NANO LETTERS
Nanoexplosion Synthesis of Multimetal Oxide Ceramic Nanopowders
2005 Vol. 5, No. 12 2598-2604
Oleg Vasylkiv* and Yoshio Sakka National Institute for Materials Science, 1-1, Namiki, Tsukuba, Ibaraki 305-0044, Japan Received October 14, 2005
ABSTRACT Herein we demonstrate a unique processing technique for engineering multicomponent ceramic nanopowders with precise morphologies by “nanoblast” calcination/deagglomeration. Multiple “nanoexplosions” of C3H6N6O6 nanoparticles embedded in preliminary engineered nanoreactors break apart the agglomerates because of the highly energetic impacts of the blast waves. Also, the solid-solubility of one component into the other is enhanced by the extremely high local temperature generated during the nanoexplosions. We applied this technique to produce nanosized agglomerate-free ceria−gadolinia solid solution powder with an average aggregate size of 42 nm. The described method opens the door to the synthesis of a wide range of multimetal oxide ceramic and metal−ceramic composite nanopowders, with precise stoichiometries and uniform morphologies.
Introduction. Single- and multicomponent ceramic and metal-ceramic composite nanosize powders enable the improvement of quality and the differentiation of product characteristics at scales currently unachievable by commercially available micrometer-, and submicrometer-sized powders.1-4 Nanostructured powders have been synthesized previously by so-called “wet chemical” methods from aqueous or nonaqueous solutions.1-27 A typical prior art synthesis procedure involves several sequential steps: (A) preparation of single- or multicomponent starting solutions of metal salts (usually aqueous solutions); (B) preparation of aqueous solutions of different reductants; (C) reductive decomposition of the starting single- or multicomponent solution to obtain the precipitate, colloidal suspension, or gel of the desired end-product phase or intermediate multicomponent product; (D) separation of the end or intermediate product; (E) deagglomeration of the synthesized (precipitated) powder prior to calcination; and (F) synthesis of the end-product powder via calcinations, that is, thermal decomposition of the intermediate products.6-32 Because of the high surface energy and chemical activity of nanoparticles,1-14 aggregation and subsequent or simultaneous formation of hard agglomeration are the main problems encountered in the preparation of such powders.1-33 Despite these difficulties, several aqueous-solution-based precipitation techniques have been employed to synthesize nanosized ceramic powders. These include the use of ammonia,16-18 oxalic acid,19-21 urea,22,23 ammonium carbonate,24 and hexamethylenetetramine26,27 precipitants. Prepara* Corresponding author. E-mail:
[email protected]. 10.1021/nl052045+ CCC: $30.25 Published on Web 10/27/2005
© 2005 American Chemical Society
tion techniques have included approaches based on sol-gel processing,10-30 reverse-micellar nanoreactors,13-15 hydrothermal synthesis,22,23,26 sonochemically and/or microwaveassisted decomposition of various aqueous (or nonaqueous) precursor solutions,22-27 salt-assisted aerosol decomposition, and combustion synthesis.28-33 However, processing multicomponent powders has proven to be extremely challenging and often results in a nonhomogeneous multiphase compound with poor morphology.14-33,38-40 Typically nucleation, growth, aggregation, and subsequent hard agglomeration of the first component occur within seconds under very mild conditions. The nucleation of the second component often starts at a higher temperature and requires more time and a different pH. The final product of such “coprecipitation” is a nonhomogeneous composite powder, which is nanocrystalline in nature but usually consists of micrometer-sized hard agglomerates with very poor morphologies.26 To achieve a solid solution, such a bior multicomponent composite powder would require an excessive calcination temperature and potentially prolonged hold times.16-29 Hence, it is not surprising that these powders also require high temperatures and long holding times for densification, with no realistic prospect of achieving a finegrained structure.18-33 The present study was aimed at establishing a nontraditional synthetic method of preparing nanosized, agglomeratefree, cerium-gadolinium oxide ceramic powders with precise morphologies and chemical compositions. During the past decade, CeO2-based materials have been investigated intensively as catalysts, structural and electronic promoters of heterogeneous catalytic reactions, and oxide-ion conducting solid electrolytes in electrochemical cells.34-39
