Name: spark plasma sintering apparatus used for the formation of strontium titanate bicrystals
Uploaded: 2017-02-09T15:00:00
Description: Watch this Scientific Journal Video about Spark Plasma Sintering Apparatus Used for the Formation of Strontium Titanate Bicrystals at JoVE.com

LH gratefully acknowledges financial support by an US National Science Foundation Graduate Research Fellowship under Grant No. 1148897. Electron microscopy characterization and SPS processing at UC Davis was financially supported by a University of California Laboratory Fee award (#12-LR-238313). Work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

1. Sample Preparation of Single Crystal Strontium Titanate
NOTE: Single crystal STO is supplied with a (100) surface polished to a mirror finish.
<ol>
	<li>Section STO into 5x5 mm2 pieces using diamond wire saw.</li>
	<li>Ultrasonically clean samples at 50-60 Hz consecutively in baths of acetone, isopropanol, and methanol for fifteen minutes each.</li>
	<li>Remove STO from methanol bath to immediately place on hot plate held at a temperature of 200 &#176;C. Heating the sample afte...

<ol>
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Jpn. 109 (7), S110-S120 (2001).</li><li>Hutt, S., Kienzle, O., Ernst, F., Ruhle, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Processing+and+Structure+of+Grain+boundaries+in+Strontium+Titanate.">Processing and Structure of Grain boundaries in Strontium Titanate.</a> Z. Metallkd. 92 (2), 105-109 (2001).</li><li>Takahisa, Y., Ikuhara, Y., Sakuma, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Current-voltage+characteristics+across+45&#9702;+symmetric+tilt+boundary+in+highly+donor-doped+SrTiO3+bicrystal.">Current-voltage characteristics across 45&#9702; symmetric tilt boundary in highly donor-doped SrTiO3 bicrystal.</a> J. Mater. Sci. Lett. 20, 1827-1829 (2001).</li><li>Hill, A., Wallach, E. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Modelling+Solid+State+Diffusion+Bonding.">Modelling Solid State Diffusion Bonding.</a> Acta Metall. 37 (9), 2425-2437 (1989).</li><li>Sato, Y., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Non-linear+current-voltage+characteristics+related+to+native+defects+in+SrTiO3+and+ZnO+bicrystals.">Non-linear current-voltage characteristics related to native defects in SrTiO3 and ZnO bicrystals.</a> Sci. Technol. Adv. Mater. 4 (6), 605-611 (2003).</li><li>Hirose, S., Nishimura, H., Niimi, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Resistance+switching+effect+in+Nb-doped+SrTiO[sub+3]+(100)+bicrystal+with+(100)+&#8764;45&#176;+twist+boundary.">Resistance switching effect in Nb-doped SrTiO[sub 3] (100) bicrystal with (100) &#8764;45&#176; twist boundary.</a> J. App. Phys. 106 (4), 043711-043716 (2009).</li><li>Hutt, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Doctoral Thesis. , (2002).</li><li>Brunner, D., Taeri-Baghbadrani, S., Sigle, W., Ruhle, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Suprising+Results+of+a+Studay+on+the+Plasticity+in+Strontium+Titanate.">Suprising Results of a Studay on the Plasticity in Strontium Titanate.</a> J. Amer. Ceram. Soc. 84 (5), 1161-1163 (2001).</li><li>Gumbsch, P., Taeri-Baghbadrani, S., Brunner, D., Sigle, W., Ruhle, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Plasticity+and+an+inverse+brittle-to-ductile+transition+in+strontium+titanate.">Plasticity and an inverse brittle-to-ductile transition in strontium titanate.</a> Phys. Rev. Lett. 87 (8), 085501-085504 (2001).</li><li>Taeri, S., Brunner, D., Sigle, W., Ruhle, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Deformation+Behavior+of+Strontium+Titanate+between+Room+Temperature+and+1800K+under+Ambient+Pressure.">Deformation Behavior of Strontium Titanate between Room Temperature and 1800K under Ambient Pressure.</a> Z. Metallkd. 95, 433-446 (2004).</li><li>Takahashi, K., Ohtomo, A., Kawasaki, M., Koinuma, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Advanced+Processing+and+Characterization+of+SrTiO3+Single+Crystals+and+Bicrystals+for+High+Tc+Superconducting+Film+Substrate.">Advanced Processing and Characterization of SrTiO3 Single Crystals and Bicrystals for High Tc Superconducting Film Substrate.</a> Mater. Sci. Eng. B. 41, 152-156 (1996).</li><li>Rhodes, W. H., Kingery, W. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Dislocation+Dependence+of+Cationic+Diffusion+in+SrTiO3.">Dislocation Dependence of Cationic Diffusion in SrTiO3.</a> J. Amer. Ceram. Soc. 49 (10), 521-526 (1966).</li><li>Yamamoto, T., Hayashi, K., Ikuhara, Y., Sakuma, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Grain+Boundary+Structure+and+Electrical+Properties+in+Nb-Doped+SrTiO&lt;sub&gt;3&lt;/sub&gt;+Bicrystals.">Grain Boundary Structure and Electrical Properties in Nb-Doped SrTiO&lt;sub&gt;3&lt;/sub&gt; Bicrystals.</a> Key Eng. Mater. 181-182, 225-230 (2000).</li><li>Fitting, L., Thiel, S., Schmehl, A., Mannhart, J., Muller, D. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Subtleties+in+ADF+imaging+and+spatially+resolved+EELS:+A+case+study+of+low-angle+twist+boundaries+in+SrTiO3.">Subtleties in ADF imaging and spatially resolved EELS: A case study of low-angle twist boundaries in SrTiO3.</a> Ultramicroscopy. 106 (11-12), 1053-1061 (2006).</li><li>Hughes, L. A., van Benthem, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Formation+of+SrTiO3+bicrystals+using+spark+plasma+sintering+techniques.">Formation of SrTiO3 bicrystals using spark plasma sintering techniques.</a> Scr. Mater. 118, 9-12 (2016).</li></ol>

