The Characterisation and Synthesis of Hafnium diboride for Aerospace Applications
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Transcript of The Characterisation and Synthesis of Hafnium diboride for Aerospace Applications
Final Report
The Characterisation and Synthesis of Hafnium diboride for hypersonic flight applications
Kishan Desai
2015 3rd Year Individual Project
I certify that all material in this thesis that is not my own work has been identified and that no material has been included for which a degree has previously been conferred on me.
Signed..............................................................................................................
College of Engineering, Mathematics, and Physical Sciences University of Exeter
ii
Final Report ECM3101/ECM3102/ECM3149
Title: The Characterisation And Synthesis Of Hafnium Diboride For Aerospace Applications
Word count: 8754 Number of pages: 40
Date of submission: Wednesday, 29 April 2015
Student Name: Kishan Desai Programme: MEng Mechanical Engineering
Student number: 620027268 Candidate number: 004466
Supervisor: Professor Shawoei Zhang
iii
Acknowledgements
I would like to thank my mentor Professor Shaowei Zhang for providing me with the opportunity to take part in an exciting research project about high temperature ceramics. Thank you for guiding me throughout the project.
Thank you Juntong Huang and Ben Jun Chen for assisting me in the laboratory and for helping me understand many different aspects of this project.
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Abstract In this experiment Hafnium diboride (HfB2) was successfully produced by borothermal
reduction. Molten salt synthesis was the synthesis technique that was used and the
temperatures of the furnaces in each experiment varied between 1100°C-1300°C. Boron
dioxide and magnesium were used as reducing agents and hafnium dioxide was used as the
source for hafnium. The powders were characterised using a scanning electron microscope, a
transmission electron microscope, energy-dispersive x-ray spectroscopy and x-ray diffraction.
The results showed that the optimal sintering conditions was 1300°C with a hold time of 12
hours. At a temperature of 1200°C and a hold time of 6 hours, phase pure HfB2 powder was
created that varied in size between 0.5µm and 2.5µm, with very little HfO2 remaining in the
sample.
Keywords: Molten Salt synthesis, Characterisation, Hafnium Diboride (HfB2), X-ray Diffraction (XRD), Scanning electron microscope (SEM), Transmission electron microscopy (TEM), Ultra-High Temperature Ceramics (UHTC)
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Table of Contents 1. Introduction and background ...................................................................................... 1
1.1. Aim .................................................................................................................................. 1 1.2. Objectives ....................................................................................................................... 2 1.3. Evolution of Project ......................................................................................................... 2
2. Literature review ........................................................................................................ 3 2.1. Material Properties of HfB2 .............................................................................................. 3
2.1.1 Thermal Properties .............................................................................................................. 3 2.1.2 Melting Point ....................................................................................................................... 3 2.1.3 Oxidation Resistance ............................................................................................................ 4 2.1.4 Strength Properties .............................................................................................................. 4 2.1.5 Density ................................................................................................................................. 4 2.1.6 Molecular properties ........................................................................................................... 4
2.2. An Introduction to Molten Salt Synthesis ......................................................................... 5 2.3. Previous Research and Synthesis Techniques ................................................................... 5
2.3.1 Borothermal/carbothermal Reduction ................................................................................ 6 2.3.2 Unconventional method (Sol-‐Gel Synthesis) ....................................................................... 6 2.3.3 Molten salt synthesis ........................................................................................................... 6
3. Methodology and theory ............................................................................................ 6 3.1. Reactions ......................................................................................................................... 6
3.1.1 Borothermal reduction ........................................................................................................ 7 3.1.2 Salt Wash ............................................................................................................................. 7 3.1.3 Acid Rinse ............................................................................................................................. 7 3.1.4 Raw materials ....................................................................................................................... 7
3.2. Research .......................................................................................................................... 7 3.3. Sintering and Molten salt synthesis ................................................................................. 8
3.3.1 An Introduction to Sintering ................................................................................................ 8 3.3.2 Sintering Process and description ........................................................................................ 8 3.3.3 Preparation for Molten Salt synthesis ................................................................................. 8
3.4. X-‐Ray Diffraction ............................................................................................................. 9 3.4.1 Preparation of the samples for XRD ..................................................................................... 9
3.5. SEM ................................................................................................................................ 10 3.6. EDS ................................................................................................................................. 10 3.7. TEM ................................................................................................................................ 10
4. Experimental work .................................................................................................... 10 4.1. Procedure ....................................................................................................................... 10
4.1.1 Preparing each sample ....................................................................................................... 10 4.1.2 Heating the samples .......................................................................................................... 11 4.1.3 Washing the salt ................................................................................................................. 11 4.1.4 Acid Rinse and Dry ............................................................................................................. 12
4.2. Calculating the mass of each reactant in the sample ....................................................... 12 4.2.1 Example calculation ........................................................................................................... 12
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4.2.2 Volatility of reducing agents .............................................................................................. 13
5. Results ...................................................................................................................... 13 5.1. Experiment 1 (Samples 1,2,3) ......................................................................................... 13
5.1.1 Initial Conditions ................................................................................................................ 14 5.1.2 Observations ...................................................................................................................... 14 5.1.3 XRD Results ........................................................................................................................ 14
5.2. Experiment 2 (Samples 4,5,6) ......................................................................................... 15 5.2.1 Initial Conditions ................................................................................................................ 15 5.2.2 XRD Results ........................................................................................................................ 16
5.3. Experiment 3 (Samples 7,8,9) ......................................................................................... 17 5.3.1 Initial Conditions ................................................................................................................ 17 5.3.2 XRD Results ........................................................................................................................ 17
