Magnetite in Glassy Matrix V. Sandu, M. S. Nicolescu, V. Kuncser, I. - - PowerPoint PPT Presentation

Magnetite in Glassy Matrix

• V. Sandu, M. S. Nicolescu, V. Kuncser, I. Ivan,

National Institute of Materials Physics-Bucharest, Magurele, 077125, Romania

• E. Sandu

„Horia Hulubei“ National Institute of Physics and Nuclear Engineering, Magurele, 077125, Romania

Acknowledgments: Romanian National Authority for Science, project 72.151/2008

NATIONAL INSTITUTE of Materials Physics 105 bis Atomistilor Str, 077125 Magurele, CP MG-7, Romania

Glass ceramic as composite material

• Is glass ceramic a composite?
• Composite materials are materials made from two or more constituent materials with

significantly different physical or chemical properties which remain separate and distinct at the macroscopic or microscopic scale.

• A nanocomposite is as a multiphase solid material where one of the phases has one,

two or three dimensions of less than 100 nanometers (nm), or structures having nano- scale repeat distances between the different phases that make up the material. It is more usually taken to mean the solid combination of a bulk matrix and nano- dimensional phase(s) differing in properties due to dissimilarities in structure and chemistry. The mechanical, electrical, thermal, optical, electrochemical, catalytic properties of the nanocomposite will differ markedly from that of the component materials.

• Recognized as composites:

– Ceramic-matrix nanocomposites – Metal-matrix nanocomposites – Polymer-matrix nanocomposites

• But not glass ceramic: vitreous matrix containing dispersed nanograined crystalline

phases.

• Crystalline grains nucleate and growth:
• during cooling down of the molten composition
• as a result of different thermal treatments

Magnetic Glass Ceramics

• Glass-ceramic containing magnetic nano-crystallite (iron oxides, barium, and

strontium hexaferrites, etc) grown within the vitreous matrix.

• Features:
• very fine single-or multi-domain magnetic particles
• superparamagnetic behavior
• dipol-dipol interaction prevents grain aglomeration
• Very flexible and cheap process that depends on:
• appropriate choice of the ingredients
• particular thermal excursion
• stable ferro fluids with a large number of carrier due to the lack of sedimentation;
• magnetic drug transport and targeting;
• magnetofectia;
• biodetection and magnetic separation;
• Hyperthermia: incorporation of the magnetic particles via endocytosis;
• MRI contrast agents;
• Component of glass-ceramics substitutes of bones.

For in vivo medical applications only iron oxides, i.e., Magnetite (Fe3O4) or maghemite (-Fe2O3) can be used

Preparation of glass-ceramics with high content of magnetite by direct crystallization

Magnetite: Fe3O4

• Inverse spinel structure:

Fe3+

A Tetrahedral positions

Fe3+

B & Fe2+ B octahedral positions

• Fe3+

A & Fe3+ B are antiferromagnetically

• rdered and the ferrimagnetism comes from

Fe2+

B spins

• Continuous exchange of electrons between

Fe2+ and Fe3+ in the octahedral positions

• Verwey transition at TV120 K
• Curie temperature Tc = 858 K

Still under dispute: Charge & orbital ordering Multiferroicity

Samples preparation

Basically: borosilicatic glass with constant B2O3 and Na2O content and small amounts of either one of Al2O3, and nucleators (Cr2O3, P2O5 ). Variation of the ratio SiO2/Fe2O3. The batches were melt into alumina crucible for 2.5-3 hours in the temperature range 1400-1500 C.

• The melts were poured onto a steel sheet and the resulting slabs were

immediately transferred to an annealer operating at 560 C for 2-4 hours. -

• Cooling down 450 C (10 C/hour)
• Cooling down to 300 C (20 C/hour) and then inertially to 25 C.

Sample code Starting oxide composition (%w) SiO2 B2O3 Na2O Fe2O3 Cr2O3 Al2O3 P2O5 C1 47 28.6 6.4 17.5 0.5

• P1

46.5 28.6 6.4 17.5

• 1

C2 36.5 28.6 6.4 24.5 0.5 3.5 P2 39.5 28.6 6.4 24.5

• 1

Why these compositions?

• 1. The silica tetrahedra network may also include other tetrahedral or

triangular groups (B, Na) and the intermediate (F2O3, Al2O3)

• 2. Cr and P have nucleating oxides with high solubility at high T but low

solubility and high diffusivity at low T:

• Cr in glass melts has two valence states Cr2+ and Cr6+ . The latter is

stabilized by the presence of alkaline oxides (Na). Cr6+ has a intense field q /r = 17.2 with a strong ordering effect on O ions which promote the separation of crystalline nuclei at low T

• P5+ is glass formating at high T. In tetrahedral coordination the charge

difference between Si4+ and P5+ leads to separation not as P2O5 but in combination with alkaline metal

• AlO4 stabillizes PO4 tetrahedra due to the special bonds, so, increases

the stability of glass relative to recrystallization. Fe3O4 crystallizes on these nucleii

Thermal expansion investigation

Sample Tir TG Tsr TD C1 464.8 495.9 509.5 553.6 P 1 425.5 461.2 474 572.4 C2 438.0 465.5 476.9 513.5 P2 419.2 464.4 479.1 546.8

C1

Glassy temperature

• vs. composition and

nucleators Expansion constant  vs composition and nucleators.

