Bulk microstructure and local elastic properties of carbon nanocomposites studied by Impulse Acoustic Microscopy technique

Experimental unit

Scanning Impulse Acoustic Microscope (SIАМ), Institute of Biochemical Physics, RAS, 2011

Operation frequency: 50 - 200 MHz  Immersion: water Scanning step: Z=15 μm Scanning field: up to 360×450 mm   Modes: 3D visualization (tomography) А, В and С-scans

Principles of acoustical bulk imaging

1. Reflection mode;
2. Impulse probe signals;
3. Time resolution of echo signals coming from different depth inside the specimen bulk
4. 1D or 2D probe beam scanning for image formation

Basic modes of data representation

 

Probe pulse

Electronic gate (given by green color) – the technique of signal processing for displaying recorded signals as acoustical B- and C-scans. 

Echo pattern - oscillogram of the reflected signal	B-scan the sweep of echo signal within the electronic gate when 1D motion of the acoustic lens	С-scan gray-scale displaying of echo-signal amplitude variations within the electronic gate in 2D scanning of the acoustic lens

Idea of nanocomposites

1. routine polymer medium as a binder (matrix material)
2. nanosized particles as filler
3. small content of nanofiller (usually 0.05 ¸ 10 weight %)
4. uniform distribution of nanofiller over the matrix material

Advantage of nanocomposite materials – new physical properties:

• DC electrical conductivity
• HF electromagnetic properties - effective EM absorption, resonance phenomena, shielding, screening, etc.
• high heat conductivity
• mechanical properties enhancing elastic moduli and strength

Graphite – Epoxy Nanocomposites

Nanofiller: diverse kinds of graphite nanoparticles

1. Exfoliated graphite (EG). Milled exfoliated graphite, particle sizes – (20 ÷ 500) μm
2. Flat micronic graphite (FMG). Fine milling and thermal treating of exfoliated graphite. Graphite stacks consisting of 30- 40 graphite atomic layers. Lateral sizes are not larger than 10-15 μm
3. Graphite nano-platelets (GNP). Intercalation / Pulverization / Additional milling Thickness ~10 nm, lateral sizes ~1 - 10 μm
4. Multiwall carbon nanotubes (MWCNT). CVD produced carbon multiwall nanotubes, Ø 20 – 40 nm, 5 - 12 μm long

Possible mechanisms of properties enhancement

1. Formation of fractal clusters of contacting nanoparticles in polymer matrix: formation of conductive nets and islands
    

electrical and heat conductivity; electromagnetic properties

 2.Restructuring polymeric matrix by the ordering of macromolecules in the vicinity carbon nanoparticles

elastic and strength properties

The paper goals

Application of the high-resolution acoustic vision technique for studying fundamental problems of nanocomposite material science:

• uniformity of nanoparticle distribution over the composite bulk for nanocomposite with different types of carbon nanofiller;
• what elastic properties– enhanced or worsened ones comparing with properties of the neat polymer binding; are realized in nanocomposites with different carbon nanofiller;

Investigated materials: Epoxy + different carbon nanoforms

Mechanisms of acoustical contrast in carbon nanocomposites

1. Acoustical imaging in epoxy + exfoliated graphite (EG) composites

Reflection or scattering of the probe beam at EG particles.

Tops of EG particles only are displayed in acoustical images.
Radiation reflected from other parts of the particle surface does not get into the aperture of the acoustic objective.

Acoustical images of epoxy + EG composites represent only the occurrence of EG particles and their position in the specimen bulk. They give no information on particle sizes and shapes.

2. Acoustical imaging in epoxy + 2D nanocarbon (FMG and GNP) composites

Scattering at micronic 2D nanoparticle agglomerates filled by air (aerogels).

1. 2D nanoparticles agglomerate into micron-sized conglomerates because of their high affinity to each other (higher than their affinity to polymer binder). 2. Trapping air in interparticle space and aerogel formation 3. Ultramicroscopic mode of imaging - receiving radiation scattered at small obstacles inside the focal zone of the probe beam. 4. High efficiency of ultrasonic scattering at small acoustically soft obstacles (scatterers filled by air)

The ultramicroscopic mode is the acoustical analog of the dark-field technique in optics

Acoustical images depict the presence and position of scatterers, not their sizes, and shapes.

What minimal objects could be seen with impulse acoustic microscope in the specimen bulk?

It is seen in the acoustic image:   air-filled fractal conglomerations with sizes ≥ 1÷2 μm stiff inclusions with sizes exceeded 20÷30 μm

3. Acoustical imaging in composites with carbon nanotubes (epoxy + MWCNT)

Acoustic contrast as a result of the non-uniform distribution of nanotubes and corresponding spatial variations of local elasticity.