5.4. Experiment 4 (10,11,12) .................................................................................................. 18 5.4.1 Initial Conditions ................................................................................................................ 18 5.4.2 Notable Observations ........................................................................................................ 19 5.4.3 XRD Results ........................................................................................................................ 19
5.5. Experiment 5 (13,14,15,16) ............................................................................................. 20 5.5.1 Initial Conditions ................................................................................................................ 20 5.5.2 XRD Results – Pre Acid Wash ............................................................................................. 21 5.5.3 XRD Results -‐ Post Acid Wash ............................................................................................ 23 5.5.4 Pre –acid conclusion .......................................................................................................... 25 5.5.5 Post acid conclusion ........................................................................................................... 25
5.6. XRD Conclusion .............................................................................................................. 26 5.7. SEM, TEM and EDS .......................................................................................................... 26
5.7.1 SEM and EDS Analysis ........................................................................................................ 26 5.7.2 TEM Analysis ...................................................................................................................... 27
6. Discussion and conclusions ....................................................................................... 29 6.1. Summary ........................................................................................................................ 29 6.2. Optimal Conditions ......................................................................................................... 29
6.2.1 Temperature and Hold time .............................................................................................. 29 6.2.2 Reactants ratio ................................................................................................................... 29
6.3. Further research ............................................................................................................. 30
7. Project management, consideration of sustainability and health and safety ............. 30 7.1. Project Management ...................................................................................................... 30 7.2. Considering the Environment ......................................................................................... 31 7.3. Health and Safety Risk Management .............................................................................. 32
References ....................................................................................................................... 33
1
1. Introduction and background Ultra high temperature ceramics have been around since the early 1960’s and have been
identified as a possible material for the leading edges of hypersonic jets and atmospheric re-
entry vehicles. With hypersonic jets potentially travelling at speeds of up to 5 times the speed
of sound (in air), a lot of research needs to be conducted now in order for the vehicles to
operate safely in the future. At the stagnation region over the leading edges of these jets, the
temperature is expected to reach over 2000°C in some cases. In these extreme conditions
there will be heavy oxidisation and there are very few materials, which are able to operate
normally in such high temperatures and yet remain structurally stable, resist oxidisation and
conduct the heat quickly away from the leading edges. [1-9]
Hafnium diboride (HfB2) belongs to this family of materials classified as UHTC and it has
been described as a potential covering material for the leading edges of hypersonic flight and
atmospheric re-entry vehicles. These ceramics such as Hafnium diboride and Zirconium
diboride (ZrB2) are possessed with good thermal and electrical conductivity, good chemical
inertness, high melting points and high strength. As a pure substance it has shown poor
oxidation resistance at high temperatures, but studies have shown that its oxidation resistance
will increase if it is fused with another composite such as SiC or TaSi2. [1-10,12]
Hafnium diboride is one of the most exciting high temperature ceramics and there has already
been a large amount of research conducted on this material in the last 2 decades or so. It has
been synthesised using many different reduction routes, but mostly by borothermal reduction.
It has been produced using several different manufacturing techniques, including spark
plasma sintering and the sol-gel method. However very little research has been conducted on
the molten salt synthesis of HfB2 using boron dioxide as a reducing agent.
1.1. Aim
The aim of this is experiment is to:
1. Synthesize a pure ceramic powder of HfB2
2. Characterise the ceramic powder using various characterisation techniques
This experiment used a synthesis technique called molten salt synthesis. This means that
reaction took place in a pool of molten salt. This particular experiment looked at the
possibilities of high temperature molten salt synthesis using boron dioxide as the reduction
2
route. High temperature molten salt synthesis occurs in the temperature range of 1050°C to
1350°C. This is because the salts used in this experiment (NaCl and KCl) both boil at around
1420°C.
This experiment started the synthesis at 1200°C and increased to 1300°C before decreasing
back down 1100°C.
1.2. Objectives
The key objectives of this experiment was to:
1. Locate the optimal temperature and hold time of the furnace for the high temperature
synthesis
2. Investigate the effects of changing the composition of the reducing agents and
reactants
3. Check if increasing the hold time would affect the purity of the finished ceramic
powder
4. Identify the average particle size of the purest sample of HfB2 produced
5. Investigate the effects of the acid wash by doing an XRD analysis on both the pre-
wash and post acid wash for an experiment
Personal Objectives of this experiment was to:
1. Gain a clear understanding of sintering processes’ and in particular, Molten salt
synthesis
2. Understand how to interpret the results of the main types of characterisation
techniques including SEM, TEM, EDS and XRD
3. Become familiar with working in a laboratory,
4. Become aware of laboratory safety as well safety of the environment
1.3. Evolution of Project
Initially the main aim of this project was to produce an extremely pure bulk sample of HfB2
through molten salt synthesis. The sample would be characterized using several techniques
such as XRD, SEM and TEM. Once the optimal conditions of the material had been found,
the next part of the project would be to combine this pure Hafnium diboride with Silicon
3
Carbide and test out the mechanical properties of this (such as oxidisation rate, density and
strength).
However the aim of this experiment changed slightly as it was understood that the initial
objectives were a little too ambitious, and the new focus was to try to optimize the reaction
conditions for MSS of phase pure HfB2.
2. Literature review
2.1. Material Properties of HfB2
2.1.1 Thermal Properties
Hafnium diboride and other ultra high temperature ceramics possess extremely good thermal
conductivity. This is advantageous because the leading edges of the hypersonic jet will heat
up due to wind resistance. So a material with good thermal conductivity is required because it
will be able to conduct the heat away from the leading edges to the cooler regions of the
aircraft.
Hafnium diboride has a thermal conductivity of 104WmK-1 and this is significantly higher
than ZrB2 a, which has a thermal conductivity 25-56WmK-1. The thermal conductivity of
HfB2 is higher than most pure metals and similar to the aluminium alloys that are currently
used to build the airframe of commercial jets. It has a relatively low co-efficient of thermal
expansion (6.3x10-6K-1) [1,3-6,15]
2.1.2 Melting Point
During hypersonic flight, the leading edges of the aircraft will experience temperatures of
over 2000°C and therefore a key characteristic of a potential material is a high melting point.
Hafnium diboride’s melting point exceeds this temperature and will start to melt at 3380°C.
This is higher than ZrB2 (which melts at 3200°C) and this is because the bonds, which hold
the atoms together, are extremely strong and require a large amount of energy to break it. [1-
6,15]
a ZrB2 is another UHTC that is being researched as a possible material for the leading edge of hypersonic jets [9]
4
2.1.3 Oxidation Resistance
The one problem with hafnium diboride is that at high temperatures (1000°C -1300°C) it will
begin to oxidise very quickly. At this temperature, the hafnium separates from the boron and
reacts with the oxygen in the air to form a thin HfO2 layer on the surface of the HfB2 layer.
However studies have shown that combining HfB2 with other composites can greatly increase
its oxidation resistance. [1,14,15,17]
2.1.4 Strength Properties
Hafnium diboride possess good strength properties, it has been known to have a bending
strength between 300-500MPa. It is also an extremely hard material, with a Vickers hardness
ranging between 20-28GPa. [15]
Hafnium diborides ability to mix with other composites and increase its strength has also
been proven. For example Sciti et al. showed that by adding as little as 15% (by volume)
TaSi2 to an ultra high temperature ceramic, the flexural strength will be 600MPa even at
elevated temperatures of up to 1500°C. This proves that HfB2, when combined with other
composites, shows good thermal stability. [8]
2.1.5 Density
One of the major negative points of hafnium diboride is the high density of the material.
