2.9 Nanoscale Phosphors Phosphors with an average particle size between 1 and 100 nm Applications of nanoscale phosphors 1. Transparent dispersions, layers, and ceramic converters 2. Security labeling (value doc uments) 3. Color converter in ILEDs and OLEDs 4. Phosphors in fluorescent lamps 5. Emissive displays (CRTs, PDPs, emissive LCDs) Problems in the application • Plasma-phosphor interaction (Ar+, Ne+, Xe+, e-) • Hg/Hg+- uptake • Chemical stability in suspension • Agglomeration in suspension • Adsorption of hydrocarbons and H2O • Strong scattering / low absorption strength • Quenching of the luminescence due to surface defects ⇒ Lower quantum yield compared to microscale phosphors • Production costs Chemical Material Technology Prof. Dr. T. Jüstel

Sheet #52

3.2.9 Nanoscale Phosphors Nanoscale phosphors Luminescence deletion occurs by energy transfer (ET) to the surface: ET

A1* Excitation

A2*

Transfer to defect states, e.g. at the surface (defect area)

Emission

A1

ET

A2

Typical transition time scale ~ 1*10-7....10-8 s Phosphors, which exhibit short intrinsic decay times (τ < 1.10-7 s), can also be very efficiently nanoscale materials ⇒ Quantum Dots (GaN, GaP, GaAs, ZnSe, CdS, CdSe, CdTe, Si, …) ⇒ Activators with 4f-5d transitions, such as Pr3+, Nd3+, Eu2+, Ce3+ However, surface quenching can be suppressed by surface modifications


Sheet #53

3.2.9 Nanoscale Phosphors Nanoscale semiconductor phosphors Problem: Surface quenching of the excited states, as excitons in semiconductors have substantial radius Semiconductro CuCl ZnSe CdS CdSe CdTe GaAs PbS

Bohr radius [nm] 1.3 8.4 5.6 10.6 15.0 28.0 40.0

Band gap [eV] 3.4 2.58 2.53 1.74 1.50 1.43 0.41

Solution: Epitaxial coating with a material with a higher band gap (Exciton Reflective Coating)


Sheet #54

3.2.9 Nanoscale Phosphors Colloidal phosphors, which form stable suspensions Example: CePO4:Tb as a nanoscale phosphor (d50 ~ 10 nm) Light output ~ I(λexc)

Emission spectrum

1,0

1,0

Emission intensity [a.u.]

Light output = QE*(1-R)

0,8

0,6

0,4

0,2

0,0 150

200

250

Wavelength [nm]

• • •

300

350

0,8

0,6

0,4

0,2

0,0 300

400

500

600

700

Wavelength [nm]

QE ~ 60% (40% Tb3+ + 20% Ce3+) Ce3+ is an [Xe]4f1 - [Xe]5d1 emitter with a decay time of about 20 - 100 ns Efficient ET to Tb3+, but not between the Tb3+ ions Chemical Material Technology Prof. Dr. T. Jüstel

Sheet #55

800

3.2.9 Nanoscale Phosphors Synthesis of nanoscale garnets and oxides Example: Hydrogencarbonate precipitation for the synthesis of Ln3Al5O12-nanoparticles • Precipitation of Ln3+ and Al3+ by addition of NH4HCO3 3 Ln3+ + 5 Al3+ + 12 OH- + H2O + 3 CO32- → [3 LnOHCO3 / 5 AlOOH]Gel + 3 H2O • Sintering at 900 °C [3 LnOHCO3 / 5 AlOOH]Gel → Ln3Al5O12 + 3 CO2 + 4 H2O XRD of Lu3Al5O12 SEM Image of Lu3Al5O12 Impulse Duv35yageu.rd

1600

400

0 10

20

30

40

50

60

70

80

Position [°2Theta]


Sheet #56

3.2.10 Nitride Phosphors Phosphors on the basis of the host lattice, which contain the nitride anion N3Advantages over oxide and sulphide phosphors • Highly compact network ⇒ high density ⇒ high chemical stability ⇒ high hardness ⇒ high thermal quenching temperature •

High charge density between the activator and the nitride anions Oxides < Oxynitrides < Nitrides < Nitridocarbide ⇒ Strong red shift of the band gap or the emission band

r [pm] Electronegativity χ Ionic bonding Si-X [%] Chemical Material Technology Prof. Dr. T. Jüstel

