The Sintering and Densification Behavior of an Iron 

Nanopowder Characterized by Comparative Methods 

by Laszlo J. Kecskes, Robert H. Woodman, Samuel F. Trevino, 

Bradley R. Klotz, and S. Gary Hirsch 

ARL-TR-2909 February 2003 

Approved for public release; distribution is unlimited. 
NOTICES 

Disclaimers 

The findings in this report are not to be construed as an official Department of the Army position unless 

so designated by other authorized documents. 

Citation of manufacturer’s or trade names does not constitute an official endorsement or approval of the 

use thereof. 

Destroy this report when it is no longer needed. Do not return it to the originator. 
Army Research Laboratory 

Aberdeen Proving Ground, MD 21005-5069 

ARL-TR-2909 February 2003 

The Sintering and Densification Behavior of an Iron 

Nanopowder Characterized by Comparative Methods 

Laszlo J. Kecskes, Robert H. Woodman, Samuel F. Trevino, 

Bradley R. Klotz, and S. Gary Hirsch 

Weapons and Materials Research Directorate, ARL 

Approved for public release; distribution is unlimited.
ii

Acknowledgments 

We note the assistance of Sara White at Nissei Sangyo America for the use of the field-emission 

scanning electron microscope, Dr. Bonnie Gersten with preliminary Brunauer, Emmett, and 

Teller (BMT), static and dynamic light scattering measurements, Kyu Cho for the x-ray 

diffraction measurement protocol development, and George Dewing for sample preparation and 

the operation of the plasma pressure compaction (P2

C). Bradley R. Klotz and Samuel G. Hirsch 

were supported in part by an appointment to the research participation program at the U.S. Army 

Research Laboratory (ARL) administered by the Oak Ridge Institute for Science and Education 

through an interagency agreement between the U.S. Department of Energy and ARL. 
iii

Contents 

Acknowledgments ii

List of Figures v

List of Tables vi

1. Introduction 1

1.1 Direct and Structural Applications of Nanosized Fe Precursor Powders........................1 

1.2 Characterization of Nanosized Precursor Powders .........................................................1 

1.3 Objectives........................................................................................................................2 

2. Experimental Procedures 3

2.1 Particle-Size Characterization Procedures ......................................................................3 

2.1.1 Electron Microscopy Techniques..........................................................................3 

2.1.2 Brunauer, Emmett, and Teller (BET) Surface Area Measurement .......................4 

2.1.3 Light Scattering Methods ......................................................................................4 

2.1.4 X-ray Diffraction (XRD) Analysis........................................................................4 

2.1.5 Neutron-Based Methods........................................................................................5 

2.2 Pressureless Sintering Experiments.................................................................................5 

2.3 P2

C Experiments..............................................................................................................6 

3. Results 6

3.1 Powder Characterization .................................................................................................6 

3.1.1 Electron Microscopy Techniques..........................................................................7 

3.1.2 Brunauer, Emmett, and Teller (BET) Surface Area Measurement .......................7 

3.1.3 Light Scattering Methods ......................................................................................9 

3.1.4 X-ray Diffraction (XRD) Analysis........................................................................9 

3.1.5 Neutron-Based Methods......................................................................................12 

3.2 Pressureless Sintering Results.......................................................................................13 

3.3 P2

C Results....................................................................................................................15
iv

4. Discussion 19

4.1 Nanopowder Characterization.......................................................................................19 

4.2 Sintering and Densification...........................................................................................20 

5. Summary and Conclusions 21

6. References 22

Appendix. Detailed Notes on the Collection, Reduction, and Analysis 

of the Neutron-Based Diffraction and Scattering Data 24

Report Documentation Page 31
v

List of Figures 

Figure 1. Electron microscopy of the Fe nanopowder. FESEM images show in 

(a) the individual nanometer-sized particles, and in (b) the dendritic agglomerates. 

Bright field TEM image of the particle substructure is shown in (c). .......................................8 

Figure 2. Type II BET surface area isotherm of the nanosized Fe. ................................................8 

Figure 3. Laser scattering results: (a) frequency distribution from SLS and 

(b) frequency distribution from DLS. ........................................................................................9 

Figure 4. XRD scan line profiles: (a) (200) peak profile for the LaB6 calibration 

standard, (b) (110) peak profile for the annealed !100-Mesh Fe powder, and 

(c) (110) peak profile for the Fe nanopowder.........................................................................10 

Figure 5. FWHM XRD peak widths for the four diffraction peaks scanned showing 

the results for all three powders...............................................................................................11 

Figure 6. ND of the (110) Fe reflection. Note the presence of the two Gaussian 

fits under the peak....................................................................................................................12 

Figure 7. SANS results confirming the existence of a bimodal distribution of crystallites in 

the nano Fe: (a) overall scattering data and (b) relative distributions of the particle sizes......13 

Figure 8. Pressureless sintered (pycnometer) density vs. temperature for the nanosized 

Fe powder.................................................................................................................................14 

Figure 9. Backscattered electron micrographs of the pressureless sintered samples: 

(a) and (b) 300 oC, (c) and (d) 500 oC, (e) and (f) 700 oC, and (g) and (h) 900 oC..................16 

Figure 10. Higher magnification SEM view of the coarsened particles and particle-particle 

neck formation at 300 oC, below the secondary recrystallization temperature........................17 

Figure 11. Grain size vs. inverse absolute temperature for the pressureless sintered samples.....17 

Figure 12. Low- and high-magnification SEM images of the P2

C samples are shown 

with 600 o

C, 1 min in (a) and (b); 600 oC, 15 min in (c) and (d); 800 oC, 5 min in (e) 

and (f); and 1000 oC, 1 min in (g) and (h). .............................................................................18 

Figure A-1. The ND of the (Materials Modification Inc.) nanosized Fe powder.........................27 

Figure A-2. The (110) peak in the diffraction pattern of the Fe nanopowder. The red and 

green solid lines are the two Gaussians used to fit the data.....................................................27 

Figure A-3. The net Gaussian variance of the two Bragg peaks plotted as functions of Q. 

The lines are the result of fits to the data using equations (A-6) and (A-8). The 

parameters resulting from the fit are given in the text.............................................................28 

Figure A-4. The percent of the total intensity from the narrow component contribution. ...........29 

Figure A-5. The SANS data and the respective fits resulting from the bimodal distribution.......29 

Figure A-6. The total particle size distribution resulting from a fit to the SANS data. 

The individual distributions of the two components are also shown.......................................30 
vi

List of Tables 

Table 1. Summary of methods and results......................................................................................7

Table 2. XRD analysis summary and results................................................................................11 

Table 3. Pressureless sintering results. .........................................................................................14 

Table 4. Corrected pressureless sintering results..........................................................................14 

Table 5. P2

C results.......................................................................................................................17 

Table A-1. Particle size of the nanosized Fe powder. ..................................................................29 
1 

1. Introduction 

1.1 Direct and Structural Applications of Nanosized Fe Precursor Powders 

Fe powders have many applications that may be enhanced with the use of nanosized precursors. 

These include the catalysis of carbon nanotubes with superior electronic properties [1] and 

magnetorheological fluids [2]. Nanosized feed material is also expected to lower the 

consolidation temperature and improve the structural properties of iron bodies prepared by 

powder metallurgical techniques [3]. For instance, it has been shown recently that dense bodies 

of nanostructured iron (Fe) exhibit high-strain-rate mechanical behavior that is unlike that 

observed in coarser-grained material [4]. A popular, and arguably the most successful, method 

for preparation of nanograined material entails the comminution of micrometer-sized powder to 

nanoscale by high-energy ball milling [5]. The nanograined iron (Fe) cited by Jia et al. [4] was 

prepared by ball milling and sinter forging [6]. However, despite the promise of such evidence, 

the fabrication of large, fully dense, nanostructured bodies remains an unrealized goal. 

A promising alternative to comminution is the direct synthesis of the nanopowder by microwave 

plasma synthesis (MPS). Fe bodies of 95% theoretical density (% TD) have been reported by 

moderate-pressure consolidation of precursor powder by plasma pressure compaction (P2

C) at 

850 °C [3]. However, while the mean particle size of the powder was measured to be ~500 nm, 

the grain size of the final part was on the order of 10 μm. The primary advantage of P2

C is the 

ability to apply large direct currents (DCs) and alternating currents (ACs) through a powder 

sample, causing the formation of a plasma arc, whereby the interior temperatures rise rapidly. 