The bonding temperature of 1,200 &#176;C was chosen to maximize diffusion as small changes in temperature can greatly impact the kinetics of all diffusion bonding mechanisms. A temperature of 1,200 &#176;C is outside the brittle-ductile transition temperature range of STO. However, the sample underwent brittle fracture at this temperature. The catastrophic failure of the STO bicrystal was not unexpected as STO has ~ 0.5% ductility at 1,200 &#176;C. Also, the sample was held at a pressure of 140 MPa throughout the heating...

Spark plasma sintering (SPS) is a technique in which application of high uniaxial pressure and pulsed direct current leads to the rapid densification of powder compacts1. This technique also leads to the successful formation of composite structures from various materials, including silicon nitride/silicon carbide, zirconium boride/silicon carbide, or silicon carbide, with no additional sintering aids required2,3,4,5. The synthesis of these composite structures by conventional hot-pressing had been challenging in the past. While application of a high uniaxial pressure and fast heating rate via the SPS technique enhances consolidation of powders and composites, the phenomenon causing this enhanced densification debated in the literature2,3,6,7. There also exists only limited information regarding the influence of electric fields on grain boundary formation and the resulting atomic structures of grain boundary cores8,9. These core structures determine the functional properties of SPS sintered materials, including electric breakdown of high voltage capacitors and the mechanical strength and toughness of ceramic oxides10. Therefore, understanding the fundamental grain boundary structure as a function of SPS processing parameters, such as applied current, is necessary for the manipulation of a material&#39;s overall physical properties. One method to systematically elucidate the fundamental physical mechanisms underpinning SPS is the formation of specific grain boundary structures, i.e., bicrystals. A bicrystal is created by manipulation of two single crystals, which are then diffusion bonded with specific misorientation angles11. This method provides a controlled way to investigate the fundamental grain boundary core structures as a function of processing parameters, dopant concentration, and impurity segregation12,13,14.
Diffusion bonding is dependent on four parameters: temperature, time, pressure, and bonding atmosphere15. Conventional diffusion bonding of strontium titanate (SrTiO3, STO) bicrystals typically occurs at a pressure below 1 MPa, within a temperature range of 1,400-1,500 &#176;C, and time scales ranging from 3 to 20 hours13,14,16,17. In this study, bonding in a SPS apparatus is achieved at significantly lower temperature and time scales in comparison to conventional methods. For polycrystalline materials, reduced temperature and time scales via SPS significantly limits grain growth, thereby providing advantageous control of a material&#39;s properties through manipulation of its microstructure.
The SPS apparatus, for a 5&#215;5 mm2 sample, exerts a minimum pressure of 140 MPa. Within the conventional diffusion bonding temperature range, Hutt et al. report instantaneous fracture of STO when the bonding pressure exceeds 10 MPa18. However, STO exhibits temperature dependent plasticity behavior, indicating bonding pressure can exceed 10 MPa at specific temperatures. Above 1,200 &#176;C and below 700 &#176;C, STO exhibits some ductility, at which stresses greater than 120 MPa can be applied without instantaneous fracture of the sample. Within the intermediate temperature range of 700-1,200 &#176;C, STO is brittle and experiences instantaneous fracture at stresses greater than 10 MPa. At 800 &#176;C, STO has minor deformability prior to fracture at stresses less than 200 MPa19,20,21. Hence, bonding temperatures for STO bicrystal formation via SPS apparatus must be selected according to the plasticity behavior of the material.