Table 1. Characteristics of Cyclotrimethylene Trinitramine (C3H6N6O6) density detonation rate pressure at front of blast wave
1.8 g/cm 8350 m/sec 33.8 GPa
Table 2. Size Distribution of Aggregate/Agglomerate Powders: First, for Nanoreactors; Next, after Multiple Nanoexplosive Deagglomeration/Synthesis of Solid Solution; and, Finally, after Subsequent Nonisothermal Calcination up to 450 °C
generated heat burned exploded gas product volume molecule size
2.307 kcal/g 1.3 kcal/g 0.9 liter/g 0.48 nm
In general, any explosive material is chemically (or otherwise) energetically unstable. Explosions usually involve a rapid and violent oxidation reaction with a sudden release of mechanical and/or chemical energy in a violent manner, accompanied by the generation of high temperature and the release of extremely hot gases. It causes pressure waves in the local medium in which it occurs.25-27 Cyclotrimethylene trinitramine or royal demolition explosive (RDX) is a highly explosive material in its pure synthesized state. It is a colorless crystalline solid with a density of 1.8 g/cm3. It is a heterocycle and is thus ring-shaped with the following structural formula: hexahydro-1,3,5-trinitro-1,3,5-triazine or (CH2-N-NO2)3.20-27 At room temperature, it is extremely stable. Thermal decomposition of RDX starts at about 170 °C, melts at 204 °C, and explodes at 233 °C (Table 1). The method we describe here has no analogues and promises to become a principal player in advanced nanotechnologies. It overcomes the main drawbacks, that is, agglomeration and compositional inhomogeneity,1,2 of the numerous chemical methods of producing multicomponent nanopowders, even those of combustion synthesis,28-33 the method most closely related to ours. Experimental Section. For the example of our multiple nanoexplosive synthesis method, Ce(NO3)3‚6H2O, Gd(NO3)3‚ 6H2O, and GdCl3‚6H2O (Wako Pure Chemicals, Japan) were weighed and dissolved in deionized water at a total concentration of 0.1 M. Hexamethylenetetramine (C6H12N4) (Wako Pure Chemicals, Japan) was used as a precipitation agent and as a source for cyclotrimethylene trinitramine (RDX) synthesis to produce three-component nanoreactors (intermediate complex agglomerates). Hexamethylenetetramine was dissolved in demineralized water at the concentration of 1 M per 1 - x M of Ce(NO3)3‚6H2O and xM Gd(NO3)3‚ 6H2O or GdCl3‚6H2O. The total volume of the hexamethylenetetramine aqueous solution was 150 mL. The process of the “nanoreactor” (i.e., complex threecomponent intermediate agglomerates) preparation includes two simultaneous major steps: First, the coprecipitation of CeO2 with Gd2O3 oxides was achieved by mixing the aqueous solution of their respective nitrates with the hexamethylenetetramine aqueous solution, which was conducted by stirring at 1800 rpm at 70 °C. An initial pH of 8.45 was measured for the hexamethylenetetramine aqueous solution at 22 °C. Mixing the hexamethylenetetramine solution with the aqueous solution of cerium and gadolinium nitrates lowered the pH of the stock solution to 7.1. Gadolinium nitrate began to react with the hexamethylenetetramine at Nano Lett., Vol. 5, No. 12, 2005
as-synthesized ceria-gadolinia nanoreactors synthesis method (nm) example 1 example 2
37-630 18-380
after multiple explosive after calcination treatment up to 450 °C (nm) (nm) 27-70 18-52
33-70 20-64