3D imaging of carbon nanocomposites. Bulk microstructure

Interior structure in the pristine epoxy resin

Specimen thickness d = 1.74 mm Operation frequency - 50 MHz

Acoustic image (C-scan) of interior structure in the middle of the specimen thickness

Imaging layer 120 μm thick is inside the specimen at the depth of 400 μm.

Specimen thickness d = 400 μm Operation frequency - 100 MHz

Acoustic image of the specimen bulk structure in a transverse section (B-scan)

Bulk microstructure of the (epoxy + 1.5 w% EG) specimen

 

Specimen thickness d = 1.35 mm Operation frequency - 50 MHz

Acoustic image (C-scan) of interior structure in the middle of the specimen thickness

Imaging layer 200 μm thick is inside the specimen at the depth of 405 μm.

Specimen thickness d = 420 μm Operation frequency - 100 MHz

Acoustic image of the specimen bulk structure in a transverse section (B-scan)

Bulk microstructure of the (epoxy + 1.5 w% FMG) specimen

Specimen thickness d = 1.42 mm Operation frequency - 50 MHz

Acoustic image (C-scan) of interior structure in the middle of the specimen thickness

Imaging layer 120 μm thick is inside the specimen at the depth of 310 μm.

Specimen thickness d = 430 μm    Operation frequency - 100 MHz

Acoustic image of the specimen bulk structure in a transverse section (B-scan)

Bulk microstructure of the (epoxy + 0.75 w% GNP) specimen

Specimen thickness d = 1.56 mm Operation frequency - 50 MHz

Acoustic image (C-scan) of interior structure in the middle of the specimen thickness

Imaging layer 130 μm thick is inside the specimen at the depth of 400 μm. 

Specimen thickness d = 360 μm Operation frequency - 100 MHz

Acoustic image of the specimen bulk structure in a transverse section (B-scan)   

Bulk microstructure of the (epoxy + 0.1 w% MWCNT) specimen

 

	Acoustic image (C-scan) of interior structure in the middle of the specimen depth. Specimen thickness d = 1.39 mm Operation frequency - 100 MHz Imaging layer 60 μm thick is inside the specimen at the depth of 280 μm.
	Imaging layer Acoustic image of bulk microstructure in a transverse section (B-scan) of the same specimen

Material	Specimen thickness d, mm	Longitudinal sound velocity cL, km/sec	Transverse  sound velocity cT, km/sec	Density ρ, g/cm3
Epoxy 100%	0.40 1.74	2.88 2.90	1.37 -	1.170 -
Epoxy+1 w % EG	0.36 1.58	3.00 2.86	1.40 -	1.186 -
Epoxy+1 w %GNP	0.43 0.93	2.99 3.05	1.41 -	1.182 -
Epoxy+1 w %TG	1.60	3.04	-	1.135
Epoxy+ 0,1 w % MWCNT	1.39	2.80	-	1.180
Average		2.99	1.99	1.171

Comparison of non-destructive techniques for 3D visualization of bulk microstructure in nanocomposites

 Available techniques of bulk microstructure visualization:

• High-resolution X-ray tomography. Spatial resolution is given by the thickness of the X-ray probe beam. Micron-scale resolution is implemented by synchrotron radiation or application of special high-power X-ray tubes.
• Impulse scanning acoustic microscopy. Spatial resolution is given by the wavelength (15 – 60 μm) of the probe ultrasound. Ultramicroscopic mode provides micron resolution of 3D imaging of the bulk microstructure.

Bulk microstructure of the (epoxy + 1.5 w% exfoliated graphite) specimen (thickness d = 1.44 mm)

synchrotron radiation; probe beam Ø 20 μm	focused ultrasound,  f = 50 MHz, λ = 30 μm

Bulk microstructure of the (epoxy + 0.1 w% MWCNT) specimen (thickness d = 1.4 mm)

synchrotron radiation; probe beam Ø 20 μm	focused ultrasound,  f = 50 MHz, λ = 30 μm

Conclusions

1. It was shown the impulse acoustic microscopy is a powerful technique for studying the internal microstructure and local elastic measuring in the bulk of nanocomposite materials.

2. It was the first time when the occurrence of complicated fractal mesostructure in the bulk of nanocomposites has been shown.

3. Efficient agglomeration of carbon nanofiller in the bulk of carbon nanocomposites has been demonstrated for 2D carbon nanoforms.

4. Local elastic measurements demonstrated sufficient elastic homogeneity of carbon nanocomposites despite occurrence of bulk mesostructure.

Ultrasonic elastic measurements demonstrate minimal influence of nanofiller on elasticity of carbon nanocomposites for wide spectrum of carbon nanoforms being used as nanofiller – from exfoliated graphite up to carbon nanotubes and thick grapheme stacks.