Anna Swiderska-Sroda

Synthesis of Metal-Ceramic Nanocomposites by High-Pressure Infiltration

A technique of preparation of novel nanocrystalline composites by infiltration of a liquid metal under high-pressure is presented. A porous nanocrystalline body is obtained by compacting nanosized powder of a high-hardness ceramic material under the pressure of 2-8 GPa. The molten metal penetrates into the open pores and crystallizes there upon cooling. As a result the second nano-phase is obtained. Practical aspects of the technique and some properties of the composites are discussed.

SiC – Zn Nanocomposites Obtained Using the High – Pressure Infiltration Technique

Two-nanophase SiC-Zn composites were synthesized under pressure up to 8 GPa at up to 1000oC using an high-pressure infiltration method. The advantage of this technique is that in a single, continuous process the ceramic nanopowder is compressed to form the matrix with nanopores; the nanopores are filled with a liquid secondary phase, (here Zn), which crystallizes as nano-scale grains. The key limitation is that the pores in the infiltrated preform have to stay open during the entire process. For this reason only powders of very hard ceramic materials can be used as a matrix. Two types of SiC nanopowders with average crystallite size of 10 nm and 60 nm and average particle size of 30 nm and 100 nm, respectively were used. The measurements of porosity of the green compacts prepared from these powders, pressed at 2.5 GPa and 8 GPa at room temperature, indicated that open porosity was maintained. The nanocomposites obtained show a “nano-nano” type microstructure with a uniform mixture of SiC and Zn phases. The volume fraction of Zn is 20 % independent of the process conditions and initial powder morphology. The process parameters and powder granularity influenced the crystal size of the secondary phase. The average grain size of Zn varied from 20 to 85 nm and was smaller in the composites obtained with the finer matrix, under higher pressure and at lower temperature. The microhardness HV02 of SiCZn nanocomposites varied from 6 to 22 GPa and increased with an increase of pressure and temperature of the infiltration process, and was significantly larger for the finer grained composites.

Combination of ECAP and Hydrostatic Extrusion for UFG Microstructure Generation in Nickel

An ultra-fine grained microstructure was obtained in high purity nickel by a combination of (a) equal-channel angular pressing (ECAP) and (b) hydrostatic extrusion (HE) with a cumulative true strain of ~11.2. The resulting microstructure was examined by light and TEM microscopy. Mechanical properties have been measured by tensile and hardness tests. It was found that HE of ECAP-ed samples leads to a significant grain size refinement (from 330 to 160nm) and to an increase in microstructural homogeneity. SPD nickel, made by a combination of the ECAP and hydrostatic extrusion methods, has high strength and ductility (i.e.: YS=1120MPa and εf = 11%). The microstructure transformation was accompanied by a strength increase of 78% compared to ECAP alone. The results obtained fit well with the Hall-Petch relationship. A combination of ECAP and HE has achieved much better properties than either single process and show it to be a promising procedure for manufacturing bulk UFG nickel.

Fabrication and Physical Properties of SiC{GaAs Nano-Composites

Nano-composites consisting of a primary matrix phase of hard nanocrystalline SiC and a secondary nanocrystalline GaAs semiconductor phase were obtained by high-pressure zone infiltration. The synthesis occurs in three stages: (i) at room- temperature the SiC nanopowder is compacted under high pressure to 8 GPa, (ii) the temperature is increased to 1240°C, above the melting point of GaAs, and the pores were filled with liquid, (iii) on cooling GaAs nanocrystallites grow in the pores. The synthesis was performed using a toroid-type high-pressure apparatus (IHPP PAS, Warsaw) and a six anvil cubic press (MAX80 at HASYLAB, Hamburg). X-ray diffraction studies were performed with a laboratory D5000 Siemens diffractometer. The phase compositionn, grain size and macrostrains in the synthesized materials were examined. The microstructure of the composites was characterized using a Scanning Electron Microscopy (SEM), and High Resolution Transmission Electron Microscopy (TEM). Far-infrared reflectivity and Raman spectroscopy measurements were used to trace built-in strains.