Si 26 1.92 -

X = O2138 3.61 51

X = N3146 3.07 28

X = C4160 2.54 9 Sheet #57

3.2.10 Nitride Phosphors Very efficient, long-wavelength absorbing emitters ⇒ Application in light emitting Diodes (LEDs) (Sr,Ba)SiN2:Eu2+ (Ca,Sr)AlSiN3:Eu2+ SrLi[Al3N4]:Eu2+ SrAlSi4N7:Eu2+ (Ca,Sr,Ba)2Si5N8:Eu2+

λem= 620 - 700 nm λem= 610 - 650 nm λem= 650 nm λem= 630 nm λem= 580 - 630 nm

H.T. Hintzen et al. K. Uheda et al. W.S. Schnick et al. W.S. Schnick et al. W.S. Schnick et al.

YSiO2N:Tb3+ Y2Si3O3N4:Tb3+ Gd2Si3O3N4:Tb3+

λem= 545 nm

B. Hintzen et al.

(Ca,Sr,Ba)Si2N2O2:Eu2+

λem= 505 - 565 nm

P.J. Schmidt et al.

SrSiAl2O3N:Eu2+

λem = 480 nm

Osram

Warm-white pcLEDs with a yellow-emitting, e.g. (Y,Gd)3Al5O12:Ce, and a red-emitting nitride phosphor, mainly (Ca,Sr)AlSiN3:Eu or (Ca,Sr,Ba)2Si5N8:Eu, are on the market since end of 2003


Sheet #58

3.2.10 Nitride Phosphors Synthesis of nitride phosphors Selected routes 1. Classic solid-state reaction (in Nb or Ta-ampoules) 2 Ca3N2 + 5 Si3N4 + N2 + Eu → 3 Ca2Si5N8:Eu

N2 atmosphere

2.

Conversion of the metals with imides/amides (high frequency furnace) Sr + Eu + Si(NH)2 → Sr2Si5N8:Eu + N2 + H2 N2 atmosphere

3.

Carbothermal reduction/nitridation, CRN method (tube furnace) SrCO3 + EuF3 + Si3N4 + C → Sr2Si5N8:Eu + CO N2/H2 atmosphere

4.

Gas-reduction/nitridation (GRN) method (tube furnace) SrCO3 + SiO2 + EuF3 + NH3 + CH4 → Sr2Si5N8:Eu + CO + H2O under NH3/CH4 Chemical Material Technology Prof. Dr. T. Jüstel

Sheet #59

3. Inorganic Functional Materials 3.3. Ceramics 3.3.1 Definition and Classification 3.3.2 General Structure 3.3.3 Properties 3.3.4 Preparation of Crystalline Ceramics 3.3.5 Raw Materials 3.3.6 Technology of Clay Products 3.3.7 Refractory Ceramic Materials 3.3.8 Binding Material (Cement) 3.3.9 Ceramic Cover Layers 3.3.1 Modern Forming Technology


Sheet #60

3.3.1 Definition and Classification By ceramics one understands solid materials, which are inorganic and non metallic, and which from a structure consisting of one or more phases (crystalline, glass-like) Structural or construction ceramics Ceramics, which have to withstand mechanical stresses and strains. High-performance ceramics Highly developed, high-performance ceramic material. Functional ceramics High performance ceramics, which are used in the inherent properties of the material for an Active function, e.g. ceramic components, which exhibit electrical, magnetic, dielectric or optical properties. Cutting ceramics High performance ceramics, which are suitable due to outstanding abrasion and thermal resistance quality as a tool for cutting processing (tricks, drilling, milling). Bio ceramics High performance ceramics for the application in the medical filed i.e. in the human body. This concerns products, which replace bones, teeth or hard tissue. Chemical Material Technology Prof. Dr. T. Jüstel