46 °C at pH 6.4, resulting in the appearance of a milkywhite precipitate. The synthesis of cerium oxide started at 22 °C; however, at temperatures from 22-50 °C, a long stirring time (100-110 h) was necessary to complete its precipitation. Fast precipitation of ceria was achieved in 5 h by stirring at 70 °C. However, the total precipitation of gadolinia with simultaneous agglomeration of fine primary particles occurred at a lower temperature within only 100600 s. Consequently, the nucleation, growth, and agglomeration of ceria that followed occurred on the surface of the gadolinia agglomerates. Second, the direct mixing of hexamethylenetetramine with nitric acid, after decomposition of the metal nitrates dissolved into the aqueous stock solution, causes the formation and simultaneous saturation (nanoscale impregnation) of cyclotrimethylene trinitramine (C3H6N6O6) into the matrix agglomerates of the cerium and gadolinium intermediate compound, that is, final formation of nanoreactors. Results and Discussion. Table 2 shows the size distribution of the primary engineered nanoreactors (complex intermediate matrix agglomerates of gadolinia-doped ceria particles impregnated with highly energetic RDX particles) as analyzed by dynamic light scattering (DLS). The intermediate agglomerates synthesized by this first example of our method ranged in size from 37-630 nm. This wide range was attributed to the nonsimultaneous precipitation of the gadolinium and cerium compounds. A diagram of the processing pathway is shown in Figure 1. Primary synthesized three-component agglomerates were preheated to 100 °C in an alumina container. A critical factor behind our methodology is preventing the undesirable ignition of the RDX at approximately 180 °C by ultrarapid heating of the three-component complex intermediate agglomerates to the thermal detonation temperature (∼230 °C) of RDX. This thermal detonation temperature, that is, the temperature at which spontaneous multiple rupture of the N-NO2 bonds occurs, depends strongly on the heating rate and may vary from 220 to 360 °C. Similar to other explosives, the explosive initiation of RDX begins in nanosized regions, so-called “hot spots”, which are capable of accumulating the mechanical energy of an impact/shock wave and transferring it into chemical energy, thus starting the blast reaction. Cleavage of the N-NO2 chemical bonds requires less energy for isolated clusters than for molecules located in the bulk of the solid. Extremely rapid detonation (10-8 sec/gram) forms gaseous products with a temperature 2599
Figure 1. Diagram of the processing pathway.
of 2000-5000 °C compressed into a volume equaling the initial volume of each explosive RDX particle. Multiple nanoexplosions occur within the volume of each multicomponent intermediate complex. The instantaneous power of each explosion (i.e., the expansion of compressed gases) is 500 MW/gram.42,43 The impact of the blast waves leads to the internal crushing, defragmentation, and plastic deformation of the surrounding matter. The rapid evolution of a large volume of gaseous products during combustion dissipates the heat of the process and limits the temperature increase, thus reducing the possibility of premature local partial sintering among the primary particles. This gas evolution also helps by somewhat limiting interparticle contact, resulting in a less-agglomerated product. Multiexplosive deagglomeration of the nanopowder occurred because of the highly energetic impact of the blast waves, whereas the short-term high temperatures generated during the explosions enhanced the solid-solubility of one component into the other. A nanosized bimetal (cerium and gadolinium) oxide composite and, very soon afterward, a solid solution of the dopant oxide (gadolinia) into the matrix oxide (ceria) were synthesized. Utilizing the methodology described here, we produced cerium-gadolinium oxide (CGO) powder with an average 2600
Figure 2. TG/DTA analysis of the thermal explosion of RDX (heating rate ) 10 °C/min).
primary crystallite size of 11 nm and an aggregate size distribution of 33-70 nm (see Table 2), with uniform morphology and precise stoichiometry. We analyzed the thermal decomposition of both RDX itself (Figure 2) and of RDX distributed within the complex intermediate agglomerates of cerium-gadolinium compounds, produced under both rapid (Figure 3) and slow Nano Lett., Vol. 5, No. 12, 2005
Figure 3. TG/DTA analysis of the thermal explosion of RDX during the multiexplosive synthesis of the ceria-gadolinia solid solution (heating rate ) 10 °C/min).
Figure 4. TG/DTA analysis of the thermal decomposition of the three-component intermediate complex agglomerates under subcritical conditions (heating rate ) 5 °C/min).