Hafnium diboride has a density of 10.5g/cm3. This is much higher than other composites and
metal alloys which make up most of todays airframes in jets. For example, Carbon Fibre
Reinforced Plastic has a density of 1.9g/cm3, and most aluminium alloys have densities
ranging from about 2.1g/cm3–3g/cm3. ZrB2 has a much lower density, which is
approximately 6g/cm3. [13,21]
2.1.6 Molecular properties
Hafnium diboride is a ceramic that has an extremely strong covalent bond. This strong
covalent bond makes the molecule structurally stable at high temperatures. It also means that
the molecule will require a large amount of energy to weaken the bond and as a result it has a
high melting point. The atoms are arranged in a crystal lattice and this is the reason why it
has good thermal and electrical conductivity. [15]
5
2.2. An Introduction to Molten Salt Synthesis
Molten salt synthesis is a synthesis technique that used in manufacturing, to produce a
material at elevated temperatures, with molten salt used as medium for the reaction. Normally
the powders are placed in a non-reactive crucible (carbon) and heated, in a furnace, to a
temperature above the melting point of the salts used. Salt is used as a catalyst in this process
because when it melts, it forms an ionised liquid that helps speed up the reaction time. [7,27]
In molten salt synthesis, normally the salt content ratio to reactants is large. This allows a
pool to form around the reactants and allow them to dissolve and react accordingly. When the
hold time at an elevated temperature is completed, the furnace will begin to cool at a set rate.
This rate can also determine the mechanical properties of the finished product. The salt
solidifies separately from the other reactants and can be easily washed out with water. In
molten salt synthesis, this salt that is washed out can be reused and this is one of the most
important advantages of the MSS method if HfB2 is to be mass-produced industrially.
The role of the molten salts as said by Kimura is [7]:
• To increase the reaction rate and lower the reaction temperature
• To control particle size
• To control particle shape
Molten salt synthesis usually occurs at temperatures in the range of 800°C to 1300°C. This is
because that is the range of melting and boiling points of the salts used. NaCl Melts at 801°C
and boils at 1465°C. KCl melts at 770°C and boils at 1420°C. [19,20]
2.3. Previous Research and Synthesis Techniques
Hafnium diboride has been synthesized in many different ways but mostly by the following
methods:
• Solid-State reduction
• Spark Plasma sintering
• Boro/Carbo thermal Reduction
• Sol-Gel Synthesis
6
2.3.1 Borothermal/carbothermal Reduction
HfB2 has previously been synthesised by both borothermal reduction. This method involves
the use of a boron based (sometimes there can be a carbon based) molecule as a reducing
agent in the reaction.
Phase pure hafnium diboride (HfB2) powder has been produced by borothermal reduction of
hafnium dioxide using amorphous boron at relatively low temperature (1600°C) in vacuum.
The synthesized HfB2 powder had an average particle size of 1.37 µm. [14]
Wei-Ming Guoa, Zhen-Guo Yang and Guo-Jun Zhang produced a sub micrometre powder of
HfB2 using borothermal reduction. They produced the powder using a conventional
borothermal route and the average particle size produced was 2.1 µm. In the same experiment
they tested a new borothermal reduction method and managed to produce a fine powder that
had an average particle size of 0.8µm [6]. In another experiment done by Guoa et al., a
hafnium diboride powder was produced that was 1-1.5µm in size. It was synthesised by
borothermal reduction using HfO2 and amorphous boron in a vacuum at 1600°C. [21]
2.3.2 Unconventional method (Sol-‐Gel Synthesis)
Hafnium diboride (HfB2) powder has been synthesized via a sol–gel-based route by
Venugopal et al. using phenolic resin, hafnium chloride, and boric acid as the source of
carbon, hafnium, and boron, respectively. The temperature in this reaction was much higher
than the temperatures used in molten salt synthesis. This method would not be suitable for the
large-scale production of Hafnium diboride. The costs of the raw materials that are used in
this experiment are much higher than the costs of the reactants in this molten salt synthesis
route. Also some of the raw materials are harmful to the environment. Particles of 1.2µm
were achieved at when the pre cursor was heated at 1600°C for 2hrs. [16]
2.3.3 Molten salt synthesis
Almost no research has been published on this technique of the synthesis of Hafnium
diboride. Toshio Kimura conducted research on the “molten salt synthesis of ceramic
powders”, but hafnium diboride was not used in that experiment. [7]
3. Methodology and theory
3.1. Reactions
7
3.1.1 Borothermal reduction
In this project, Hafnium diboride was synthesised using the following reaction process:
𝐻𝑓𝑂! + 𝐵!𝑂! + 5𝑀𝑔 !"#$!!"#
𝐻𝑓𝐵! + 5𝑀𝑔𝑂 (1)
The Mg acts as a reducing agent in the redox reaction. The Mg reduces the Oxidising agent
(HfO2 & B2O3) to become Magnesium oxide (MgO). The reaction takes place in a pool of
molten salt (KCl + NaCl). All the raw materials in this reaction are in a powder form.
3.1.2 Salt Wash
After the reaction takes place in the furnace and it is removed, the salt is washed out by
repeatedly rinsing the material that comes out of the furnace. This process is sped up with the
help of a centrifuge. The procedure is explained in detail in Section 4.
3.1.3 Acid Rinse
After the salt is washed out, the magnesium oxide is still remaining in the sample and so it is
removed using an acid (in this case it was Hydrochloric acid/HCl).
2𝐻𝐶𝑙 +𝑀𝑔𝑂 → 𝑀𝑔𝐶𝑙! + 𝐻!𝑂 (2)
3.1.4 Raw materials
The materials used in this experiment each had a purity rating and it is important to note this
because slight changes in the purity rating of any material may severely alter the results.
• Hafnium (IV) Oxide, HfO2, was 98% pure.
• Boron Trioxide, B2O3, was 99% pure.
• Magnesium, Mg, was >99% pure.
3.2. Research
The research for this project was conducted and collected through a variety of different
sources. Most of the research that was presented in the literature review came from online
journals that were written by researchers. Using the Boolean search tool provided by EBSCO
E-Journals and the University of Exeter, I was able to gain a solid understanding of my
project and I was able to identify my objectives. All research was written down on paper, or
in a logbook and later copied to this document.