The plasma causes the evolution of impurities, and a simultaneous application of moderate 

pressure densifies the sample material. Detailed descriptions of P2

C, also known as plasma 

activated sintering (PAS), have been detailed by Groza [7] and Jones et al. [8]. 

1.2 Characterization of Nanosized Precursor Powders 

In order to assess the contributions of nanopowders compared with larger-sized precursor 

powders, the particle morphology, size, and distribution must be known or evaluated. A number 

of books and articles address the topic of particle size measurement [9–11]. Only a few of these 

reports have dealt with the difficulties and limitations encountered in determining the particle 

size of nanopowders [12]. 

Many of the currently available particle size measurement techniques were designed for 

micrometer- and submicrometer-sized particles. While established off-the-shelf, canned 

methods offer ease of operation and minimal sample preparation, they still have limited 

applicability to nanopowders. For instance, field-emission scanning electron microscopy 

(FESEM) and other electron or optical microscopy techniques analyze small samples that may 

not be representative of the powder. In gas adsorption methods such as Brunauer, Emmett, and 
2 

Teller (BET) analysis, measurement can be done on a representative sample. However, 

adsorption of nitrogen or capillary condensation in interparticle voids can result in erroneous 

measurements [13]. 

Light scattering methods (e.g., dynamic light scattering [DLS] and static laser scattering [SLS]) 

have been developed for rapid measurement of particle size and size distributions of 

submicrometer-sized powders. Such methods measure either the spatial or the temporal 

variation of scattered light. In SLS, particles suspended in a fluid are exposed to a laser, and the 

amount of light scattered and the scattering angle are measured. Once calibrations of the 

response coefficients of the scattering light and detectors are known, the measured intensity 

distribution of the scattered light can be converted into a particle size distribution. It may be 

noted that it is difficult to collect light at high scatter angles associated with nanoparticles. As a 

result, the static method is limited to a practical size range of 20 nm–1000 μm. 

In contrast, DLS is used for finer particles with a range of 3 nm–6 μm. In this method, the 

Brownian motion of the dispersed particles in the suspending fluid causes a Doppler shift of the 

incident beam. Consequently, the scattered light has a different temporal distribution than that of 

the incoming light. Iterative fitting operations of known frequency distribution functions (of 

particles with various known sizes) are applied to that of the unknown frequency distribution to 

obtain the unknown particle size. 

Another challenge posed by nanosized powders is the preparation of dispersions. Such powders 

often do not wet or deagglomerate in the dispersing fluid medium. Pretreatment to deflocculate 

the nanopowder may require dispersants that need to be selected based on the surface chemistry 

of the powders. The powder suspension often requires a homogenization procedure (e.g., 

ultrasonics) to further break up aggregates. However, this process could also break up physically 

welded agglomerates, skewing the true particle size distribution of the sample. 

Crystallography or diffraction-based techniques (e.g., x-ray diffraction [XRD] and neutron 

diffraction [ND]) determine crystallite size. If the nanopowder is polycrystalline, the result may 

be different than the apparent particle size [14]. Small angle neutron scattering (SANS) or smallangle x-ray scattering (SAXS) [12] can theoretically provide information about aggregate size, 

particle morphology, size distribution, surface area, total pore volume, and the thickness of a 

surface layer. However, the size range is limited from 1 nm to ~300 μm (1 nm to 2 μm for 

SAXS). This means that a prescreening for larger particles should be done. If the particles are 

very uniform in both size and shape, it is difficult to determine the size and shape of the powders 

from SANS alone. 

1.3 Objectives 

The primary objective of this effort was to use a wide range of analytical techniques such as 

FESEM, TEM, BET, DLS, SLS, XRD, and SANS/ND in conjunction with one another to 

characterize the particle size, particle morphology, and/or distributions of the MPS nanosized Fe 
3 

powder. Additionally, it was hoped that during the use of these methods, one would emerge as 

an effective screening tool for the evaluation of nanopowders. In other words, one that requires 

the least amount of preparation and analysis, but is most efficient in conveying an overall 

description of the sample. 

To further examine the suitability of such nanopowder as raw material for the preparation of 

nanograined bodies, compacts of MPS Fe nanopowder were pressureless sintered in an H2

atmosphere at various temperatures. The primary objective was to correlate the precursor 

powder characteristics and properties with sintering behavior and final microstructure. 

Additionally, evidence of enhanced low-temperature sintering, if any, was of interest. However, 

results showed that without the application of pressure, there was difficulty in attaining full 

density samples. Subsequently, several additional attempts were made to further densify the Fe 

using P2C. Density of the sintered compacts was determined by mercury pycnometry, and the 

microstructure examined by scanning electron microscopy (SEM). The sintering results in 

context of the precursor characteristics, with implications for the production of bulk samples, 

are discussed. 

2. Experimental Procedures 

A representative sample of Fe nanopowder, derived from microwave plasma synthesis [3], was 

obtained from Materials Modification Inc. (MMI), Fairfax, VA. The microwave synthesis of the 

nanopowder entailed the controlled decomposition of Fe(CO)5 (iron pentacarbonyl) at 

700 °C. To prevent pyrophoric oxidation, freshly synthesized powder was quenched in liquid 

nitrogen (LN2). 

2.1 Particle-Size Characterization Procedures 

2.1.1 Electron Microscopy Techniques 

FESEM was performed on a Hitachi S4700 F-SEM (Nissei Sangyo America, Gaithersburg, 

MD). Several attempts to obtain optimum imaging conditions resulted in selection of an electron 

energy of 5 kV. Lower kilovolt settings did not have the required resolution; higher kilovolt 

settings tended to penetrate into the particles too deeply, resulting in the loss of surface detail. 

Both lower SE(L) and upper SE(U) secondary electron detectors were used with a working 

distance ranging from 11.6 to 3 mm. The sample was prepared by sprinkling Fe onto a colloidal 

carbon-covered aluminum stub. The loose, excess powder was blown off with an air gun. 

The TEM used was a JEOL JEM-3010 (Japan Electron Optics Laboratory, Peabody, MA). 

Samples were prepared by placing a dash of powder in 2 mL of ethanol and sonicating it for 

~2 min, then a carbon-coated, standard 200-mesh (65 μm) copper grid was dipped into the 
4 

suspension. The powder sample was examined in bright-field imaging mode at an accelerating 

voltage of 150 kV. 

2.1.2 Brunauer, Emmett, and Teller (BET) Surface Area Measurement 

Nitrogen gas adsorption analysis was performed on an ASAP-2010 Accelerated Surface Area 

and Porosimetry System (Micromeritics, Norcross, GA). Samples were outgassed overnight at 

200 °C under vacuum. Two separate samples were subjected to six-point BET surface area 

analysis. Additionally, a full adsorption isotherm was collected from one of the samples. 

The adsorption data offered no evidence of microporosity in the powder. Therefore, calculation 

of the equivalent area diameter yielded a meaningful value for particle size. Equation (1) depicts 

that SBET, the surface area from the BET measurement, is inversely related to the particle radius: 

 r = 3 [ρ(SBET)]-1, (1) 

where r is the sphere radius and ρ is the density of Fe. 

2.1.3 Light Scattering Methods 

SLS was performed on an LA-910 (Horiba, Irvine, CA) using a flow cell. DLS was performed 

on a Horiba LB-500 (Horiba, Irvine, CA) in a stationary quartz cell. Preliminary attempts to 

disperse Fe in H2O with a microfluidizer (Microfluidics, Newton, MA) or titanium microtip 

ultrasonic probe for 10–60 s were unsuccessful. In a more effective dispersion method, 

0.1 weight-percent of sodium hexametaphosphate [(Na(PO3))6] was dissolved in deionized water. 

Approximately 0.023 g of nanosized Fe powder was dispersed in 20 mL of the base solution. 

The powder dispersion was sonicated for 10 min at 80 W, boiled for 5 min, and sonicated again 

for 10 min to break up the powder sample. Two SLS samples and one DLS sample were taken 

from this suspension. 