<table border="0" cellpadding="0" cellspacing="0" width="1429">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:373px;">Strontium titanate single crystal (100)</td>
			<td style="width:187px;">MTI Corporation</td>
			<td style="width:427px;">STOa101005S1-JP</td>
			<td style="width:443px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:373px;">Buffered oxide etch, hyrofluoric acid 6:1</td>
			<td style="width:187px;">JT Baker</td>
			<td style="width:427px;">&#160;MBI 1178-03</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:373px;">Scanning electron microscope (SEM)</td>
			<td style="width:187px;">FEI</td>
			<td>Model: 430 NanoSEM</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">SPS apparatus&#160;</td>
			<td>Sumitomo Coal Mining Co</td>
			<td>Model: Dr. Sinter 5000 SPS Apparatus</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">High Temperature Furnace</td>
			<td>Thermolyne</td>
			<td>Model: 41600</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Ultrasonic Cleaner</td>
			<td>Bransonic</td>
			<td>Model: 221</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:373px;">Mechanical polisher</td>
			<td style="width:187px;">Allied High Tech Products</td>
			<td>15-2100-TEM</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:373px;">Diamond lapping film</td>
			<td style="width:187px;">3M</td>
			<td>660XV&#160;</td>
			<td>1 um to 9 um Grit Size</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:373px;">Diamond lapping film</td>
			<td style="width:187px;">3M</td>
			<td style="width:427px;">661X</td>
			<td>0.5 um to 0.1 um Grit Size</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:373px;">Colloidal silica</td>
			<td style="width:187px;">Allied High Tech Products</td>
			<td>180-20000</td>
			<td>.05 um Grit Size</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:373px;">Sputter coater</td>
			<td style="width:187px;">QuorumTech</td>
			<td>Model: Q150RES</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Focused ion beam (FIB) instrument&#160;</td>
			<td style="width:187px;">FEI</td>
			<td>Model: Scios dual-beamed focused ion beam (FIB) instrument&#160;</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Nanomill TEM specimen preparation system</td>
			<td style="width:187px;">Fischione Instruments</td>
			<td>Model: 1040</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Transmission electron microscope (TEM)&#160;</td>
			<td style="width:187px;">JEOL</td>
			<td>Model: JEM2500 SE&#160;</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Scanning transmission electron microscope (STEM)</td>
			<td style="width:187px;">FEI</td>
			<td>Model: TEAM 0.5&#160;</td>
			<td></td>
		</tr>
	</tbody>
</table>

spark plasma sintering apparatus used for the formation of strontium titanate bicrystals

A spark plasma sintering apparatus was used as a novel method for diffusion bonding of two single crystals of strontium titanate to form bicrystals with one twist grain boundary. This apparatus utilizes high uniaxial pressure and a pulsed direct current for rapid consolidation of material. Diffusion bonding of strontium titanate bicrystals without fracture, in a spark plasma sintering apparatus, is possible at high pressures due to the unusual temperature dependent plasticity behavior of strontium titanate. We demonstrate a method for the successful formation of bicrystals at accelerated time scales and lower temperatures in a spark plasma sintering apparatus compared to bicrystals formed by conventional diffusion bonding parameters. Bond quality was verified by scanning electron microscopy. A clean and atomically abrupt interface containing no secondary phases was observed using transmission electron microscopy techniques. Local changes in bonding across the boundary was characterized by simultaneous scanning transmission electron microscopy and spatially resolved electron energy-loss spectroscopy.