8
3.3. Sintering and Molten salt synthesis
3.3.1 An Introduction to Sintering
The furnace used in this experiment consisted of a long quartz tube surrounded by heating
elements. The reaction (in equation 1) takes place inside a non-reactive crucible at elevated
temperatures. The high temperature and pressure inside the carbon based crucible causes the
elements to fuse and form a ceramic. Inside the furnace there is usually a non-reactive gas
that is pumped into the heating chamber. This ensures that the oxygen from the atmosphere
does not react with the magnesium. In this experiment argon gas was used as a non-reactive
gas.
3.3.2 Sintering Process and description
After all the reactants were measured using an electronic scale. They were crushed into a fine
powder using a mortar & pestle and they were placed into a carbon crucible. The crushing
increases the surface area of the particles and aids the reaction process. The crucibles were
wrapped in carbon paper and carefully placed, in the correct order, inside the quartz tube of
the furnace. The quartz tube was then sealed on both ends of the cylinder and checked for
leaks. At one end of the quartz tube, argon gas is pumped in via a pipe, and at the other end
an escape tube is connected to a pipe that has an open end, which is submersed in a bottle of
water. The furnace is sealed when the bubbles of the argon gas escape using the water route.
The furnace is then ready to be switched on. The hold time, hold temperature and heating
rate are all set using the control panel and the machine is turned on.
Once the temperature reading inside the furnace has cooled down to a safe temperature, it is
acceptable to open the seals on each side of the quartz tube. It is important to remember the
order that the samples are placed in and ensure that no mistakes are made.
3.3.3 Preparation for Molten Salt synthesis
In this experiment both KCl and NaCl were used. The ratio of the composition of salts was
50:50. In MSS as mentioned before, the molten salt content needs to be much larger than the
rest of the samples. The total mass of all the reactants in this experiment was between 4g-4.5g
and the total mass of the salts was 20g. Therefore the salt to reactant ratio was 20:4 or 5:1.
9
3.4. X-‐Ray Diffraction
X-Ray diffraction is a characterisation method that can be used to evaluate the chemical
composition of a sample of material.
The advantages of using XRD is that it is a relatively cheap method to use, and more
importantly X-Rays are not absorbed by air so there is no need to create a vacuum. XRD does
not work well with lighter elements, but that is not a problem in this experiment. [23]
An XRD machine works by sending a beam of X-Rays into the layer of the prepared sample
at an angle. There is a detector that is placed on the opposite side, which detects the angle at
which the X-ray beams diffract. The detector measures the angle at which the X-rays diffract,
and the intensities of the waves at these peaks are also recorded. [23]
Diffraction occurs when a wave of the electromagnetic spectrum passes through a slit that is
similar in size to the wavelength of the wave. X-ray wavelengths are similar in length to the
distance between atoms and layers in a crystal structure and so diffraction can occur. The
diagram below shows how the diffraction works. The beam is fired at an angle of incidence,
and the detectors measure the reflected angle as seen in Figure 1 below.
Figure 1 shows the how the x-rays diffract in a crystal structure [28]
Using Bragg’s equation, it is possible to be able to calculate the space (d) between the layers
of atoms in a crystal structure. Bragg’s equation is shown below.
2𝑑 sin𝜃 = 𝑛𝜆 (3)
3.4.1 Preparation of the samples for XRD
After the samples had been washed by water, acid and then dried in an oven, they were
crushed into a fine powder by the mortar & pestle. A small area of the fine powder was
carefully placed on a transparent glass disk and pressed gently to form a thin flat circular
sample. This was placed in the XRD machine for further analysis.
10
3.5. SEM
SEM/Scanning electron microscopy uses a beam of electrons to view objects on a very small
scale. It is able to magnify several thousand times more than an optical microscope because it
uses the miniscule de Broglie wavelength of an electron to create an image of the contours of
tiny particles. High-energy electron beams are used to energise the specimen and the signals
that are given off are detected and analysed so that an image can be constructed. [24]
3.6. EDS
EDS or energy-dispersive X-ray spectroscopy is another materials characterisation technique
and this was used in this experiment in conjunction with SEM. The EDS in this experiment
used high-energy focused electrons to excite the electrons within the materials sample. The
electrons from the high-energy beam replace the electrons in the shell of the sample and this
gives of energy in the form of X-Rays. These X-rays are detected using an X-ray detector and
characteristics of the X-rays given off; provide details with what materials are contained
inside the sample.
3.7. TEM
Transmission electron microscopy is a method of characterisation that (like SEM) uses a
beam of electrons rather than light to magnify the image. It is different to SEM because TEM
sends electrons through a specimen and the resultant image is then magnified onto an
imaging device. TEM happens in a vacuum and therefore the sample needs to be prepared
carefully. The preparation for TEM is a very complex procedure because the sample has to be
only a few nanometres thick. TEM can produce a much sharper image than even SEM and in
this experiment it will be used to try and identify the crystal structure. [25]
4. Experimental work
4.1. Procedure
4.1.1 Preparing each sample
1. All of the reactants were weighed using a scale, which was accurate to 1/1000th of a
gram.
2. The material was scooped out of each bottle using disposable micropipettes that had
11
been cut at the thick end to form a spoon shape. It is important to avoid contamination
of the base materials by regularly changing the disposable micropipettes when
changing from measuring one material to another.
3. The powdered materials were weighed in a disposable plate. The scale was set to zero
each time after the empty plate is placed on the scale.
4.1.2 Heating the samples
1. Place the sample in a carbon crucible and place in the centre of the furnace.
2. Seal the furnace and pump in argon gas to stop the Mg from reacting with the oxygen
in the air.
3. Select the maximum temperature in the furnace and the required hold time.
4. Set the temperature to increase slowly at a rate of 5°C per minute until it reaches the
predetermined maximum temperature.
5. Once cooled down, use a rod to push out the carbon crucibles from the other side.
4.1.3 Washing the salt
1. The by-product of the heated sample contains many impurities. One is of these is the
salt and the other is Magnesium Oxide. To remove the salt, remove all the powdered
material from the carbon crucible and place it in a beaker.
2. Add 100ml of water in the beaker and shake well.
3. The Hafnium Diboride and Magnesium Oxide are insoluble in water so they will not
dissolve in the water and be washed away.
4. Place the beaker in an ultrasonic tub of liquid, which sends a high frequency through
the beakers and helps to separate the solution from the insoluble particles. Switch the
machine on and leave to mix for 30-50 minutes.
5. Take the beaker out of the ultra sonic and leave it to rest for several hours. The
particles that cannot dissolve in the water will settle at the bottom of the beaker.
6. Pour out as much water as possible and empty all the remaining contents into a 50ml
centrifugal bottle.
7. Fill the centrifugal bottle and place it in the centrifuge. Turn on the machine for 9
12
minutes at 6000 rpm.