2.1.4 X-ray Diffraction (XRD) Analysis 

XRD analysis for determination of particle size was performed using Cu-Kα radiation on a fixed 

optics, APD-1700 Automated Powder Diffractometer System (Philips, Natick, MA). Generator 

settings were 45 kV and 40 mA. For the Fe powder, the four most intense peaks, [(110), 44.67°], 

[(200), 65.02°], [(211), 82.33°] and [(220), 98.94°], were scanned. Preliminary scans were made 

to determine the dwell time, step size, and scan range such that the net peak height under each 

peak was at least 10,000 counts. All peaks were scanned with a step size of 0.010° and 2Θ range 

of at least ±3°. The instrumental broadening was determined by using two commercially 

available powders: !100-Mesh Fe (Alfa-Aesar, Ward Hill, MA) and an LaB6 XRD-broadening 

standard (National Institute of Standards and Technology, Gaithersburg, MD). The !100-Mesh 

Fe powder was annealed at 450 °C for 2 hr in H2 to remove any residual stress. Specifically, 

characteristic peaks of LaB6 near the four nanosized Fe peaks were scanned: [(200), 43.52°], 

[(220), 63.22°], [(320), 83.85°], and [(410), 99.64°]. Because of the narrowness of the LaB6

peaks, a finer step size of 0.005° was used. For the !100-Mesh Fe powder, the same 
5 

aforementioned peaks as those for the nanosized Fe powder were scanned. It was assumed that 

the measured peak broadening of the LaB6 peaks was attributable to instrumental broadening 

only. Despite the annealing treatment, the !100-Mesh Fe showed a greater broadening than 

LaB6. Consequently, its use was discontinued. After subtracting the instrumental broadening 

contribution from the nanosized Fe peaks, Warren’s or Scherrer’s equation [14, 15] was used to 

determine the particle size. 

2.1.5 Neutron-Based Methods 

Both SANS and neutron diffraction (ND) were performed at the Center for Neutron Research 

(CNR) at the National Institute of Standards and Technology (NIST) (Gaithersburg, MD) on the 

30-m SANS and BT1 powder diffractometer instruments, respectively. 

Briefly, ND was performed on a multidetector instrument where the neutrons are scattered by 

single crystal monochromators [Cu (311) and Si (531) with wavelengths, λND, of 0.15405 and 

0.15904 nm, respectively] at a fixed scattering angle. The neutron detector was composed of an 

array of 32 detectors separated by 5° in scattering angle. The widths of the collimators before 

and after the monochromator, and before the detectors are 7, 20, and 7 min of arc, respectively. 

The measurement was taken with 0.05° steps in 2Θ. Measurements of particle size, local strain, 

and lattice strain distributions were obtained from the intensity, IND, vs. QND [4πsin(Θ)/λ] data. 

A more detailed description of the instrument, data collection, and analysis is provided in the 

Appendix. 

For SANS, a mechanical velocity selector rendered the “cold neutrons” from the source 

monochromatic and a monochromator provided wavelength resolution from 10% to 20% and 

wavelengths, λSANS, of 0.5–1.2 nm. The incident direction was defined by two circular pinholes. 

The scattered neutron direction was defined by a 5-mm spatial resolution, two-dimensional (2-D) 

detector located perpendicular to the incident direction. The angular range available (and 

therefore the reciprocal wave vector range, Q) was determined by the position of the detector 

with respect to the sample. The detector, placed at 1–15 m from the sample, could be laterally 

displaced by 25 cm. The distance between the incident pinholes was adjusted to match the 

resolution determined by the sample detector configuration. The sample was kept under vacuum 

so that the scattering length density of spheres was taken to be that of Fe (8.02 H 10!4

 m!2). The 

data were first analyzed using a uniform density sphere model. The data were then reanalyzed 

by a model of bimodal particle size distribution, as guided by the diffraction results. Each mode 

was represented by a Schulz distribution [14] whose parameters were a scale factor, ISANS, a 

mean particle size, r, and a polydispersitivity, P. In the fitting process only ISANS and P were 

allowed to vary. The value of the r used was obtained from the ND results. 

2.2 Pressureless Sintering Experiments 

The bulk tap density was determined by first weighing the as-received Fe powder, then dividing 

the weight by the filled container volume. 
6 

Green compacts were made from the Fe nanopowder at a range of compaction pressures under 

ambient laboratory conditions in a uniaxial die. Only about 1 g of powder was used for each 

compact. During handling, the Fe nanopowder was very fluid, readily flowed, forming a dust 

cloud. Extreme care had to be used to prevent the pyrophoric oxidation of the powder. In many 

instances the powder rapidly oxidized into red, ferric oxide, Fe2O3. The density of the green 

compact was measured by dividing its mass by the calculated volume, based on measurements of 

the compact thickness and diameter. After a trial and error determination of the optimum 

pressure to achieve the highest compact density without an oxidation reaction, several compacts 

were pressed at 55 MPa (8000 psi) for the pressureless sintering studies. 

Pressureless sintering was performed at 300°, 500°, 700°, or 900 °C for 30 min under a 10 L/min 

flow of dry H2. The sintered pellets were halved; one half was used to determine pellet density. 

The other half was mounted and polished for SEM examination; the polished surfaces were 

etched with 2% Nital to reveal the microstructure. Initial density measurements of the sintered 

samples, using Archimedes’ principle, were discontinued after the discovery of very large errors, 

introduced by the smallness of the specimen volumes. The sample densities were then 

remeasured using mercury pycnometry in an Autopore IV Model-9510 Mercury Porosimeter 

Analyzer (Micromeritics, Norcross, GA). 

2.3 P2

C Experiments 

Similarly sized green compacts were loaded in a 1.27-cm inner diameter graphite die assembly. 

A two-color optical pyrometer was focused on the center of the die to measure the external 

temperature as the power level through was raised. It is likely that the interior temperatures were 

higher. In each P2C run, DC current was ramped at 100 A/min, such that the target temperature 

was reached in a few minutes. At the same time, a constant pressure of 127 MPa was applied to 

the samples. More detailed descriptions of the apparatus and its operation can be found in Klotz 

[16]. Higher pressures caused the dies to crack or shatter. When a predetermined time elapsed 

at the selected temperature, the current was ramped down to zero A and the pressure released. 

After cooling, the P2

C samples were extracted and analyzed in the same manner as those 

obtained from pressureless sintering. 

3. Results 

This research program was divided into three parts: characterization, pressureless sintering, and 

plasma pressure compaction. Each component of the work will be described separately. 

3.1 Powder Characterization 

Results of the characterization effort are summarized in Table 1. In turn, each method and the 

corresponding data are now described in detail. 
7 

Table 1. Summary of methods and results.

Method Particle Size 

(nm) 

Comments 

FESEM/TEM 50–80; 300–1000 Bimodal 

XRD 20 — 

SANS/ND 24 and 64 Bimodal, Gaussian 

BET 60 Type II Isotherm 

SLS 500–8000 Bimodal 

DLS 70 Gaussian 

3.1.1 Electron Microscopy Techniques 

Figures 1a and 1b depict FESEM images taken at 5 kV. The particle size appears to be 

50–80 nm. It was difficult to locate any isolated Fe particles. Instead, the Fe grains appear to be 

fused to each other into chain-like tentacles that form dendritic agglomerates with sizes of 

0.3–1 μm. It is also worthwhile to note that the high contact angles seen in the particle-particle 

necks reflect late-stage particle-particle sintering. Despite the high transparency of the particle 

chains (Figure 1c), TEM examination of the powder was nevertheless able to discern a finer 

substructure within the apparently coarser 50–80 nm grains. The approximate size, or lower 

limit, of the subgrains was at least 20 nm. 

3.1.2 Brunauer, Emmett, and Teller (BET) Surface Area Measurement 

Figure 2 shows the BET plots for the two samples. Both are linear with least-squares correlation 

coefficients >0.999. The linear BET plot corresponds to a BDTT Type II or Type IV isotherm. 

Microporous samples frequently exhibit a BDTT Type I isotherm also known as the Langmuir 

type [17]. Type I isotherms do not produce a linear fit in a BET plot. Furthermore, the value for 

the BET c constant was ~42 for each sample. If the powders were microporous, the value of this 

parameter would be higher (~200 or greater), reflecting the higher apparent adsorption energy in 

a microporous solid [17]. The linear BET plots and the values of the c constant both indicate 

that the powders are free of fine pores and that the effective diameter calculated from SBET is 

indicative of particle size. 