(Figure 4) heating conditions. This enabled us to identify the explosive decomposition conditions of the RDX impregnated into the matrix compound of the nanoreactors and confirm them by comparison to the explosive decomposition conditions of RDX itself. The results for thermogravimetric (TG) and differential thermal analysis (DTA) of the thermal explosion of RDX itself under supercritical conditions (heating rate ) 10 °C/min) are shown in Figure 2. Three stages can be identified in the thermal decomposition. At the heating rate of 10 °C/min, the ignition of the RDX started at around 180 °C. The ignition temperature depended directly on the heating rate: the higher the heating rate used, the smaller the gap between the theoretical (∼180 °C) and the experimentally measured ignition temperature. The melting point of RDX is 202-205 °C. A slight endothermic peak can be detected irrespective of the extremely short melting time (2 s for the RDX particles filled into the container with no preliminary densification) prior to the explosion. The thermal detonation and explosion of the RDX occurred at ∼230 °C. Just at the beginning of the ignition reaction, the TG analysis showed a significant increase in the mass of the sample (approximately 10.8 wt % at the heating rate of exactly 10 °C/min.). This phenomenon is explained by the capturing of external oxygen from neighboring space by the reacting species. The ignition transformed into thermal Nano Lett., Vol. 5, No. 12, 2005
Figure 5. TEM micrograph of ceria-gadolinia composite agglomerates (a) produced under subcritical conditions (i.e., at a subcritical rate with calcination to 450 °C) and (b) micrograph of cerium-gadolinium oxide (CGO) nanoaggregates produced by applying the nanoexplosive synthesis method.
Figure 6. XRD patterns of (a) a ceria-gadolinia composite nanopowder produced under subcritical conditions (seen in Figure 5) and (b) a ceria-gadolinia solid solution synthesized by the multiple nanoexplosion method.
detonation instantly (within nanoseconds), that is, the RDX exploded. Even the temperature as detected by the thermocouple of the TG/DTA system increased momentarily by about 100 °C. Figure 3 shows TG-DTA analysis results for the thermal explosion of RDX during the thermal decomposition of the multicomponent intermediate complex agglomerates under supercritical conditions with a heating rate of 10 °C/min. The reaction results observed here are not, however, limited to this particular rate. The strong exothermal peak detected by differential thermal analysis at similar temperature and time as for RDX itself (seen in Figure 2) confirms the occurrence of multiple nanoexplosions of RDX particles distributed uniformly within the volume of the matrix agglomerates (nanoreactors). TG-DTA analysis results for the thermal decomposition of the three-component intermediate complex under subcritical conditions, with a heating rate of 5 °C/min, are shown in Figure 4. There is no significant exothermal peak in the DTA curve. We can assume that the subcritical time and temperature conditions prevented multiple thermal detonations in the hot spots, and, consequently, only slow ignition of the RDX particles occurred. Under such subcritical 2601
Figure 7. DSC analysis of the thermal decomposition of the RDX itself (red line), cerium-gadolinium intermediate agglomerates impregnated with RDX (dark blue line), three-component intermediate complex agglomerates washed to eliminate the residual RDX (black line), and RDX-free cerium-gadolinium intermediate agglomerates.
conditions, this method may be considered as a combustion route. Figure 5a shows a TEM (JEM-2000-FX, JEOL, Tokyo, Japan; operated at 200 kV) micrograph of the ceriagadolinia composite agglomerates produced by “ignition” (i.e., under subcritical temperature/time conditions of calcination to 450 °C). Dense agglomerates of very fine primary crystallites of gadolinia appear in the TEM image as large, black, nonuniform particles. Nanoarea TEM-EDX spectra of these composite nanoaggregates synthesized by the combustion route (under subcritical conditions) show the existence of both cerium and gadolinium inside the aggregates. Nanoaggregates of a ceria-gadolinia solid solution synthesized by the multiple nanoexplosive method are shown in the TEM micrograph in Figure 5b. Figure 6 shows the XRD patterns of (a) the ceria-gadolinia composite nanopowder produced by the combustion route under subcritical conditions and (b) the ceria-gadolinia solid solution produced by multiple nanoexplosive synthesis. In b, the weak reflections associated with Ce2O3 and Gd2O3 have disappeared.Moreover,theXRDpeaksattributedtotheGd20Ce80O1.95 composite are relatively broad, indicating that the powder is composed of very fine crystallites. Subsequent nonisothermal calcination up to 350-450 °C may lead to removal of the products of the explosive decomposition of RDX. Thus, such treatment acts to preserve both the compositional homogeneity and morphology of the powder. In the second example of our multiple nanoexplosive synthesis method, GdCl3‚6H2O was used to replace gadolinium nitrate. Prior to ceria synthesis, the precipitation of gadolinium oxide from gadolinium chloride aqueous solution was performed by spraying hexamethylenetetramine into the solution under very rapid stirring conditions (2000 rpm) at a reduced temperature of 3 °C. The resulting powder consisted of agglomerates of primary particles 3-4 nm in size. A stock suspension of these gadolinia particles in cerium 2602
Figure 8. SEM micrograph of ceria-gadolinia composite agglomerates blasted by means of in-situ explosion of single threecomponent intermediate agglomerate under the beam of a scanning microscope.