Sheet #61

3.3.1 Definition and Classification Classification of the ceramics based on their chemical or mineralogical Composition is better Silicate or clay ceramics (classical structural ceramics) • Arrangement of several crystalline phases and glass phases (silicate) • Most important components: Silicates ⇒ kaolinite Al4[Si4O10](OH)8, talc Mg3Si4O10(OH)2, montmorillonite, feldspar Additive ⇒ corundum Al2O3, zircon ZrSiO4 Oxide ceramics • Fine-grained structure consisting of crystalline and usually binary oxide phase and only small amounts of glass phase • Binary oxides: Al2O3, MgO, ZrO2, TiO2 • Mixed oxide ceramics: (Ba,Pb)(Ti,Zr)O3, Al2O3/ZrO2 Non-oxide ceramics • Ceramic materials based on compounds of boron, carbon, nitrogen, and silicon • SiC, Si3N4, AlN, BN, …


Sheet #62

3.3.2 General Structure Ceramics consist of more or less randomly oriented crystalline grains (crystallites), amorphous areas (glass phase), and cracks or pores (Micro)structure = crystallites + gas phases + pores + cracks Structure of a mix-carbide ceramics consists of crystallites (dark) and pores (light areas)

Microstructure of a dense Al2O3 ceramics consisting of microcrystallites

20 µm

The structural composition is of crucial importance for the mechanical and physical properties of a ceramic component Chemical Material Technology Prof. Dr. T. Jüstel

Sheet #63

3.3.3 Properties Due to their ionic or covalent bonds ceramic materials possess a number of characteristic properties 3 • • • • •

low thermal and electrical conductivity high hardness and brittleness high melting point (> 1500 °C) high chem. and therm. stability low density

Material Al2O3 SiC Si3N4 SiAlON ZrO2

Density[g/cm ] 4.0 3.1 3.2 3.2 5.8

Tensile strength [N/mm2] 210 175 560 420 455

SiAlON = Si3-xAlxN4-xOx

Ceramics, which functional and non-mechanical properties are not in the foreground, exhibit however different properties, like e.g. FeO, ZnO Semiconductor Superconductor YBa2Cu3O7- x Ion conductor ß-NaAl11O17 CrO2, Y3Fe5O12 Magnets (Pb,La)(Zr,Ti)O3 Pressure sensors Scintillators for CT Gd2O2S:Pr Luminescence converter for LEDs Y3Al5O12:Ce Solid state laser Lu3Al5O12:Nd Chemical Material Technology Prof. Dr. T. Jüstel

Sheet #64

3.3.4 Preparation of Crystalline Ceramics The basic characteristics or the microstructure of a ceramic component depend on selected raw materials and on the production process General flow chart Powder synthesis → Solid/solid reactions Precipitation reactions Decomposition reactions Solid/gas reactions Gas/gas reactions

Chapter 2.1 – 2.3 Chemical Material Technology Prof. Dr. T. Jüstel

Powder preparation → Mixing Deagglomeration Spray drying Freeze-drying Addition of additives Granulation

Shaping → Densification Dry pressing Sintering Casting Gas pressure sint. Extruding HIP Injection molding Pressing Impregnation Hot pressing Infiltration Manual shaping

Green body

Final ceramic Sheet #65

3.3.4 Preparation of Crystalline Ceramics Production of oxide ceramics for ceramic light sources (CDM light sources )

Pressure

d ~ 20 µm

1. Al2O3 powder d = 0.6 µm Chemical Material Technology Prof. Dr. T. Jüstel

2. Forming (pressing) ρRD ~ 45%

3. Sintering (1900 ºC) (densification) ρRD = ~100% Sheet #66

3.3.4 Preparation of Crystalline Ceramics Production of a high temperature superconductor ceramics from „YBaCu“ Y2O3 + 4 BaCO3 + 6 CuO → 2 YBa2CuII/III3O7-x + 4 CO2↑ Manufacturing process 1. Mixing and grinding of the starting materials BaCO3, Y2O3, CuO in acetone 2. Sintering at 890 °C in air 3. Pulverization 4. Sintering bei 930 °C in air 5. Powderization ⇒ YBaCu-powder 6. Forming ⇒ YBaCu-green body 7. Sintering ⇒ YBaCu-ceramic → Further processing into cables


Sheet #67

3.3.5 Raw Materials For the ceramics production either naturally occurring raw materials, further treated raw materials or inorganic chemicals are used Group Non-processed Raw materials