8. Pour out the water, and repeat step 7 a minimum of 3 times.
4.1.4 Acid Rinse and Dry
1. Pour the contents of the salt wash into a 250ml bottle.
2. Pour about 200ml of Hydrochloric acid into the 250ml bottle. Close the lid and shake.
3. Place the bottle in the ultrasonic for 60 minutes.
4. Unscrew the cap slightly (to ensure there is no pressure build up from the reactions)
and leave to settle overnight
5. Repeat the salt wash steps to remove the acid and reduce the PH levels.
6. After the final salt wash, place the samples in an oven at 80°C until dry
4.2. Calculating the mass of each reactant in the sample
The mass of each reactant remained the same throughout the experiment (except experiment
5) but the initial calculation was based on the number of moles required of each reactant.
The total mass of the three reactants HfO2, B2O3 and Mg were supposed to add up to 4g in
this experiment. The ratio of moles required for these reactants were 1:1:5 (From equation 1).
Based on this information, the amount of mass of each reactant can be calculated. The first
step is to calculate the mass of 1 mole of each substance, and this is:
• 210.49g for HfO2
• 69.59g for B2O3
• 24.31g for Mg
But Mg needs 5 moles for every 1 mole of the other reactants. So Mg = 121.55g (5 Moles).
4.2.1 Example calculation
To calculate the mass of each reactant required for each sample, the following formula was
used:
𝑟.𝑁𝑅𝑁×100
13
Where N is the number of moles associated with each reactant and R is the mass of 1 mole of
a single reactant in grams.
For example the total mass of Mg required was calculated as shown below.
(24.31×5)210.49×1 + 69.59×1 + (24.31×5) × 100
This gives 30.3%. So Mg will be 1.21g (30.3% of 4g).
Using this formula the other two reactants were calculated and shown below.
HfO2 = 2.09g and B2O3 = 0.693g
4.2.2 Volati l ity of reducing agents
When the reactants are reacting inside the furnace, in the pool of liquid salt, some of the
magnesium and some of the boron trioxide will be vaporised and lost. This is because both
Mg and B2O3 are highly volatile [17,18]. To test this hypothesis, extra Mg and B2O3 were
added in each batch. In each experiment (except experiment 5) there were 3 samples used per
batch, the table below shows how much of each reactant was used in each experiment.
Sample A corresponds to the first sample in each experiment. For example in experiment 2,
sample A corresponds to sample 6, and sample B corresponds to sample 7.
Sample HfO2 B2O3 Mg
A 2.09g 0.693g 1.21g
B 2.09g 0.9g (+30%) 1.21g
C 2.09g 0.9g (+30%) 1.45g (+20%)
The salt content remained constant throughout the experiment and a combination of both
NaCl and KCl was used. For each experiment, 10g of NaCl and 10g of KCl was used.
5. Results
5.1. Experiment 1 (Samples 1,2,3)
14
5.1.1 Init ial Conditions
The temperature of the furnace was set at 1200°C and the only variable between the 3
samples was the composition of the reducing agents. The furnace heated up from room
temperature to 1200°C at a rate of 5°C /min. The hold time was set to 6 hours, and once this
was done, the furnace was allowed to cool naturally.
Sample
No. HfO2 B2O3 Mg NaCl KCl Temperature Hold Time
1 2.0921 0.6943 1.2125 10.021 10.035
1200°C 6 hr. 2 2.0913 0.905 1.213 10.017 10.031
3 2.0943 0.904 1.452 10.013 10.033
5.1.2 Observations
In the first experiment there was a slight problem with one of the samples. When I removed
sample number 2 from the oven, it was cracked. Some of the material had been lost but there
was enough of the sample left inside the crucible to characterise.
5.1.3 XRD Results
In each experiment, the sample that showed the most promising results has the largest image.
Figure 2 (left) shows the XRD results of sample 1. (Right) shows the XRD results of sample 2.
Position [∞2Theta] (Copper (Cu))
10 20 30 40 50 60 70
Counts
0
10000
20000
30000
PdB1
Position [∞2Theta] (Copper (Cu))
10 20 30 40 50 60 70
Counts
0
10000
20000
30000
40000
PdB2#
HfB2
HfO2
HfB2
HfO2
15
Figure 3 Shows the XRD results of sample 3.
The XRD Results show that the third sample produced the most pure HfB2 out of the 3
samples tested. It had the largest amount of pure HfB2 and the least amount of HfO2.
In conclusion, at a temperature of 1200°C and a hold time of 6 hours, the best sample
contained 20% extra magnesium and 30% extra boron dioxide.
5.2. Experiment 2 (Samples 4,5,6)
5.2.1 Init ial Conditions
In the previous experiment, the carbon crucible that contained sample 2 had cracked. It was
caused because one of the samples got stuck inside the quartz tube, and the rod applied too
much force, which cause it to break. To counter this problem, the carbon crucibles were
wrapped in carbon paper. This would ensure that the force from the rod would be distributed
evenly over the carbon paper.
The temperature of the furnace was set at 1300°C and the only variable between the samples
was the composition of the reducing agents. The furnace increased its temperature from room
temperature to 1300°C at a rate of 5°C /min. The hold time was set to 6 hours, and once this
was achieved, the furnace was cooled naturally.
Position [∞2Theta] (Copper (Cu))
10 20 30 40 50 60 70
Counts
0
20000
40000
60000
3
HfB2
HfO2
16
Sample No.
HfO2 B2O3 Mg NaCl KCl Temperature Hold Time
4 2.093 0.6931 1.2133 10.0143 10.0018
1300°C 6 hr 5 2.0935 0.9017 1.235 10.0013 10.0101
6 2.0955 0.9011 1.454 10.0012 10.0167
5.2.2 XRD Results
Figure 4 (left) shows the XRD results for sample 4. (right) shows the XRD results for sample 5.
Figure 5 Shows the XRD results for sample 6.
Position [∞2Theta] (Copper (Cu))
10 20 30 40 50 60 70 80
Counts
0
20000
40000
4
Position [∞2Theta] (Copper (Cu))
10 20 30 40 50 60 70 80
Counts
0
10000
20000
30000
40000 5
Position [∞2Theta] (Copper (Cu))
10 20 30 40 50 60 70 80
Counts
0
20000
40000
60000
6
HfB2
HfO2
HfB2
HfO2
Kotoite
HfB2
HfO2
17
Once again the third sample (which contained 20% extra Mg and 30% extra B2O3) in the batch produced the best results. The highest peak was at 42.13° and the height of the peak was 55901 counts.