Results of the BET surface area analysis were 13870 and 15080 m2

/kg. Using a hard-sphere 

model and a 7870 kg/m3

 density for Fe, the equivalent radius was found to be 28 and 25 nm, 

respectively, which corresponds to a diameter range of 50–60 nm. This particle size appears 

consistent with the FESEM results. 
8 

Figure 1. Electron microscopy of the Fe nanopowder. FESEM images show in (a) the individual nanometersized particles, and in (b) the dendritic agglomerates. Bright field TEM image of the particle 

substructure is shown in (c). 

Figure 2. Type II BET surface area isotherm of the nanosized Fe. 

0.00

0.05

0.10

0.15

0.20

0.00 0.10 0.20 0.30 0.40 0.50

Relative Pressure [P/Po]

1/[VA*(Po/P-1)]

(a) (b)

(c) 
9 

3.1.3 Light Scattering Methods 

Frequency distributions of particle size for SLS and DLS are shown in Figures 3a and 3b. From 

the various SLS measurements (Figure 3a), Fe particles appear bimodal with particles sizes that 

are much larger than the size determined from the previous methods. Results of five consecutive 

DLS measurements spread over a 20-min time span indicated a single particle size distribution of 

a mean particle size of 70 ± 7 nm. 

Figure 3. Laser scattering results: (a) frequency distribution from SLS and (b) 

frequency distribution from DLS. 

3.1.4 X-ray Diffraction (XRD) Analysis 

Typical XRD line-broadening profiles are shown with the LaB6 (200) peak in Figure 4a, the 

!100-Mesh, annealed Fe (110) peak in Figure 4b, and the corresponding nanosized Fe (110) Fe 

0

1

2

3

4

5

6

7

8

9

0.01 0.10 1.00 10.00 100.00 1000.00

Diameter [μm]

Frequency [%]

H2O; LA900

(NaPO3)6; LA910;#1

(NaPO3)6; LA910;#2

(a) 

0

2

4

6

8

10

12

14

16

18

20

22

0.01 0.10 1.00

Diameter [μm]

Frequency [%]

Run: #1

Run: #2

Run: #3

Run: #4

Run: #5

(b) 
10

Figure 4. XRD scan line profiles: (a) (200) peak profile for the LaB6

calibration standard, (b) (110) peak profile for the annealed 

!100-Mesh Fe powder, and (c) (110) peak profile for the 

Fe nanopowder. 

10

100

1000

10000

100000

42 43 44 45 46 47 48

2Θ [deg]

Intensity [Counts]

(a) 

10

100

1000

10000

100000

42 43 44 45 46 47 48

2Θ [deg]

Intensity [Counts]

(b) 

10

100

1000

10000

100000

42 43 44 45 46 47 48

2Θ [deg]

Intensity [Counts]

(c) 
11

peak in Figure 4c. The as-measured full width at half maximum (FWHM) for the three powders 

are graphed in Figure 5. Note that the !100-Mesh, annealed Fe and LaB6 gave slightly largersized minimum peak widths. The remainder of the analysis, summarized in Table 2, entailed 

subtracting the instrumental FWHM from each of the four nanosized-Fe peaks. Using the 

Scherrer equation [15] yielded an average crystallite size of ~20 ± 3 nm. In contrast, Warren’s 

method [14] yielded a slightly smaller size. Aside from the TEM results, these values were 

considerably less than those obtained by any of the other methods. 

Figure 5. FWHM XRD peak widths for the four diffraction peaks scanned showing the 

results for all three powders. 

Table 2. XRD analysis summary and results. 

Peak 2Θ

(deg) 

2Θ

(rad) 

F 

(deg) 

F 

(rad) 

F –F(LaB6) 

(rad) 

[ ( ) ] 2

6

2 F − F LaB

(rad) 

Scherrer 

Size 

(nm) 

Warren 

Size 

(nm) 

LaB6

(200) 43.53 0.760 0.089 0.002 NA NA NA NA 

(220) 63.24 1.104 0.085 0.001 NA NA NA NA 

(320) 83.86 1.464 0.094 0.002 NA NA NA NA 

(410) 99.65 1.739 0.099 0.002 NA NA NA NA 

Nanosized Fe 

(110) 44.75 0.781 0.393 0.007 0.005 0.007 28 22 

(200) 65.09 1.136 0.499 0.009 0.007 0.009 23 19 

(211) 82.38 1.438 0.585 0.010 0.009 0.010 21 18 

(220) 98.98 1.728 0.716 0.012 0.011 0.012 20 17 

Note: NA = not applicable. 

0.0000

0.1000

0.2000

0.3000

0.4000

0.5000

0.6000

0.7000

0.8000

20 40 60 80 100

2Θ [deg]

2

Θ FWHM [deg]

 MMI nano-Fe

 LaB6 Standard

-100Mesh Fe Standard; Annealed
12

3.1.5 Neutron-Based Methods 

In the measured ND pattern obtained from the Fe sample with the Cu (311) monochromator, 

each peak consists of two components: a narrow and broad peak at the same positions as shown 

in Figure 6 for the (110) Fe reflection. From ND, the narrow peak particle diameters were found 

to be 63 nm with a root mean square (RMS) strain of 0.028; the broad peak particle diameter was 

found to be 24 nm with an RMS strain of 0.165. The larger particles constituted 37% of the 

sample and comparatively did not have residual strain. 

Figure 6. ND of the (110) Fe reflection. Note the presence of the two Gaussian fits 

under the peak 

Figure 7a shows the SANS results of the fit to a bimodal distribution model. In the figure, the 

data are shown as open circles and the fits as solid lines. The two distributions are found by the 

information represented by the broad concave curvature at Q~0.001 nm and a convex curvature 

at Q~0.006 nm. The particle size distributions, P, for each of these components are shown in 

Figure 7b. The total distribution is included as well. The scale factors indicate that the large 

component is 47% of the total rather than the ratio resulting from the ND. However, this is not 

considered important enough to assert that the two data sets (ND and SANS) are not consistent. 

It is clear from the results previously mentioned that the sample consists of at least a bimodal 

distribution of particle sizes that are not very narrow. The range of Q that was obtained here 

does not exclude the existence of even larger particles. The capabilities of each of the 

measurements are also clearly demonstrated. The diffraction can detect mean particle size and 

the effects of strain, whereas the SANS is sensitive to the particle size distribution. Each on its 

own is very useful but in combination provides a quite complete description of the sample. A 

detailed description of how the ND and SANS data were obtained is given in the Appendix. 

.

0

200

400

600

800

1000

1200

1400

1600

1800

28 29 30 31 32 33 34

Q [1/nm]

Intensity [Counts]
13

Figure 7. SANS results confirming the existence of a bimodal distribution 

of crystallites in the nano Fe: (a) overall scattering data and (b) 

relative distributions of the particle sizes. 

3.2 Pressureless Sintering Results 

Pressureless densification results are displayed in Figure 8 and summarized in Table 3. 

Apparently, full densification by pressureless sintering could not be achieved. Relatively high 

sample densities of ~90% TD were determined by the Archimedes method. However, upon 

remeasurement with mercury pycnometry, it was found that the densities were erroneous. The 

source of error was traced to a rather large error in the measurement of the sample volume; the 

correct densities (obtained with mercury pycnometry) are listed in Table 4. 

The bulk density of the powder was 0.13 gcm!3

, or 1.6% TD of Fe. Green density of the 

compacts was ~40% TD. Furthermore, the powder did not compact normally, but proved to be 

“springy.” That is, the compacted powder exhibited sufficient springback to be ejected from the 

die during filling. Furthermore, the powder proved to be pyrophoric, sometimes combusting 

during die filling or compaction. 

1E-02

1E-01

1E+00

1E+01

1E+02

1E+03

1E+04

1E+05

0.01 0.10 1.00 10.00

Q [1/nm]

Intensity [1/cm]

Narrow Component

Broad Component

Total

(a) 

0.000

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

0 10 20 30 40 50 60 70 80

r [nm]

P(r)

Total

12-nm Particles

32-nm Particles

Narrow Component

Broad Component

Total

(b) 
14

Figure 8. Pressureless sintered (pycnometer) density vs. temperature for the nanosized 

Fe powder. 

Table 3. Pressureless sintering results.