Bonding temperature, time, and misorientation angle were all altered to determine optimum parameters needed for the maximum possible bonded interface fraction of the STO bicrystal (Table 1). The interface was considered &#39;bonded&#39; when the grain boundary was not visible during SEM imaging (Figure 2a). A &#39;non-bonded&#39; interface was exhibited when a dark image contrast or voids were present at the boundary location (Figure 2b)....

Watch this Scientific Journal Video about Spark Plasma Sintering Apparatus Used for the Formation of Strontium Titanate Bicrystals at JoVE.com

Spark Plasma Sintering Apparatus Used for the Formation of Strontium Titanate Bicrystals

A viable technique for the formation of strontium titanate bicrystals at high pressure and fast heating rate via the spark plasma sintering apparatus is developed.

A spark plasma sintering apparatus was used as a novel method for diffusion bonding of two single crystals of strontium titanate to form bicrystals with ...

The goal of this procedure is to form strontium titanate bicrystals using a Spark Plasma Sintering apparatus in order to create grain boundary structures with well defined misorientations as a function of processing parameters, such as high pressure and heating rate. We need to understand whether grain boundaries generated during SPS are different from those produced during more traditional ceramic processing techniques. This will then help us generate new ceramic materials with unprecedented properties.The main advantage of this technique is to form specific grain boundaries with well defined misorientations for the systematic understanding of microstructure formation during spark plasma sintering. To begin sample preparation, use a diamond wire saw to cut five millimeter by five millimeter pieces of a strontium titanate single crystal with a 1-0-0 plane surface polished to a mirror finish. Clean the samples with an ultrasonic cleaner in baths of acetone, isopropanol, and methanol for 15 minutes each at 50 to 60 Hertz.Heat a hot plate to 200 degrees Celsius during the cleaning. Upon completing the ultrasonic cleaning in methanol, immediately place the samples on the hot plate to dry. Etch the samples in a six to one solution of ammonium fluoride and 49%hydrofluoric acid for 10 minutes.Rinse the etched samples in deionized water and isopropanol in sequence. Dry the samples with filtered dry air. Discard any samples that developed a rainbow patina on the optically flat side.First, place a 30 millimeter circle of graphite paper on a 30 millimeter diameter graphite plunger. Then, stack two STO single crystal samples with their optically flat sides facing each other on the center of the paper covered plunger. Rotate the top single crystal around the 1-0-0 axis to the desired misorientation angle.Carefully slide a 30 millimeter diameter graphite die over the crystals and plunger. Insert another 30 millimeter circle of graphite paper and plunger into the die on top of the crystal stack. Place the assembly on the lower graphite spacer of the SPS apparatus.And then place the other graphite spacer on top of the assembly. Use the z-axis control buttons to bring the upper punch down to apply a three kilonewton uniaxial force to the assembly. Insert a K type thermocouple into the side of the graphite die to reach the sample.Set the chamber pressure to approximately 10 Pascal. Then on the instrument program control, set the bond temperature, time, and heating rates for the experiment. Select sinter conditions of a DC pulse of 12 seconds on, two seconds off, and then start the sintering program.Once the program ends, allow the sample to cool to 150 degrees Celsius. Transfer the gray black reduced sample to a high temperature furnace. Anneal the sample at 1200 degrees Celsius for 140 hours in air at ambient pressure to obtain the off-white oxidized bicrystal.Use a diamond wire saw to slice the bicrystal into five millimeter by one millimeter pieces. Polish the cross-sections with decreasing grit sizes of diamond lapping film until the scratches are uniform at the smallest grit size. Then start a two minute polish of the cross-sections with continuously poured colloidal silica and a matte cloth.15 seconds before polishing ends, switch from pouring colloidal silica to pouring DI water onto the platen. Immediately rinse the polish sample in DI water for one minute. Clean the polished sample in consecutive ultrasonic cleaning baths of acetone, isopropanol, and methanol for 15 minutes each.Immediately after cleaning, dry the sample on a hot plate at 200 degrees Celsius. Melt the clean sample polished side up on a sample stub coated with colloidal graphite. Sputter coat the sample surface with two to three nanometers of carbon.First, place the focused ion beam copper grid in consecutive ultrasonic cleaner baths of acetone and isopropanol for one hour. Then, plasma clean the copper grid for 10 minutes. Place the STO bicrystal sample and the clean copper grid in the FIB apparatus with the