Powder Precursors for Nanoceramics: Cleaning and Compaction

Thermal surface purification in an inert gas flow and densification processes of SiC and diamond nanocrystalline powders with specific surface in the range of 60 – 300 m2/g and average grain sizes from 5 to 15 nm in diameter were examined. Termogravimetric Analysis (TGA) linked with mass spectrometry of outgassing products show that surface impurities desorb at up to 450°C. Further heating above 450°C leads to oxidation of the powder surface. Small Angle X-Ray Scattering (SAXS) and gas porosimetry (ASAP) was applied to investigate densification of the nanocrystalline powders. Compaction under 1GPa or higher pressure was found necessary for obtaining the ceramic matrix with porosity in the nanometer range.

SiC, TiC and ZrC Nanostructured Ceramics: Elaboration and Potentialities for Nuclear Applications

Carbide ceramics as SiC, TiC or ZrC are potential candidates for high temperature applications such as fourth generation nuclear plants because of their refractory or low activation under neutron irradiation properties. Nevertheless, the typical drawbacks of hard ceramics (brittleness) could limit their use in these applications. In order to overcome these problems, one possibility is to decrease the grain size down to the nanometric scale. Enhancement of the mechanical properties is actually expected in such nanostructured ceramics (ductility) and moreover, these nanomaterials could also take advantage of their strong grain boundaries density to withstand severe irradiation conditions. If one wants to quantify the expected enhancement of the properties, the first challenge that has to be faced is the elaboration of the nanostructured ceramics samples. That means being able to synthesize the pre-ceramics nanopowders in weighable amounts, and then finding an efficient way to sinter them aiming at the maximum densification together with avoiding grain growth. In this contribution, we present SiC, TiC and ZrC nanopowders synthesis by laser pyrolysis and inductively coupled plasma, together with their densification by different techniques (Hot Isostatic Pressing, Spark Plasma Sintering, High Pressure Flash Sintering). We also report the latest findings obtained on the behavior of SiC nanostructured ceramics under low energy ion irradiation. Raw micrometric SiC and ZrC powders were used as precursors in the inductively coupled plasma experiment. The production was as high as 1 kg.h-1, with nanograins ranging from 10 to 100 nm in size depending on the synthesis conditions. For the laser pyrolysis method, gaseous precursors (SiH 4 , C 2 H 2 ) were used for SiC while liquid alkoxides precursors were used for TiC and ZrC respectively. For SiC, the production rate can reach 100 g.h-1 (laboratory scale) with grain sizes ranging from 10 to 50 nm with narrow size distribution. For TiC and ZrC nanopowders, the production rate is lower than for SiC because of the use of liquid precursors that leads to a worse yield. In this latter case, the carbide phase is obtained after carburization of the laser pyrolyzed TiO2 (or ZrO 2 ) / free carbon nanocomposites. The final carbide nanograins size is in the 50 – 80 nm range. After sintering, the obtained pellets show different characteristics depending on the starting powder and the sintering technique. With the right sintering conditions, the densification reaches 95 % without any sintering additives, with no (or limited) grain growth and no modification of the crystalline structure. Concerning the properties of the obtained nanostructured ceramics, the SiC pellets, together with the as-synthesized nanopowders, were submitted to low energy ion irradiation in order to compare their behavior to conventional SiC materials.

Investigation of the Microstructure of SiC-Zn Nanocomposites by Microscopic Methods: SEM, AFM and TEM

SiC-Zn nanocomposites with about 20% volume fraction of metal were fabricated by infiltration process under the pressure of 2-8 GPa and at the temperature of 400_1000oC. SiC nanopowders used in the experiments formed loosely agglomerated chains of single crystal nanoparticles. The dimension of the agglomerates was in the micrometer range, the mean grain size was up to tens of nanometers. Microstructural investigations of the nanocomposites were performed at a different resolution levels using scanning, transmission electron microscopy and atomic force microscopy techniques (SEM, TEM, AFM, respectively). SEM observations indicate a presence of nano-dispersed, uniform (on the micrometer scale) mixture of two phases. TEM observations show that distribution of SiC and Zn nanocrystallites is uniform on the nanometer scale. High-resolution TEM images demonstrate an existence of thin (on the order of tens of Angstroms) Zn layers separating SiC grains. AFM images of the mechanically polished samples show a smooth surface with the roughness on the order of the SiC grain size (10-30 nm). After ion etching of some samples the AFM topographs show surface irregularities: periodically spaced hillocks 50-100 nm in height. The size of the SiC grains remains equal to that of the initial powder crystallites. The size of the Zn grains varies in the range of 20-100 nm depending on the initial SiC grain size and the composite fabrication conditions.