Temperature 

(°C)

Archimedes Density 

(gcm-3)

Density 

(% TD)

300 7.25 92.1 

500 7.00 88.9 

700 6.50 82.6 

900 7.15 90.9 

Table 4. Corrected pressureless sintering results. 

Temperature 

(°C)

Pycnometry Density 

(gcm-3)

Density 

(% TD)

Grain Size 

(μm)

300 3.52 44.7 0.05 

500 4.52 57.4 1 

700 5.87 74.6 5 

900 5.87 74.6 50 

Regardless of the sintering temperature, all four of the consolidated pellets delaminated, though, 

the appearance of the compacts improved with increasing temperatures. At the lower two 

temperatures, the pellets were heavily delaminated, black, and rough. The extent of 

delamination seemed to decrease with increasing temperature. At the higher two temperatures 

(700° and 900 °C), the pellets had only one or two lateral delaminations, but were blistered, with 

a silvery exterior surface. Interior surfaces were also shiny. 

3.00

3.50

4.00

4.50

5.00

5.50

6.00

200 400 600 800 1000

Temperature [oC]

Density [g/cm3

]
15

Shown in Figure 9, low- and high-magnification SEMs of the polished and etched surfaces of the 

pellet cross sections, depict the evolution of the sintered microstructure. At 300 °C, there is very 

little or no coarsening of the individual Fe nanoparticle grains (Figures 9a and 9b). Note that the 

dendritic aggregate structure of the Fe clusters is still mostly present. However, in some isolated 

areas, there is extreme coarsening. Figure 10 shows an enlarged view of the onset of sintering 

with bridging and necking between adjacent particles as the dendrite arms collapse. As shown in 

Figures 9c and 9d, at 500 °C, a banded structure appears, with alternating low- and high-density 

regions that most likely are associated with the macroscopic delamination of the sample. At this 

temperature, the onset of the dendritic nanoparticle coarsening is also evident. In contrast, as 

depicted in Figures 9e and 9f, once the temperature exceeds 700 °C, any remaining evidence of 

the initial dendritic structure and morphology is lost. Instead, gross rearrangement, with 

extensive secondary recrystallization and growth, occurs. Nevertheless, submicrometer voids or 

closed pores remain at the triple points between the recrystallized grains. Impurities, such as 

oxides, would be expected to segregate at the grain boundaries, and thus cause high contrast. 

The absence of such contrast at the grain boundaries indicates this powder is relatively free of 

contamination. Finally, in Figures 9g and 9h, heterogeneous grain growth at 900 °C of the 

recrystallized Fe grains dominates the pellet's microstructure. (There are a few polishing 

scratches in some of the softer grains.) Submicrometer voids also coalesce into 10-μm void 

agglomerates or void bands. 

Despite a lack of densification, as was shown in the Table 4, the grain size of the consolidated Fe 

pellets dramatically increases with temperature. Shown in Figure 11, a semi-logarithmic graph 

of the average grain size vs. inverse sintering temperature appears fairly linear. Assuming an 

Arrhenius behavior for atom diffusion and associated grain growth, a least squares fit to the data 

yields an activation energy of 62 ± 6 kJ/mol. This is only one-fourth of that for the self-diffusion 

of Fe [18]. 

3.3 P2

C Results 

Unlike the pressureless sintered samples, the P2

C samples were not delaminated. Results of the 

experiments are listed in Table 5. As apparent, the variations of time or temperature have a 

relatively minor effect on the overall density of the samples. Under moderate pressure, the 

initially isotropic agglomerates form highly oriented (transverse to the compaction axis), 

filamentary structures of alternating dense and porous strata (Figure 12). Low- and highmagnification images at 600 °C (Figures 12a and 12b) show that the interior of these filaments 

consists of dendritic aggregates that sintered into a coarsened, yet porous spongy structure. Note 

the formation of anomalously more dense regions. A comparison of the two samples at 600 °C, 

shows that at low temperatures the effect of time is negligible on the microstructure (Figures 12c 

and 12d). At higher temperatures, similar image pairs, especially at 800° and 1000 °C 

(Figures 12e–12h), show that these filamentary layers grow denser. The interior of these bands 
16

Figure 9. Backscattered electron micrographs of the pressureless sintered samples: (a) and (b) 300 oC, (c) and 

(d) 500 o

C, (e) and (f) 700 oC, and (g) and (h) 900 oC. 

(a) (b)

(c) (d)

(e) (f)

(g) (h)
17

Figure 10. Higher magnification SEM view of the 

coarsened particles and particle-particle neck 

formation at 300 oC, below the secondary 

recrystallization temperature. 

Figure 11. Grain size vs. inverse absolute temperature for the pressureless sintered samples. 

Table 5. P2

C results.

Temperature 

(°C)

Time 

(min)

Pycnometry Density 

(gcm!3

)

Density 

(% TD)

600 1 6.20 78.8 

600 15 6.25 79.4 

800 5 5.90 75.0 

1000 1 6.06 77.0 

0.01

0.10

1.00

10.00

100.00

0.0008 0.001 0.0012 0.0014 0.0016 0.0018

1/T [1/K]

Grain Size [

μm]
18

Figure 12. Low- and high-magnification SEM images of the P2C samples are shown with 600 oC, 1 min in 

(a) and (b); 600 o

C, 15 min in (c) and (d); 800 oC, 5 min in (e) and (f); and 1000 oC, 1 min in (g) 

and (h). 

(b)

(c) (d)

(e) (f)

(g) (h)

(a)
19

consists of 2- to 3-μm grains, interspersed with isolated submicrometer-sized porosity. Particles 

in less dense regions coarsen as well, resulting in a random distribution of isolated fine, 

nanosized droplets. 

4. Discussion 

4.1 Nanopowder Characterization 

During the collection and evaluation of the particle size data it was quickly recognized that not 

all of the instruments provide first-hand or raw data output. Though statistically unsatisfactory, 

the electron optics-based observations instantaneously revealed the particle morphology and size 

information. In contrast, the BET and laser scattering techniques operate with factory-installed 

algorithms and geometrical models that require post-measurement interpretation related to the 

validity of the model. Lastly, the crystallographic methods required much more extensive userbased data fitting and analysis. Consequently, they do not readily lend themselves to the rapid 

assimilation and interpretation of the raw data. 

Table 1 reported the summary of the mean particle sizes determined from all of the 

characterization methods. From the summary, it appears that the apparent, morphologically 

distinct Fe particle size is on the order of nanometers, ranging from roughly 25 to 80 nm. This is 

generally consistent with previous analyses of the powders [3]. However, the particles have a 

distinct fine structure and appear to cluster into much larger, micrometer-sized dendritic 

agglomerates. The evidence from FESEM of particle-particle necks along the dendritic arms, 

indicative of near-late-stage sintering, was left unexplained in the previous article [3]. 

This is quite understandable, notwithstanding the size of the particles, the enhanced sinteribility 

of nanopowders, and the excess heat available during the microwave sintering process. 

However, the available heat is limited because the internal crystal size within the particles 

appears to be smaller than their external dimensions. In other words, there is little or no 

indication of annealing within the grains. 

As expected, FESEM yielded an adequate first-hand physical description of “typical” particle 

aggregates, though only semi-quantitative. The information gained with FESEM was more 

useful than that obtained with TEM because of the loss of surface detail in the latter. It may be 

noted that TEM also revealed a fine structure within the particles. Only, BET, the surface 

adsorption-based technique, identified the actual or functional particle size with a reasonable 

statistical variation. However, because BET is insensitive to macroscopic morphological 

arrangement of the particles, it failed to indicate the nature of agglomeration. 
20

Neither SLS nor DLS could correctly identify the size or nature of the agglomeration. In part, 

this was attributed to the intrinsic properties of the Fe nanopowder agglomerates, the limitations 

in preparing the powder suspension, and the inherent inability to interpret scattering data from an 

open agglomerate structure by the analysis software. Further discrepancies between the SLS and 

DLS results may also be likely accountable by the differences in sampling methods. In SLS, the 

sample dispersion is circulated during the measurement; whereas, in DLS, it is not. That is, 

while in the SLS the agglomerates remain suspended, in the stationary cell of the DLS, the 

heavier particles will settle out. As a result, DLS would not detect any of the larger 

agglomerates. Further experiments such as sedigraphy may be needed to alleviate the disparity. 