Substances Rock clay Raw bauxite

Clay minerals AlO(OH).xFe2O3.ySiO2

Industrially processed Raw materials

Wollastonite Zircon Rutile Kaolinite Dolomite

CaSiO3 „chain silicate“ ZrSiO4 TiO2 Al4[Si4O10](OH)8 (Ca,Mg)CO3

Industrial inorganic Chemicals

Al2O3, MgO, SiC, ZrO2, UO2, Y2O3, Gd2O3, BeO, Ta2O5 BaTiO3, Pb(Ti,Zr)O3, Al2TiO5, MFe2O4 (M = Mn, Ni, ...) Si3N4, BN, AlN, B4C, TiB2, TiN, MoSi2


Sheet #68

3.3.6 Technology of Clay Products Tone products are used for the production of pipes, bricks, tiles, pottery etc a

Raw materials • Clay, e.g. kaolinite Al4[Si4O10](OH)8 • Initial bonding agent (mostly water) • Ceramic particle (mostly SiO2-quartz powder) • Fluxing agent during the following thermal treatment, e.g. felspar [(K,Na)2O·Al2O3·6SiO2] Forming technology a. Pressing b. Isostatic pressing c. Extrusion d. Manual forming e. Slip casting

b

c

d

e Ref.: D.R. Askeland, Materialwissenschaften, Spektrum Akademischer Verlag GmbH, Heidelberg, Berlin, Oxford, 1996 Chemical Material Technology Prof. Dr. T. Jüstel

Sheet #69

3.3.6 Technology of Clay Products Drying and firing of clay products a)

Decrease in volume during drying

Evaporation of water stored between clay plates

b)

Density increase when burning

1. 2. 3.

Dehydrogenation of the bonded water in kaolinite Melting of the flux and the silicate Formation of a glass phase in the clay mineral gaps

Source: D.R. Askeland, Materialwissenschaften, Spektrum Akademischer Verlag GmbH, Heidelberg, Berlin, Oxford, 1996 Chemical Material Technology Prof. Dr. T. Jüstel

Sheet #70

3.3.7 Refractory Ceramic Materials Lining of furnaces and other high temperature equipment Classification of materials is done according to their chemical behavior (→ IEP) Material class Silica brick Fire brick

SiO2 [%] Al2O3 [%] MgO [%]

Cr2O3 [%]

95 - 97 10 - 45

acid

50 – 80

Magnesite Olivine

Fe2O3 [%]

83 - 93 43

2–7

neutral

57

Chromite

3 - 13

12 - 30

10 - 20

12 - 25

30 – 50

Chromite magnesite

2-8

20 - 24

30 - 39

9 - 12

30 - 50

Special refractory materials • Graphite (stable under oxygen exposure) → Graphite furnaces • Zirconium compounds: ZrO2, ZrO2·SiO2 • Silicon carbide: SiC reacts at the surface of SiO2 → Passivation to about 1500 °C Chemical Material Technology Prof. Dr. T. Jüstel

Sheet #71

basic

3.3.9 Ceramic Cover Layers Ceramic substances serve frequently also as a protective coating → Glazes and enamel Glaze: Protective layer for ceramic Enamel: Protective layer for metallic This concerns clay products, which easily glass during sintering, e.g. CaO.Al2O3.2SiO2 → Transparent glaze/enamel Additive of further minerals leads to colored protective coating Color White Blue Green Yellow Red

Additive ZrSiO4 Co2O3 Cr2O3 PbO Se, CdS


Sheet #72

3.3.10 Modern Forming Technology For the production of modern ceramics from high purity raw materials one uses special forming technology •

Pressing and sintering (hot pressing) HIP-technique (Hot Isostatic Pressing): green body is sintered in a pressure chamber under inert gas (N2)

•

Reaction sintering 3 Si (powder or green body) + 2 N2 → Si3N4

•

Sol-Gel-technique → Chapter 2.3.2


Mechanical properties of Si3N4-ceramic Production process

Compression strength [N/mm2]

Bending strength [N/mm2]

Slip casting

140

70

Reaction sintering

770

210

Hot pressing

3500

875 Sheet #73

3. Inorganic Functional Materials 3.4. Ion Conductor 3.4.1 Ion Conduction in the Solid State 3.4.2 Alkali Halides: Hole Transport 3.4.3 Silver Chloride: Interstitial Conduction 3.4.4 Solid Electrolytes 3.4.5 ß-Alumina 3.4.6 Silver Ion Solid Electrolytes 3.4.7 Anion Conductors 3.4.8 Applications