5.3. Experiment 3 (Samples 7,8,9)
5.3.1 Init ial Conditions
The aim of Experiment 3 was to investigate the effect of increasing the hold time. The
previous experiments results, where the sample was heated to 1300°C and held for 6 hours
produced a sample of HfB2, which was less pure than the first experiment of 1200°C for 6hr.
So for this next experiment the temperature remained the same, but the hold time was
increased to 12hrs. The aim of this experiment was to check if increasing the hold time would
increase the purity of HfB2.
Sample No.
HfO2 B2O3 Mg NaCl KCl Temperature Hold Time
7 2.0953 0.694 1.2138 10.0182 10.0272
1300°C 12 hr 8 2.0975 0.906 1.245 10.0063 10.0233
9 2.0945 0.9042 1.4537 10.0202 10.0212
5.3.2 XRD Results
Figure 6 (left) shows the XRD results for sample 7. (right) shows the XRD results for sample 8.
Position [∞2Theta] (Copper (Cu))
10 20 30 40 50 60 70 80
Counts
0
20000
40000
60000
7#
Position [∞2Theta] (Copper (Cu))
10 20 30 40 50 60 70 80
Counts
0
20000
40000
8#
HfB2
HfO2
HfB2
HfO2
18
Figure 7 Shows the XRD results for sample 9
For this experiment, the sample with the highest purity was sample 9. This sample had 20%
extra MgO and 30% extra Boron dioxide. There was very little Hafnium dioxide left over in
sample 9, but there was a small amount of MgO found. This was probably because the acid
wash was not thourough enough. The Hafnium dioxide was still not fully erradicated and so
for the next experiment the temperature was lowered. In conclusion, the results showed that
increasing the hold time did infact increase the purity of HfB2.
5.4. Experiment 4 (10,11,12)
5.4.1 Init ial Conditions
In the previous experiment, the sample produced was still not pure even though the hold time
was increased and there was a possibility that this was occurred because the temperature was
set too high in the furnace. 1300°C is extremely close to the boiling point of both salts used
throughout this experiment (both have a b.p. of around 1400°C), so there is a possibility that
some of the liquid salt was evaporating and escaping from the carbon crucible. So for this
next experiment, the temperature of the furnace was reduced to 1100°C and the hold time set
back to 6 hours as in experiment 1.
Position [∞2Theta] (Copper (Cu))
10 20 30 40 50 60 70 80
Counts
0
20000
40000
60000
9#
HfB2
HfO2
MgO
19
Sample No.
HfO2 B2O3 Mg NaCl KCl Temperature Hold Time
10 2.0933 0.6935 1.2127 10.0051 10.0036
1100°C 6 hr 11 2.0942 0.9042 1.2123 10.0014 10.004
12 2.094 0.9012 1.4531 10.0049 10.0013
5.4.2 Notable Observations
During the acid wash, the reaction of the metal oxide and the acid creates bubbles and if the
bottle is sealed shut; there will be a build of pressure inside the bottle. In this experiment,
sample number 12’s bottle was shut too tightly and left overnight to react and there was a
large build up of pressure in the bottle. The next morning, the bottle had exploded and the
entire sample could not be recovered. No one was harmed during the process and the senior
PhD students in the laboratory cleaned the acid.
5.4.3 XRD Results
Figure 8 Shows the XRD results of sample 10
Position [∞2Theta] (Copper (Cu))
10 20 30 40 50 60 70 80
Counts
0
10000
20000
30000
17#
HfO2
HfB2
20
Figure 9 Shows the XRD results of sample 10
In this experiment, the hafnium diboride powder was successfully produced however the
purity of the sample used in the XRD was low. There was a large amount of hafnium dioxide
found in the sample and this was probably occurring because at this lower temperature the
ceramic powder did not have enough energy to fuse efficiently. In conclusion, this
temperature and hold time produced the worst results so far. A large amount of hafnium
dioxide was found in the finished product.
5.5. Experiment 5 (13,14,15,16)
5.5.1 Init ial Conditions
In this experiment the objective was slightly different to the previous ones. The aim of this
experiment was to investigate the effects of changing the mass of one of the reactants. In this
case 4 samples were prepared (13-16) and in sample 13 the normal amount of B2O3 was
added. In the samples after that, (i.e. samples14, 15,16) the B2O3 was altered with 15%, 30%
and 45% of extra B2O3 added. The Mg content remained constant in all the samples at 1.45g.
The temperature of 1100°C and hold time of 6 hours was chosen again because sample 12
was destroyed in the previous experiment. So for this experiment, sample 15 has the exact
same initial conditions of sample 12 from the previous experiment and could be used as a
comparison.
Position [∞2Theta] (Copper (Cu))
10 20 30 40 50 60 70 80
Counts
0
10000
20000
30000
40000 18#
HfO2
HfB2
21
Sample No.
HfO2 B2O3 Mg NaCl KCl Temperature Hold Time
13 2.0951 0.6938 1.4537 10.0185 10.00235
1100°C 6 hr 14 2.0905 0.7963 1.4537 10.003 10.005
15 2.0977 0.904 1.4532 10.0036 10.0048
16 2.094 1 1.4547 10.0034 10.001
5.5.2 XRD Results – Pre Acid Wash
Figure 10 Shows the pre-acid wash XRD results of sample 13
Position [∞2Theta] (Copper (Cu))
10 20 30 40 50 60 70 80
Counts
0
10000
20000
30000 q13#
HfB2
HfO2
MgO
22
Figure 11 Shows the pre-acid wash XRD results of sample 14
Figure 12 Shows the pre-acid wash XRD results of sample 15
Position [∞2Theta] (Copper (Cu))
10 20 30 40 50 60 70 80
Counts
0
10000
20000
30000
q14#
Position [∞2Theta] (Copper (Cu))
10 20 30 40 50 60 70 80
Counts
0
10000
20000
30000
40000
Q15#
HfB2
KCl
MgO
Ag 3SI
HfB2
HfO2
MgO
23
Figure 13 Shows the pre-acid wash XRD results of sample 16
5.5.3 XRD Results -‐ Post Acid Wash
Figure 14 Shows the post-acid wash XRD results of sample 13
Position [∞2Theta] (Copper (Cu))
10 20 30 40 50 60 70 80
Counts
0
10000
20000
30000
40000 q16#
Position [∞2Theta] (Copper (Cu))
10 20 30 40 50 60 70 80
Counts
0
10000
20000
13#
HfB2
HfO2
MgO
HfB2
HfO2
Mg3(BO3)2
24
Figure 15 Shows the post-acid wash XRD results of sample 14
Figure 16 Shows the post-acid wash XRD results of sample 15
Position [∞2Theta] (Copper (Cu))
10 20 30 40 50 60 70 80
Counts
0
10000
20000
30000
14#
Position [∞2Theta] (Copper (Cu))
10 20 30 40 50 60 70 80
Counts
0
10000
20000
30000
15#
HfB2
HfO2
HfB2
HfO2
Mg3(BO3)2
25
Figure 17 Shows the post-acid wash XRD results of sample 16
5.5.4 Pre –acid conclusion
The XRD results of the pre acid wash show that each sample had excessive amounts of
magnesium oxide. This was expected (as seen in equation 1) and this is the reason why
Hydrochloric acid will be required in the manufacturing process of HfB2. The XRD machine
detected a small amount of Silver sulphide iodide in sample 15 but this is probably an error in
the peak readings, or it is just a contamination and it can be ignored.