nitrate aqueous solution and hexamethylenetetramine was heated to 70-90 °C while stirring at 2000 rpm for 6 h. The primary particles of gadolinia assembled into agglomerates covered with as-synthesized ceria. The size distribution of these engineered nanoreactors (gadolinia-doped ceria agglomerates impregnated with simultaneously synthesized particles of RDX) composing the composite powder synthesized by this second example of nanoexplosive method was 18-380 nm, as indicated in Table 2. After drying, the powder was heated in an alumina container (similar to the first example). Next, these nanoreactors were heated ultrarapidly to the thermal detonation temperature (∼230 °C) of RDX. Thermal detonation and subsequent multiple nanoexplosions of RDX particles occurred in each threecomponent agglomerate. The nanopowder deagglomerated because of the highly energetic impact of the blast waves, whereas the short-term high temperature generated in the local areas surrounding each exploded particle of RDX caused an increased solubility of gadolinia into the ceria matrix component. This second example of the nanoexplosive method produced a solid solution powder of gadolinium oxide in cerium oxide with an average primary crystallite size of 11 nm, an aggregate size distribution of 18-52 nm just after multiple nanoexplosion synthesis, and 20-64 nm after following calcination up to 450 °C, with uniform morphology and precise stoichiometry. DSC (differential scanning calorimetry) analysis of the thermal decomposition of the RDX itself, cerium-gadolinium intermediate agglomerates as-impregnated with RDX, three-component intermediate complex agglomerates washed to eliminate the residual RDX, and RDX-free ceriumgadolinium intermediate agglomerates are shown in Figure 7. The heat effect of the explosive decomposition of the three-component intermediate agglomerates (dark blue line) is just slightly weaker in comparison to the heat effect of RDX itself (red line). Ceria-gadolinia composite agglomerates blasted by means of in-situ explosion of a single threecomponent intermediate agglomerate under the beam of a scanning electron microscope (SEM) are shown in the SEM Nano Lett., Vol. 5, No. 12, 2005
Figure 9. TEM-EDX mapping of CGO nanoexplosively synthesized aggregates.
micrograph of Figure 8. The area spanned by the hightemperature and blast impact violently exceeded the initial volume of the single complex precursor agglomerate. We attributed this phenomenon to the existence of a large quantity of RDX particles not contained inside the agglomerates (in suspension or on the exterior of the agglomerates). Multiple explosions of the exterior particles are responsible for the additional agglomeration of the neighboring particles, diminishing the effect of deagglomeration due to explosion of embedded RDX particles. To prevent local sintering of the neighboring ultrafine crystallites, we had to minimize the heat impact of the RDX explosive decomposition. We applied a washing technique to remove the excessive amount of RDX particles. The heat effect of explosive decomposition of cerium-gadolinium precursor agglomerates impregnated with RDX particles (black line) is a factor of 10 times weaker in comparison to the heat effect of RDX itself (red line) and only 20% in average of the heat effect of unwashed powder. The powder produced by this explosive decomposition of well-washed complex intermediate agglomerates is shown in Figure 5b. We can assume that mostly RDX particles embedded into cerium-gadolinium intermediate agglomerates remained after washing. This circumstance allowed production of a CGO nanosized powder with excellent nanosized morphology. The TEM-EDX mapping image of CGO nanoexplosively synthesized aggregates (see Figure 9) shows the uniform distribution of gadolinium within the ceria matrix. These results are extremely promising, and further research is currently underway to develop and optimize this methodology for other multimetal (more than two) oxide nanopowders, as well as metal-oxide composite nanopowders. Acknowledgment. This study was performed through Special Coordination Funds for Promoting Science and Technology from the Ministry of Education, Culture, Sports, Nano Lett., Vol. 5, No. 12, 2005
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NL052045+
Nano Lett., Vol. 5, No. 12, 2005