Cr2O3 P2O5

X-ray Difrraction

• Intensive process of recrystallization
• ccurs in all samples
• Attests to the efficiency of Cr2O3 and P2O5 as a

nucleating agents.

• unique crystalline phase: magnetite.
• minor phase: F2O3, (in C1).

Grain size as extracted from the peak (311)

FTIR-data

Morphology C1 C2

P1 P2

Mössbauer Spectroscopy

Two magnetic components: magnetically ordered component: sextet central paramagnetic component: doublet. Hyperfine parameters  Fe coordination: Octahedral Fe2.5+  Hhfo = 46 T Tetrahedral Fe3+  Hhft = 49 T Ideal Magnetite R = N(Oct)/N(Tetra) = 2 Sextets: magnetically ordered Fe C-1: R = 2.1 & Hhfo = 45.9 T  almost ideal magnetite. C-2: R = 1.7 & Hhfo = 46.1 T  octahedral postion equally underoccupied P-1: R = 1.7 & Hhfo = 45.8 T  octahedral postions equally underoccupied P-2: R = 2  well structured magnetite. The paramagnetic doublets: Fe ions dispersed in the glassy matrix. C-1: 16% of the total Fe ions C-2: 10% as Fe3+. P-1: 25 %: 22% Fe3+ , octahedral coordination, 3% is Fe2+. P-2: 41 %: 32 % Fe3+ , 9 % Fe+,  >2, tetrahedral coordination

Cr2O3-multi domain

dc-Magnetization

R = 2.1, d = 121 nm Slightly octahedrally dominant R = 1.7, d = 79 nm tetrahedrally dominant

• AFM ordering of Fe3+ and Fe2+ in O
• FM ordering of Fe3+ in T
• J. Wang et al., Mat. Chem. Physi. 13 6 (2009)
• domains

P2O5-single domain

R = 1.7, d = 33 nm Tetrahedrally dominant R = 2.0, d = 26 nm

ac-susceptibility

• The peaks shift with  but not

enough to attribute of activated

• processes. Not TB but TV.
• ” increases with  : rotation of the

magnetic moments and the change

• f the ionic order within walls 

friction

• Low T shoulder ???
• Slower  dependence
• ” decreases with 
• The

shoulder at low T evolves to a peak (21.12 K at 30 Hz)

• A

second shoulder is present at high temperatures (85 K at 30 Hz). C1 C2 dc-susceptibility

ac-susceptibility

• Sharp Verwey transition TV =

122 K

• negligible dependence of 
• Low T peak in ”
• ”

decreases

• n

average with 

• Is it related to R = 1.7???

(electronic processes between Fe3+ and Fe2+)

• Broad ”
• ” increases 
• Single domain nanoparticles

P1 P2 dc-susceptibility

Conclusions

1. At high Fe content, borosilicatic glass can be crystallized with the formation of magnetite containing glass ceramic composite 2. Cr2O3 and P2O5 are good nucleators for magnetite crystallization but between 12 and 44 % of Fe ions remain dispersed in the glassy matrix as paramagnetic ions. 3. Cr2O3 - promotes large grains and leaves small amunt of Fe ions within glass solution. However it leads to structural unbalanced occupation of the tetrahedral and octahedral sites. Specifically, at low Fe content is supported the underoccupation of the tetrahedral sites whereas at high Fe content the octahedral sites are Fe-deficient. 4. P2O5 promotes small crystallites agglomerated in large almost spherical grains with a well defined Verwey transition, but with lower values of the magnetization due to a reduced contribution of the Fe ions to the growth of crystalline, hence magnetically ordered, phase. An important amount of Fe ions is left dispersed in the glassy matrix. When the amount of Fe is increased, it results a structurally ideal magnetite but with a huge amount (41% ) of paramagnetic Fe. 5. The magnetic response is complex and depends on the degree of vacancies in the structure of magnetite as well as on the location of these vacancies. However, the problem is more complex and a clear response would require also the analysis of the role of the structural and ferroelastic domains in the dynamic behavior. 6. The absolute value of the magnetization is the result of participation of Fe ions to the formation of magnetite phase, therefore, the samples with low content of paramagnetic phase display the highest specific magnetization.

Thank you

Location of our magnetite