5.5.5 Post acid conclusion
After the acid wash, all the samples had the magnesium oxide completely removed from the
XRD results, showing that the acid wash is effective. However sample 13,15 and 16 all
contained a small amount of Magnesium borate (Mg3 (BO3) 2). Sample 14, which had 15%
extra B2O3, contained only HfB2 and HfO2. This does not however mean that it was the most
pure out of all of the samples. Sample 16 had the highest peaks of HfB2 and as explained by
Zhou Jian and Wang Hejing in their experiment ‘The Physical Meanings of 5 Basic
Parameters for an X-Ray Diffraction Peak and their Applications’, the peak intensity
determines the amount of phase in the mixture. [11] In conclusion, the results showed that
this temperature of 1100°C was less effective at producing pure hafnium diboride than the
higher temperatures of 1200°C and 1300°C. The results also showed that adding 45% extra
B2O3 increased the purity of HfB2.
Position [∞2Theta] (Copper (Cu))
10 20 30 40 50 60 70 80
Counts
0
10000
20000
30000
40000
16#
HfB2
HfO2
Mg3(BO3)2
26
5.6. XRD Conclusion
The results of the XRD graphs showed that within each experiment, the sample with the most
amounts of B2O3 and Mg produced the purest HfB2. Based on the results of XRD graphs
only, Sample 9 produced the best results, it had 3 large peaks of HfB2 with the maximum
peak at 42.13° and a peak intensity height of 60689 counts. Sample 3 showed the second best
results, its maximum peak was at 42.13° and the intensity was 49611 counts. Sample 6 had
higher peaks of HfB2 than sample 3 but it contained much more hafnium dioxide so it was
deemed less pure.
5.7. SEM, TEM and EDS
The next step in this is experiment is to determine the particle size of the ceramic powder.
The way to do this would be using SEM and TEM. SEM and TEM cannot be done on all the
samples because that would be a waste of money and time. Instead the results of the XRD
graphs were used to determine which samples were the best in each group.
5.7.1 SEM and EDS Analysis
The SEM results confirmed that HfB2 was created and that it was indeed a fine powder.
Sample 3 of experiment 1 can be seen below.
Figure 18 Shows the SEM image of sample 3 (1200°C/6hrs) [A] magnified 2000x [B] magnified 8000x.
The results show that Hafnium diboride was successfully produced and image B shows that
the particles of the powder were very fine, with most of the particles of powder sized in the
range of 0.5µm - 2.5µm. Image A had an EDS analysis completed on this section and the
27
results can be found below. The EDS confirms that the white cloudy substance in the image
consists of mostly hafnium, boron and oxygen. The results can be seen in table 1 below.
Spectrum In stats. B O Hf Au Total
Spectrum 1 Yes 4.91 4.81 73.01 17.27 100.00
Spectrum 2 Yes 5.32 5.31 76.57 12.80 100.00
Mean 5.12 5.06 74.79 15.04 100.00
Std. deviation 0.29 0.35 2.52 3.16
Max. 5.32 5.31 76.57 17.27
Min. 4.91 4.81 73.01 12.80
Table 1 Shows the EDS Results from the scan of image A in figure 18
As expected there is both hafnium and boron detected in this sample. There is a little oxygen, which could come from the hafnium dioxide. There was an unusual amount of Gold found in both spectrums that were analysed. This gold is an impurity that has crept in during some stage in the manufacturing process.
5.7.2 TEM Analysis
A scan was completed using TEM and this was done on sample 3 (1200°C/6hrs) and the results can be seen below.
Figure 19 shows where the SEM image was scanned and tested by EDS
28
Figure 19 shows the TEM scan of sample 3 (A) magnified 125000x. (B) magnified 251000x
Figure 20 Shows a magnified TEM scan of sample 3 magnified 1250000x
From figure 19 and 20, it can be seen that the particles are extremely fine (10-9m). On average, the particle size of the powder is between 40-70 nm. The darker regions correspond to more layers in the crystal structure and similarly the lighter regions correspond to fewer layers in the crystal structure.
This result is unusual and many thousand times small than the results of the SEM scan. In the literature review section, there was no other synthesis technique that produced a powder this fine. In several different experiments, the powders of HfB2 produced were always measured in micrometres and not nanometres.
29
6. Discussion and conclusions
6.1. Summary
A fine power of hafnium diboride has been successfully produced using high temperature
molten salt synthesis. The fine ceramic powders purity depended on the initial conditions of
the experiment.
From the list of objectives described in the introduction a summary of the report can be
concluded:
1. The optimal sintering conditions for this synthesis was located and it was at 1300°C
for 12 hours
2. Changing the composition of the reactants had an effect on the purity rating of the
hafnium diboride. It was shown that a 20% increase in Mg content, and a 30%
increase in B2O3 increased the purity of HfB2.
3. At 1300°C, increasing the hold time from 6 hours to 12 hours reduced the amount of
unwanted hafnium dioxide left in the finished sample.
4. The powders particle size for HfB2, as shown by the SEM image of sample 3, ranged
between 0.5µm - 2.5µm.
5. Hydrochloric acid was an effective in the removal of magnesium oxide.
6.2. Optimal Conditions
6.2.1 Temperature and Hold t ime
The optimal temperature for molten salt synthesis in this experiment was 1300°C with a hold time of 12 hours. This was based on the results of the XRD, however an SEM and TEM scan still needs to be completed on this sample in order to fully confirm this. In this experiment the lowest temperature of the furnace (1100°C) produced the least pure HfB2. The results showed that increasing the temperature and hold time increases the purity of HfB2.