Sheet #74

3.4.1 Ion Conduction in the Solid State Conductivity in the solid requires the mobility of the cations or anions At room T, majority of solid compounds are very poor conductors, i.e. insulators Increased conductivity • in certain crystal structures (mostly layer structures) Example: NaAl11O17 (ß-Al2O3) by generation of defects by means of temperature increase (intrinsic) Example: NaCl RT σ < 10-12 Ω-1cm-1 σ = A.exp[-E/RT] 800 °C σ ~ 10-3 Ω-1cm-1 (Arrhenius equation) Just below the melting point of a solid, the conductivity increases strongly

•

by the insertion of dopings (extrinsic)

Increasing doping level

log σ

•

1/T [K-1] Chemical Material Technology Prof. Dr. T. Jüstel

Sheet #75

3.4.2 Alkali Halides: Hole Transport In alkali halides the smaller cations are more mobile than the anions → Hole transport Hole transport = Migration of cation lattice vacancies V Conductivity σ = A.c(V)

The number of cation vacancies V can be increased by increasing the temperature or by the incorporation of extrinsic cations with a higher charge than those of the Na+ ions: NaCl + x MnCl2 → Na1-2xMnxVNaxCl (x = 0.0 – 0.5)


Process

Activation energy [eV]

Migration of Na+

0.65 – 0.85

Migration of Cl-

0.90 – 1.10 Sheet #76

3.4.3 Silver Chloride: Interstitial Conduction In AgCl Frenkel defects are decisive, i.e. silver ions are located at interstitial sites Ag Cl Ag Cl and are coupled to corresponding lattice vacancies → Cation migration via interstitial sites i Formation of Frenkel defects AgCl → Ag1-xLxAgixCl Process Formation of Frenkel defects Migration of the cation vacancies Migration of the Ag+ ions


Activation energy [eV] 1.24 0.27 – 0.34 0.05 – 0.16

Cl

Ag

Cl

Ag

Ag

Cl

Ag

Cl

Cl

Ag

Cl

Ag

Ag

Cl

Ag

Cl

Cl

Ag

Cl

Ag

Ag

Cl

Cl

Ag

Cl Agi Cl Ag

Sheet #77

3.4.4 Solid Electrolytes Solid electrolytes concerns usually halides or oxides Solid electrolytes can also be interpreted as a phase between the crystalline and liquid phase

Normal crystalline solid

→

Solid electrolyte

→

Liquid

TemperatureT ↑ Defect concentration c(vacancies) ↑ Conductivity σ ↑ Chemical Material Technology Prof. Dr. T. Jüstel

Sheet #78

3.4.5 ß-Alumina ß-alumina possesses by the inclusion of Na+ cations a layered structure and thus is a two-dimensional conductor α-alumina “ß-alumina” γ-alumina

Spinel block

Al2O3 NaAl11O17 Al2O3

Conduction layer

Na+ can also be substituted by Li+, K+, Ag+ and Tl+

Spinel block The cations in the conduction layer have a high ion mobility (especially small cations) Cation Na+ Ag+ K+ Tl+

Activation energy [eV] 0.16 0.17 0.30 0.36


Conduction layer Spinel block Elementarzelle Sheet #79

3.4.5 ß-Alumina Thermodynamic stability of β-alumina Structural influence of cations in the intermediate layer Conduction layer thickness M12k

4,9

ß-Alumina

4,8

Rb K

4,7

Ba

Na

4,6

Ag Pb

4,5

Sr La

4,4

Nd

Ca

Magnetoplumbite

4,3 1,1

1,2

1,3

1,4

1,5

Ionic radius [A]

• • •

Stability limit of the β-Alumina phase is M12k > 4.6 Å Incorporation of small cations destabilizes the ß-alumina phase (Sr2+, Ca2+) Incorporation of large cations stabilization the ß-alumina phase (Rb+, K+) Chemical Material Technology Prof. Dr. T. Jüstel

Sheet #80

1,6

3.4.6 Silver Ion Solid Electrolytes α-AgI and RbAg4I5 are extraordinarily good ion conductors 146 °C

ß-AgI ⇌ α-AgI

Structure of α-AgI • Body-centered cubic (bcc) arrangement of anions • The Ag+ ions are statistically distributed over 36 trigonal and tetrahedral positions ⇒ High mobility of Ag+ anions

log σ [Ω-1cm-1]

σ(α -AgI) ~ 1 Ω-1cm-1!