Both XRD and SANS/ND measurements, supported by TEM observations, resulted in a smaller 

particle size. This is most likely a measure of the crystal size within the particles. However, the 

ND/SANS with its greater penetration depth and resolution was able to identify at least two 

modes of crystallites within the sample. 

4.2 Sintering and Densification 

This study has shown that MPS does result in a powder with features on a nanometer scale. 

Aside from the large aggregates observed in FESEM, the BET surface area, and XRD 

measurements are consistent with a nanosized powder, as is the pyrophoricity of the powder 

during handling. However, despite little or no interior grain substructure as revealed in Figure 1 

and supported by XRD analysis, the individual particles are sintered into stiff dendritic 

agglomerates. 

The most striking properties of this MPS powder vis-á-vis a conventional powder are the 

unexpectedly low bulk green density, poor compaction, and sintering behavior. The high 

specific surface area of the primary particles cannot explain either the markedly low bulk density 

or the excessive springback. Both, however, can be understood as being due to the presence of 

the dendritic structures. The inability to compact the particles beyond a green density of 

40% TD can also be attributed to these aggregates. 

At temperatures below 500 °C, pressureless sintering cannot effectively remove the residual 

porosity from the as-pressed green compacts. Above 500 °C, nanoparticle grain growth is 

unavoidable, which is quickly superceded by secondary recrystallization, and, as a result, the 

initial precursor morphology is completely lost. This temperature is consistent with the 

conventional, secondary recrystallization temperature of 450 °C, TR, for pure Fe [18]. 

It has been postulated that, due to increased interfacial and surface free energies in nanosized 

particles, sintering would be more enhanced at lower temperatures [3, 19]. The well-bonded 

particles within the dendritic arms seem to support this argument. The small value of QS seems 

to support this argument; however, the observed coarsening is more consistent with the behavior 

of larger, conventionally sized powders [20]. That is, no enhanced solid-state sintering, 

rearrangement, or concomitant densification took place at low temperatures. Instead, gain in 
21

densification is seen only above TR. This fact and the samples’ structural heterogeneity 

(i.e., banding) are also a likely result of the initially dendritic agglomerates. It is believed that 

below TR, as dictated by the lower QS, adjacent particles begin to rapidly neck, sinter, coalesce to 

form aggregate clusters. However, the spatial extent, bimodal particle size, and relative stiffness 

of the dendritic arms cause these clusters to become too widely spaced, thereby prevent any 

further possible collapse of the partly coarsened structure. Only at higher temperatures, with 

higher driving forces in place, the agglomerates collapse and an increase in density is realized. 

Thus, even if sufficient pressure would be brought to bear on the green body as to break up the 

agglomerates, it expected that a greater porosity would remain than in compacts made from 

conventional powders. Such porosity in the green compact is the most likely source of the large 

isolated pore clusters seen in Figures 8g and 8h. The use of the P2C provides a little gain in 

densification, but with an added extrinsic, current-induced structural asymmetry that is imparted 

to the sintered body. 

These results are inconsistent with those of Kalyanaraman et al. [3], where the starting material 

was prepared in an identical process, but consolidation was by the P2

C technique. In that study, 

heating a similar Fe starting material to temperatures around 900 °C at 65 MPa resulted in near 

full densification with grain sizes of >10 μm and a well-dispersed, uniform, 2- to 3-μm-sized 

residual porosity. However, the micrographs and density, reported in that reference, do not 

exhibit the heterogeneity seen here. The reason for this discrepancy is not immediately evident, 

but might be elucidated by further study. 

Nevertheless, as has been said many times of powder characterization and consolidation, “What 

is generally required is not the size of the particles, but the value of some property of the 

particles that is size dependent. The final criteria are that the method shall measure the 

appropriate property of the particles [9].” For this Fe nanopowder, the extent and stiffness of the 

dendritic agglomeration was this property. Indeed, the thick interparticle necks within the 

agglomerates seen in Figure 1b are testimony to the enhanced sinterability of nanopowders. 

Unfortunately, the sinterability occurs at an inconvenient time during powder synthesis, whereby 

further processibility is compromised. It is suspected that modifications of MPS are required to 

sidestep this difficulty. 

5. Summary and Conclusions 

The size distribution of MPS Fe nanopowder was evaluated by several analytical methods. The 

nanopowder was found to consist of dendritic agglomerates that were difficult to disperse. 

Individual grains within the dendrite structures were dense and spherical. Further scrutiny and 

analysis indicated the existence of a finer subgrain structure with a bimodal size distribution 

within the particles. The estimated average particle size was 60–80 nm with 20-nm subgrains, 

while the overall agglomerate size was ~ 0.3–1 μm. 
22

Consistent with many examples in larger-size powders, particle size determination in 

nanopowders was found to depend on the method used. Only FESEM and SLS provided 

information on the extent and morphology of agglomerates. Data from other methods were 

either complementary (BET and DLS) or provided detail (XRD and ND/SANS) beyond the 

required size and distribution information. Nevertheless, it is suggested that care be taken in 

making any of the measurements because, when viewed independently, the results may be 

misleading or erroneous. 

The bimodal dendritic structure of the Fe nanopowder hindered the ability to consolidate into 

full-density structures. The initial size and morphology of the Fe was lost at a threshold 

temperature of 500 °C. An examination of the pressureless sintering behavior of this powder 

indicated that the dendritic structure of the nanopowder hinders solid-state sintering into a fully 

dense body. No enhanced sintering below the recrystallization temperature was observed. Using 

P2C (high-DC current and low-pressure), the structure is stratified with a suppression of grain 

growth, however, without an increase in density. The implications are twofold: on one hand, if 

the nanostructure is to be retained, sintering must be conducted below this temperature. On the 

other hand, direct sintering of nanopowders to dense bodies will require more careful control of 

the MPS process to avoid formation of low-density agglomerates. While it may be possible to 

introduce further processing steps, for now, conventional pressing and pressureless sintering of 

such agglomerates to full-density bodies is extremely difficult. 

6. References 

1. Peigney, A., C. Laurent, O. Dumortier, and A. Rousset. “Carbon Nanotubes-Fe-Alumina 

Nanocomposites: Part I: Influence of the Fe Content on the Synthesis of Powders.” Journal 

of the European Ceramic Society, vol. 18, pp. 1995–2004, 1998. 

2. Phule, P. P. “Synthesis of Novel Magnetorheological Fluids.” Materials Research Society 

Bulletin, vol. 23, no. 8, pp. 23–25, 1998. 

3. Kalyanaraman, R., S. Yoo, M. S. Krupashnkara, T. S. Sudarshan, and R. J. Dowding. 

“Synthesis and Consolidation of Iron Nanopowders.” Nanostructured Materials, vol. 10, no.

8, pp. 1379–1392, 1998. 

4. Jia, D., K. T. Ramesh, and E. Ma. “Failure Mode and Dynamic Behavior of Nanophase Iron 

Under Compression.” Scripta Materialia, vol. 42, no. 1, pp. 73–78, 2000. 

5. Koch, C. C. “Synthesis of Nanostructured Materials by Mechanical Milling: Problems and 

Opportunities.” Nanostructured Materials, vol. 9, no. 1, pp. 13–22, 1997. 
23

6. He, L., L. F. Allard, K. Breder, and E. Ma. “Nanophase Fe Alloys Consolidated to Full 

Density From Mechanically Milled Powders.” Journal of Materials Research, vol. 15, no.

4, pp. 904–912, 2000. 

7. Groza, J. R. “Field-Activated Sintering.” ASM Handbook; Powder Metallurgy, ASM, 

Materials Park, OH, vol. 7, pp. 583–589, 1998. 

8. Jones, G., J. R. Groza, K. Yamazaki, and K. Shoda. “Plasma Activated Sintering (PAS) of 

Tungsten Powders.” Materials and Manufacturing Processes, vol. 9, no. 6, pp. 1105–1114, 

1994. 

9. Allen, T. Particle Size Measurement. New York: Chapman and Hall, 1997. 

10. Webb, P. A., and C. Orr. Analytical Methods in Fine Particle Technology. Norcross, GA: 

Micromeritics Instrument Corporation, 1997. 

11. Geiger, G. “Selecting the Right Particle Size Analyzer.” American Ceramic Society 

Bulletin, vol. 75, no. 7, pp. 44–48, 1996. 