6.2.2 Reactants ratio
The reactant ratio of 1:1:5 between HfO2: B2O3: Mg was correct as suggested in the literature. However increasing the B2O3 and Mg slightly content did prove to be effective. 20% extra Mg produced a more pure ceramic powder than just the normal amount of Mg. A 30% increase in B2O3 also increased the purity of hafnium diboride for all temperatures in the experiment. These extra additions of the reactants were necessary due to their high volatility as discussed in the literature review, and this was expected.
30
6.3. Further research
In this experiment there is still plenty of work to be completed. A test should be conducted to try and find the optimal amount of Mg required. This test will be similar to experiment 5 which altered the content of boron trioxide between each sample.
There should have also been a TEM and SEM scan of sample 9 (1300°C/12hrs) that was created in this experiment, because the XRD graphs showed that it produced the most phase pure hafnium diboride. It was not completed due to the time constraints of this project.
A much more detailed test should be conducted to try and locate the optimal hold time.
A similar test can also be done using temperatures between 800°C and 1100°C.
7. Project management, consideration of sustainability and health and safety
7.1. Project Management
This project lasted a full academic year and it would have been quite easy to manage my time
ineffectively. To prepare for this, I used the project management knowledge I gained from the
second year module “Management and Management Science (ECM2102)”, and created a
Gantt chart on Microsoft Project.
According to my proposed Gantt chart, I was required to complete the synthesis of 1
experiment within 1 week of January 12th. However, the synthesis of the first experiment
took 2 full weeks and I was already behind schedule for the second experiment. To counter
this, a few hours was spent just trying to optimise the synthesis process and managing my
other work around this project. The initial Gantt chart that was proposed at the beginning of
the experiment can be seen below:
31
Figure 21 Shows the initial Gantt chart that was used to plan the project.
For my research, my experimental results and my meetings with supervisor/PhD students, I
used a hardcover logbook. The logbook contained most of the important information that was
used in this experiment and helped me keep a record of the results of each experiment as I
completed it.
Other management concerns included data protection. In other words what would happen if a
virus or a computer malfunction suddenly corrupted and destroyed the file. To counter this,
the word document for this project was saved on an external hard drive, Microsoft one drive,
drop box and saved on the desktop of a laptop. These files were updated daily.
7.2. Considering the Environment
One of the main reasons why molten salt synthesis is used in the experiment is because the
salt can be recovered and recycled. The water that is thrown out during the washing process
can be easily distilled, and the salt can be collected. The water that is recovered at the
distillation process can be re-used to wash the next batch of samples, as it will be pure.
The hydrochloric acid in this experiment reacts with the magnesium oxide to form
magnesium chloride and water. Magnesium chloride (MgCl2) is a salt typically known as an
ionic halide [26]. It is extremely soluble in water. The magnesium chloride that is the by-
product of this reaction can be easily separated from the water using distillation and it can
either be sold, or it can be electrolysed to give a pure magnesium metal as shown below:
𝑀𝑔𝐶𝑙!(!) → 𝑀𝑔(!) + 𝐶𝑙!(!)
32
Since Magnesium is used as a reactant it is extremely advantageous that it can be recycled.
This is also extremely important when considering the global environmental impact.
Chlorine gas is used extensively in industry, during the manufacturing process of certain
plastics. So it will also not be wasted and could also potentially be sold to another industry.
If further research is conducted, and this manufacturing process is optimised, then molten salt
synthesis would be an extremely good candidate as the chosen method that manufacturers
will use in industry because a large amount of the raw materials can be recycled.
7.3. Health and Safety Risk Management
Health and safety was taken very seriously in this project. Most of the experimental work took place inside a laboratory where there were several high temperature furnaces, high-voltage machines, and plenty of harmful chemicals stored inside this room. Other than that, there were also many experiments being conducted by other students, which involved the use of strong acid and other harmful chemicals. In order to prepare for all risks in this environment, a risk assessment form was completed and it is shown below in Table 2.
ID Risk item Effect Cause
Like
lihoo
d
Seve
rity
Impo
rtan
ce
Action to minimise risk
1 Use of HCl to
remove the MgO from the HfB2
Chemical burns,
Eye Injuries
Spillage 3 8 24
Wearing the appropriate safety
equipment (Personal Protective
equipment PPE)
2 Taking the carbon crucible out of the
furnace
Burns
Pressurised shut, can
explode when de pressurising
2 9 18
Wearing the appropriate safety
equipment (Personal Protective
equipment PPE)
3 Cables, Obstacles Spilling chemicals Tripping 3 4 12
Staying alert, clearing the
workspace before usage
4 Electrocution Electric shock - Death
Ultrasonic, Furnace, XRD 1 10 10
Wearing rubber soled shoes, taking extra precautions
Table 2
33
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[2] Zhang, G., Guo, W., Ni, D. and Kan, Y. (2009). Ultrahigh temperature ceramics (UHTCs) based on ZrB 2 and HfB 2 systems: Powder synthesis, densification and mechanical properties. J. Phys.: Conf. Ser., 176, p.012041.
[3] Brown-Shaklee, H., Fahrenholtz, W. and Hilmas, G. (2010). Densification Behavior and Microstructure Evolution of Hot-Pressed HfB2. Journal of the American Ceramic Society, 94(1), pp.49-58.
[4] Deligoz, E., Colakoglu, K. and Ciftci, Y. (2010). Lattice dynamical and thermodynamical properties of HfB2 and TaB2 compounds. Computational Materials Science, 47(4), pp.875-880.
[5] Fahrenholtz, W., Hilmas, G., Talmy, I. and Zaykoski, J. (2007). Refractory Diborides of Zirconium and Hafnium. Journal of the American Ceramic Society, 90(5), pp.1347-1364.
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[11] Jian, Z. and Hejing, W. (2003). The physical meanings of 5 basic parameters for an X-ray diffraction peak and their application. Chinese Journal of Geochemistry, 22(1), pp.38-44.
[12] Zhang, S., Hilmas, G. and Fahrenholtz, W. (2007). Pressureless Sintering of ZrB2-SiC Ceramics. Journal of the American Ceramic Society, 91(1), pp.26-32.
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34
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[17] Shugart, K., Liu, S., Craven, F. and Opila, E. (2014). Determination of Retained B 2 O 3 Content in ZrB 2 -30 vol% SiC Oxide Scales. Journal of the American Ceramic Society, 98(1), pp.287-295.
[18] Kim, J., Maeda, M., Zhao, Y., Shi, D., Dou, S., Choi, S. and Kiyoshi, T. (2008). In situ processed MgB2 conductors: Core densification due to mechanical deformation. Physica C: Superconductivity, 468(15-20), pp.1813-1816.
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