Material with the highest conductivity at RT is so far RbAg4I5 ⇒ σ(RbAg4I5) ~ 0.25 Ω-1cm-1 1/T [K-1] Chemical Material Technology Prof. Dr. T. Jüstel

Sheet #81

3.4.7 Anion Conductors Anion conductors are defect semiconductor with defects in, e.g. ZrO2 Formation of the anion lattice vacancies

Phase diagram ZrO2-CaO

ZrO2 + x CaO → Zr1-xCaxO2-xLx (0.1 < x < 0.2) ZrO2 + x/2 Y2O3 → Zr1-xYxO2-x/2Lx/2

Other anion (oxide O2-) conductors • HfO2, ThO2 • TiO2-x, VO2-x • WO3-x, MoO3-x


Sheet #82

3.4.8 Applications Requirements of a good ion conductor •

Many similar ions must be mobile

•

For the mobile ions many empty sites, which can be occupied, must be available

• • •

The empty and the occupied positions must have a comparable potential energy The structure must possess 3-dim. framework with open channels, through which mobile ions can move The anion network must be easily polarizable

Technical applications • • •

Fuel cells/water vapor electrolysis Measurement of oxygen partial pressure Batteries, e.g. – Na(l)|Na-ß-Al2O3|S(l) – Li(l)|LiI|I2-PVP(Iodpoly-2-vinylpyridine) Chemical Material Technology Prof. Dr. T. Jüstel

Sheet #83

3.5 Biomaterials Definitions Biological materials: Materials, which are used naturally by living organisms. Bio(compatible) materials : Artificial (man-made) materials that are used in place of biological materials, e.g. implant materials Biomimetic materials: Artificial materials, which recreate the structure of the biological materials Why is the research concerning biological materials of great importance? 1. 2. 3.

Generate understanding How living things are using materials to adapt to the environment? Application in the material science Which construction ideas can be derived? ⇒ Biomimetic materials Application in the medicine How can biological materials be handled or replaced? Chemical Material Technology Prof. Dr. T. Jüstel

Sheet #84

3.5 Biomaterials Classification Static structural materials • Internal and external skeleton (support function) • Cell walls, fibers, hair, nails, tendons, spider silk, nacre,... Membranes • Structural material with passive mass transport; cell membrane • intracellular membranes of organelles, particularly the nuclear membrane Active functional materials • Muscles, composed of filaments Alternative classifications • animal or vegetable • chemical composition Chemical Material Technology Prof. Dr. T. Jüstel

Sheet #85

3.5 Biomaterials Static structural materials Endoskeleton

Exoskeleton

Fibers (silk)

Cell walls (wood)


Sheet #86

3.5 Biomaterials Membranes and active functional structural materials Cell membrane

Muscles

Muscle fibers in the SEM unstretched (top) and stretched (bottom) The molecular Actin-myosin motor

Muscles (active) Tendon (passive) Chemical Material Technology Prof. Dr. T. Jüstel

Sheet #87

3.5 Biomaterials Hierarchical structure (bottom-up) 1. Nanoscopic (0.1 - 1 nm) Molecules: common polymers and inorganic substances • Carbohydrates: cellulose, chitin • Proteins: polyalanine (spider silk), collagen, keratin, actin and myosin (muscle fibril) • Inorganic compounds: hydroxyapatite (bone), calcite (nacre), SiO2, Fe2O3 • Complex compounds: lignin (various types) 2. Mesoscopic (1 – 100 nm) Structural units: order, such as helices or crystals • hard, ordered units (crystals) in a softer, disordered Matrix ⇒ Composite materials: mechanical properties change • Cellulose microfibrils, mineralization of bone and tendon • Protein crystals in spider silk, lamellar phase of membranes • Helices as the basic unit of many fibers, such as tendons (collagen)


Sheet #88

3.5 Biomaterials Hierarchical structure (bottom-up) 3. Microscopic (0.1 - 100 µm) Cells, tissues • Plant cell walls, fiber cells, wood cells • Muscle filaments 4. Macroscopic (from 0.1 mm) Architecture • Annual rings • Bone • …


Sheet #89

3. Inorganic Functional Materials

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