sample stage set at seven millimeters.In the FIB instrument software, identify a region of interest along the grain boundary. Navigate to patterning control and select the rectangle patterning tool with the CE depth surface application for electron beam deposition of carbon. Insert the carbon gas injection needle and deposit a 15 by two by two micrometers cubed protective layer of carbon over the region of interest.Then, retract the injection needle. Return to patterning control and choose the rectangle patterning tool with a W dep application for ion beam deposition of tungsten. Insert the tungsten gas injection needle and deposit a protective layer of tungsten of the same dimensions over the region of interest.Retract the injection needle. In patterning control, select the regular cross section patterning tool with the silicon application property. Mill a 22 by 27 by 15 micrometer cubed trench above the protected ROI and a 22 by 25 by 15 micrometer cubed trench below the ROI.Then, in patterning control, use the silicon clean cross section patterning tool to clean the trenched lamella surface. Next, use the silicon rectangle patterning tool to cut a two micrometer wide J pattern with two micrometer wide rectangles into the sample. Switch to the easy lift dialog box and set the micromanipulator to the park position.Lower the micromanipulator and weld the sample to the tip of the micromanipulator by tungsten deposition. Then, extend the J patten into a U pattern. Lift the lamella from the bulk sample and rotate the micromanipulator 180 degrees to set the grain boundary perpendicular to the ion beam.Tungsten welds with a lamella to the copper grid. Then, cut the micromanipulator free from the lamella using the rectangle patterning tool. Deposit a 15 by two by four micrometer cubed layer of carbon and a 15 by two by eight micrometer cubed layer of tungsten over the area of interest.Then, with the ion beam, thin the sample first to approximately 200 nanometers with a cleaning cross section pattern. And then, with a rectangle pattern, thin the sample to electron transparency. Remove the copper FIB grid from the FIB apparatus and place the grid onto a NanoMil sample holder.Then place the sample holder into the NanoMil apparatus. Using the NanoMil apparatus software, mil the sample at 500 electron volts for 10 minutes to remove amorphous damage caused by the FIB. Place the cleaned TEM sample in a sample holder and insert the sample and holder into the STEM.In the STEM software, use the capture tool to obtain an image of the sample grain boundary. Strontium titanate bicrystals were formed from single crystal pairs at various misorientation angles using a Spark Plasma Sintering apparatus. The interface was considered to be bonded wherever the grain boundary was not observable in SEM imaging.Areas of dark contrast in the images are either attributed to voids or colloidal graphite diffusion between the single crystals. The maximum bonded interface fraction was achieved with zero degrees of misorientation, a bonding temperature of 600 or 700 degrees Celsius, a bonding duration of 90 minutes, and an annealing time of 100 hours. Increasing the angle of misorientation to approximately 45 degrees necessitated a higher bonding temperature and a longer annealing time to achieve a 45%bonded interface.High resolution TEM and high angle annular dark field STEM show an atomically abrupt grain boundary with no observed second phases or intergranular film. Electron energy loss spectroscopy at the interface showed decreased crystal field splitting of the titanium L2 and L3 edges and decreased intensity in the oxygen K edge compared to the bulk crystal, suggesting a greater concentration of oxygen vacancies at the interface. Following the same procedure, we can also form doped grain boundaries using the SPS.This will help us to address whether the electric field will change grain boundary segregation. After watching this video, you should have a good understanding of how to form bicrystals with atomically abrupt interface structures using SPS. Don't forget that hydrofluoric acid can be extremely hazardous and precautions, such as personal protective wear, should be taken.

Research

JoVE Journal

Engineering

Investigation of Early Plasma Evolution Induced by Ultrashort Laser Pulses

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Encapsulation and Permeability Characteristics of Plasma Polymerized Hollow Particles

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Characterization of Recombination Effects in a Liquid Ionization Chamber Used for the Dosimetry of a Radiosurgical Accelerator

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In Situ Synthesis of Gold Nanoparticles without Aggregation in the Interlayer Space of Layered Titanate Transparent Films

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Combinations of metal oxide semiconductors and gold nanoparticles (AuNPs) have been investigated as new types of materials. The in situ synthesis of AuNPs ...