12. Singhal, A., G. Skandan, and R. J. Dowding. “A New Bench-Top Characterization Tool for 

Nanoparticulate Systems.” Scripta Materialia, vol. 44, pp. 2203–2207, 2001. 

13. Jiqiao, L., and H. Baiyun. “Particle Size Characterization of Ultrafine Tungsten Powder.” 

International Journal of Refractory Metals & Hard Materials, vol. 19, pp. 89–99, 2001. 

14. Klug, H. P., and L. E. Alexander. X-ray Diffraction Procedures. New York, NY: John 

Wiley & Sons, pp. 655–656 and 660–661, 1978. 

15. Cullity, B. D. Elements of X-ray Diffraction. Reading, MA: Addison Wesley, pp. 284, 

1978. 

16. Klotz, B. R. “Characterization of Boron Carbide Consolidated by the Plasma Pressure 

Compaction (P2

C) Method.” Master’s thesis, Worcester Polytechnic Institute, 

Worcester, MA, 2001. 

17. Gregg, S. J., and K. S. W. Sing. Adsorption, Surface Area, and Porosity, 2nd Edition. 

New York, NY: Academic Press, 1982. 

18. Askeland, D. R. The Science and Engineering of Materials. Boston, MA: PWS-Kent 

Publishing Company, p. 123, 1989. 

19. Groza, J. R., and R. J. Dowding. "Nanoparticulate Materials Densification." Nanostructured 

Materials, vol. 7, no. 7, pp. 749–768, 1996. 

20. Goetzel, C. G. Treatise on Powder Metallurgy: Vol. 1. New York, NY: Interscience 

Publishers, Inc., p. 515, 1949. 
24

Appendix. Detailed Notes on the Collection, Reduction, and Analysis of the 

Neutron-Based Diffraction and Scattering Data 

A.1 Neutron Diffraction 

The neutron diffraction measurements were performed with the BT1 multi-detector instrument 

located on the National Institute of Standards and Technology (NIST) Center for Neutron 

Research (CNR) reactor. The neutrons from the reactor are scattered by a single-crystal 

monochromator at a fixed scattering angle. This, now monochromatic, beam illuminates the 

sample. The neutrons scattered by the sample are detected by an array of 32 well-shielded 

detectors separated from each other by ~5° in scattering angle. This feature allows efficient 

collection of data. The instrument provides a choice of three monochromators. This results in 

different resolutions as function of scattering angle. The two monochromators used in the 

measurement were Cu (311) producing a wavelength of 1.5405 Å and Si (531) producing a 

wavelength of 1.5904 Å. The collimators before and after the monochromator and before the 

detectors were 7, 20, and 7 min of arc, respectively. The instrumental resolution was well 

measured as a function of scattering angle. The data were collected in 0.05° increments of 

scattering angle 2Θ. The Bragg law, relating the scattering angle at which a reflection occurs 

2Θhkl, the characteristic length of that reflection dhkl, and the wavelength λ is given by 

λ = 2 dhklsinΘhkl, (A-1) 

where (hkl) are the Miller indices of the reflection. It is more convenient to describe the 

independent variable as Q, the scattering vector, instead of 2Θ. Q is defined as 

 Q = 4πsinΘ/λ . (A-2) 

The Bragg law then is 

 Q = 2π/d. (A-3) 

The measured peak shape is described by a Gaussian function 

( ) ( ) ( ( ) ) 2 2 IND Q =[IND0 / σ 2π ]exp− Q−Q0 / 2σ , (A-4) 

where Qo is the peak position, and σ

2

 is the variance. This variance in a diffraction pattern is 

determined by several factors. The most important of these are the instrumental resolution, and 

the size and strain of the particles. These two latter determine the intrinsic peak variance σI

2

. The 

resolution determines σR

2

. The measured variance σ

2

 is related to these two by quadrature 

σ

2 

 = σR

2

 + σI

2

. (A-5) 
25

The Q dependence of an intrinsic variance due to crystalline size has been developed by 

Scherrer1 and that for lattice distortions by Wilson.2 These treatments are well summarized by 

Klug and Alexander.3 For particles of size S, the line is broadened such that 

σS

2

(Q) = π

2

K2/2ln(2)S2, (A-6) 

where K is a shape parameter (whose value is close to 1 [which is the value used in the 

calculations]). In the case of lattice distortions, given a local strain, e, the peak will be shifted, in 

Q, from its unstrained value of Qo by an amount δQ such that 

 e = Q/Qo. (A-7) 

The variance of the lattice strain distribution e2

 will cause a broadening of the peak 

corresponding to 

σe

2

(Q) = e2/Qo

2

. (A-8) 

Thus, the intrinsic variance is the sum of these two variances. In the present measurements, the 

values of S, e, and e2

 are obtained from the data. 

A.2 Small Angle Neutron Scattering (SANS)

In all scattering measurements in which coherent interference effects are measured, the length 

scale, d, of inhomogeneities that produce the interference effects is given by 

 Q = 2π/d. (A-3) 

In order to attain sensitivity to a preferred size scale (i.e., 1 nm–300 μm), either large 

wavelengths or small angle measurement capability or both are needed. The 30-m SANS 

instruments available at CNR meets this requirement. A mechanical velocity selector renders the 

“cold neutrons” from the reactor source monochromatic. The monochromator provides a 

wavelength resolution of 10%–20%, and wavelengths from 5 to 12 Å. The small angle 

measurement capability is provided by the ability to define the direction of the incident and 

scattered neutrons. The incident direction is defined by two circular pinholes, which can be 

separated from each other by 3–15 m. The scattered neutron direction is defined by a twodimensional detector whose sensitive area is arranged perpendicular to the incident direction. 

The spatial resolution of the detector is 5 mm. The angular and therefore Q range available in a 

measurement is determined by the position of the detector with respect to the sample. The 

detector can be placed at distances from 1–15 m from the sample and can be displaced 25 cm 

 1 Scherrer, P. Gott. Nachr., Vol. 2, p. 98, 1918. 

2 Wilson, A. J. C. “On Variance as a Measure of Line Broadening in Diffractometry .2. Mistakes and Stain.” Proceedings of 

the Physical Society of London, vol. 81, no. 519, p. 41, 1963. 

3

 Klug, H. P., and L. E. Alexander. X-ray Diffraction Procedures, New York, NY: John Wiley & Sons, pp. 655, 656, 660, 

661, 1978. 
26

laterally. The distance between the incident pinholes is adjusted to match the resolution 

determined by the sample-detector configuration. 

The measured scattering pattern is a result of interference effects from the inhomogeneities in 

neutron scattering power present in the sample. In the present case, because the sample consists 

of Fe particles, these inhomogeneities result from the different scattering of neutrons by the Fe 

and the vacuum. It is for this reason that the size of the Fe particles is accessible by this 

technique. Formulating a model of the structures, of which the sample consists of, or is 

suspected to consist of, usually carries out the analysis of the scattering data. A calculation of 

the scattering resulting from this model is compared to the measured scattering, and the relevant 

parameters are adjusted until satisfactory agreement is achieved. Unless the particles are very 

uniform in both size and shape, the data are not capable of establishing unique information 

concerning the size and shape. The information is hidden in the integration. In this case, it is as 

useful to choose one shape as another. So, we choose the simplest (from the viewpoint of the 

calculation). The simplest is that of spheres of uniform density. In that case, the scattering 

cross-section per unit volume is given as 

 ISANS(Q) = 9(∆ρ)

2

ΦV[sin(Qr) - Qrcos(Qr)]2[Qr]-3, (A-9) 

where r is the sphere radius, V is the volume of the sphere, Φ is the volume fraction of the 

spheres in the sample, and ∆ρ is the scattering length density of the spheres. Actually, ∆ρ is the 

difference in the scattering length of the two media. However, because one of the media is the 

vacuum for which ρ = 0, ∆ρ is that for Fe (∆ρ = 8.02x10-6 Å-2). If the sample consists of a 

distribution of spheres of different radii, then equation (A-9) is to be folded with the function 

describing this distribution. 