Folic acid–conjugated mesoporous silica particles as nanocarriers of natural prodrugs for cancer targeting and antioxidant action

Naturally derived prodrugs have a wide range of pharmacological activities, including anticancer, antioxidant, and antiviral effects. However, significant barriers inhibit their use in medicine, e.g. their hydrophobicity. In this comprehensive study, we investigated simple and effective nanoformulations consisting of amine-functionalized and conjugated with folic acid (FA) mesoporous silica nanoparticles (MSNs). Two types of MSNs were studied: KCC- 1, with mean size 324 nm and mean pore diameter 3.4 nm, and MCM - 41, with mean size 197 and pore diameter 2 nm. Both types of MSNs were loaded with three anticancer prodrugs: curcumin, quercetin, and colchicine. The nanoformulations were tested to target in vitro human hepatocellular carcinoma cells (HepG2) and HeLa cancer cells. The amine-functionalized and FA-conjugated curcumin-loaded, especially KCC-1 MSNs penetrated all cells organs and steadily released curcumin. The FA-conjugated MSNs displayed higher cellular uptake, sustained intracellular release, and cytotoxicity effects in comparison to non-conjugated MSNs. The KCC-1 type MSNs carrying curcumin displayed the highest anticancer activity. Apoptosis was induced through specific signaling molecular pathways (caspase-3, H2O2, c-MET, and MCL-1). The nanoformulations displayed also an enhanced antioxidant activity compared to the pure forms of the prodrugs, and the effect depended on the time of release, type of MSN, prodrug, and assay used. FA-conjugated MSNs carrying curcumin and other safe natural prodrugs offer new possibilities for targeted cancer therapy.

Combining Hard with Soft Materials in Nanoscale Under High-Pressure High-Temperature Conditions

Nano-composites with a primary nanocrystalline ceramic matrix and a secondary nanocrystalline material (metal or semiconductor) were synthesized by infiltration of an appropriate liquid into ceramic compacts under pressures of up to 8 GPa and temperatures of up to 2000 K. The purpose of our work was to obtain nanocomposites which constitute homogenous mixtures of two phases, both forming nano-grains of about 10 nm in size. The high pressure is used to bring the porosity of the compacted powders down to the nano-scale and force a given liquid into the nano-sized pores. The advantage of the infiltration technique is that in a single, continuous process, we start with a nano-crystalline powder, compress it to form the matrix of the composite, and crystallize and/or synthesize the second nanomaterial in the matrix pores. The key limitation of this technology is, that the pores in the matrix need to stay open during the entire process of infiltration. Thus the initial powder should form a rigid skeleton, otherwise the so-called self-stop process can limit or block further flow of the liquid phase and hinder the process of the composite formation. Therefore powders of only very hard ceramic materials like diamond, SiC, or Al2O3, which can withstand a substantial external load without undesired deformation, can be used as the primary phase. With this technique, using diamond and SiC ceramic powders infiltrated by liquid metals (Al, Zn, Sn, Ag, Au) or semiconductors (Si, Ge, GaAs, CdTe), we obtained nanocomposites with the grain size in the range of 10 – 30 nm. Our work addresses the key problem in manufacturing bulk nanocrystalline materials, i.e. the preservation of nano-scale during the fabrication process. In this paper we discuss basic technical and methodological problems associated with nano-infiltration based on the results obtained for Zn-SiC composites. We show that by selecting appropriate ceramic nanopowders and applying different pressure-temperature conditions we can, to large extend, control the size of the grains of metals (in the range of several to several tens of nm). Tentative mechanism of nanoinfiltration of liquids into ceramic matrices is proposed.