A.3 Data Reduction and Analysis 

As explained (equation [A-9]), a successful, rather an easy and straightforward, measure of the 

particle size with SANS will result if (1) the measured intensity saturates as Q tends to zero and 

(2) that several interference fringes are observed. The first indicates that the maximum particle 

size has been detected and the second that a narrow particle size distribution exists. A 

measurement of the scattering from the sample, with the 30-m SANS spectrometer at the NIST 

reactor, yielded ambiguous results. It was clear that the sample did not consist of spheres with a 

narrow distribution of radii. It was not clear what the true distribution was. Subsequently a 

high-resolution neutron diffraction measurement revealed the source of the difficulty and a 

characterization of the powder. 

The measured diffraction pattern obtained from the Fe sample with the Cu (311) monochromator 

is presented in Figure A-1. All of the diffraction peaks correspond to the body-centered cubic Fe 

structure. The pattern obtained with the Si monochromator is not shown but has been used to 

confirm the results presented below. Each of the measured peaks consists of two components: a 

narrow large peak and a substantially broadened peak centered at approximately the same 
27

Figure A-1. The ND of the (Materials Modification Inc.) nanosized Fe powder. 

position. Figure A-2 presents the (110) reflection. In that figure is also plotted the two 

components of which the feature consists. Both peaks are broader than the resolution. It is clear 

from Figure A-1 that the sample consists of at least two substantially different size particles. 

Figure A-2. The (110) peak in the diffraction pattern of the Fe nanopowder. The red and 

green solid lines are the two Gaussians used to fit the data. 

This is the reason the SANS measurements were ambiguous. All the peaks obtained with both 

monchromators were treated in this manner, i.e., they were fit with two Gaussian functions. This 

analysis results in values of the integrated intensity, measured width and position of each of the 

two components as a function of Q. The integrated intensity reflects quantitatively the amount of 

0

200

400

600

800

1000

1200

1400

1600

1800

10 20 30 40 50 60 70 80

Q [1/nm]

Intensity [Counts]

0

200

400

600

800

1000

1200

1400

1600

1800

28 29 30 31 32 33 34

Q [1/nm]

Intensity [Counts]
28

each component; the width is treated as indicated in order to obtain the size and strain variance 

conditions of that component and the position to obtain the residual strain. These results are 

presented in Figures A-3 and A-4. Figure A-3 presents the intrinsic standard deviation σI of the 

two components as a function of their position in Q. The solid lines are the result of the fit to the 

data with equations (A-6) and (A-8) as previously discussed. The parameters that produced the 

fits are presented in Table A-1. Here and in other values given, the numbers in parentheses 

correspond to one standard deviation in the least significant figure. With the exception of the 

value for the smallest Q, the data of Figure A-4 yields consistent values for the volume fraction 

of the large particles (narrow Q component) in the sample. Within one standard deviation, there 

is no residual strain in either component. The large particles constitute 37(5)% of the sample. 

Figure A-3. The net Gaussian variance of the two Bragg peaks plotted as functions of Q. 

The lines are the result of fits to the data using equations (A-6) and (A-8). The 

parameters resulting from the fit are given in the text. 

With this knowledge, we now return to the analysis of the SANS data. Because of the length 

scale to which the SANS data are sensitive (hundreds of Angstroms), only particle size and not 

information on strains can be obtained. We formulate a model that consists of a bimodal 

distribution of particle sizes as guided by the diffraction results. Each component is represented 

by a Schulz distribution4 whose parameters are scale factor, ISANS, mean particle size, r, and 

polydispersitivity, P. The model was then used to fit the SANS data. In the fitting process, only 

the scale factor and polydispersitivity were allowed to vary. The two r values used were those 

produced by the diffraction results. The result of the fit is shown in Figure A-5. In this figure, 

the data is shown as open circles, the model fit as a black line, and the contribution from each of 

 4

 Schulz, G. V. Zeitschrift für physikalische Chemie. Vol. 43, p. 25, 1935. 

0.00

0.10

0.20

0.30

0.40

0.50

0.60

30 40 50 60 70 80

Qo [1/nm]

σI [1/nm]

Narrow Component

Broad Component
29

Figure A-4. The percent of the total intensity from the narrow component contribution. 

Table A-1. Particle size of the nanosized Fe powder. 

Component Particle Size 

(Å)

Narrow 630(17) 

Broad 243(31) 

Figure A-5. The SANS data and the respective fits resulting from the bimodal distribution. 

30

35

40

45

50

55

60

30 40 50 60 70 80

Qo [1/nm]

Frequency [%]

Narrow Component

1E-02

1E-01

1E+00

1E+01

1E+02

1E+03

1E+04

1E+05

0.01 0.10 1.00 10.00

Q [1/nm]

Intensity [1/cm]

Narrow Component

Broad Component

Total
30

the distributions by blue (small particles) and red (large particles). The shape of the data may be 

seen as innocuous; this is of course the fault of the sample whose properties we intend to 

discover. There are two features that contain the essential information. These are a broad 

concave curvature at Q ~0.01 Å and a convex curvature at Q ~0.06 Å. The requirement that 

these be reproduced results in the determination of the polydispersitivity. In any case, the 

resulting particle size distribution is shown in Figure A-6 in which the distribution for each 

component is also shown. The scale factors indicate that the large component is ~47% of the 

total rather than the ratio resulting from the diffraction data. We do not consider this to be 

debilitating in so far as being able to assert that the two data sets are consistent and that the 

model is faithful. 

Figure A-6. The total particle size distribution resulting from a fit to the SANS data. The 

individual distributions of the two components are also shown. 

0.000

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

0 10 20 30 40 50 60 70 80

r [nm]

P(r)

Total

12-nm Particles

32-nm Particles

Narrow Component

Broad Component

Total
31

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4. TITLE AND SUBTITLE 

The Sintering and Densification Behavior of an Iron Nanopowder Characterized by 

Comparative Methods 

5c. PROGRAM ELEMENT NUMBER 

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1L622105AH84-622195A 

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6. AUTHOR(S) 

Laszlo J. Kecskes, Robert H. Woodman, Samuel F. Trevino, 

Bradley R. Klotz, and S. Gary Hirsch

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7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 

U.S. Army Research Laboratory 

Weapons and Materials Research Directorate 

Aberdeen Proving Ground, MD 21005-5069 

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ARL-TR-2909 

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Approved for public release; distribution is unlimited. 

13. SUPPLEMENTARY NOTES 

14. ABSTRACT 

Fe nanopowder, derived from microwave plasma synthesis (Materials Modifications Inc., Fairfax, VA), was obtained and 

characterized for particle size and size distribution. The methods used included static light scattering (SLS) and dynamic light 

scattering (DLS), surface area and size by Brunauer, Emmett, and Teller (BET) analysis, small angle neutron scattering 

(SANS), neutron diffraction (ND), x-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and 

transmission electron microscopy (TEM). Based on these methods, it was concluded that the Fe powder was composed of 

nanosized particles, but in micrometer-sized aggregates. DLS indicated a mean agglomerate size with a single mode 

distribution of 70 ± 6 nm. In contrast, SLS revealed a wide bimodal distribution ranging from 0.5 to 20 μm. The mean particle 

sizes that resulted from BET and XRD analyses were 60 and 20 nm, respectively. SANS, in combination with ND, determined 

that the powder had a bimodal distribution of mean size 24 and 64 nm. TEM and FESEM confirmed that the powder is 

composed of 50- to 80-nm particles that is found in large, dendritic particle agglomerates which are on the order of 

micrometers. 

Subsequent to characterization, the sinterability and densification behavior of the Fe nanopowder was examined by further 

experimentation wherein powder compacts were pressureless sintered under hydrogen. Results showed that without pressure, 

the powder could not be sintered to full density, however, the densification of the Fe powder strongly depended on the 

temperature. There was little or no coarsening below 500 °C. In contrast, significant grain growth was observed in the samples 

above that temperature. In further experiments, using a plasma pressure compaction (P2

C) apparatus, the powder was 

consolidated to final densities near 80% of the theoretical full density. The observed limitations in the densification and 

associated porous microstructures of the pressureless-sintered and P2

C-densified compacts were rationalized in terms of the 

b h i f h bl 15. SUBJECT TERMS d lik d d i i h h h f h i d i il

nanopowder, characterization, microscopy, surface area measurement, laser scattering, Fe, density, microstructure, 

consolidation, sintering, plasma-activated sintering, pressureless sintering, grain growth 

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