Nanomaterials Monitoring Reactions

Syracuse University Chemists have designed a nanomaterial that changes colour when it interacts with ions and other small molecules during a chemical reaction which enables them to monitor the progress of chemical reactions qualitatively with the naked eye and quantitatively using simple instruments.

Many chemical reactions that occur in aqueous solution involve colourless species. In order to determine how fast the chemical reaction occurs,  Chemists have traditionally tried to "freeze" the reaction at certain points, purify the solution and determine the amounts of unreacted reactants and products produced present at each stage.
Syracuse University Chemists have taken a different route. They are using nanoparticles that react with the byproduct of a reaction. The nanoparticles they used are known as perovskites.
Perovskites are typically composed of metal ions and oxygen. The structure shown below is for a typical perovskite, calcium titanium oxide (CaTiO₃):

Each pale-blue titanium atom is surrounded by 6 red oxygen atoms. The darker-blue calcium atom occupies the space between titanium oxide octahedrons.
The perovskites the researchers used were a bit different to the one shown above. Metal ions were surrounded by halide ions rather than oxygen.
At the nanolevel, perovskites are photo-luminescent, that is, they emit light when "excited" by a laser or a lamp. The colour they emit is largely determined by the concentration of their ions in solution., and it is this property which the researchers used to monitor chemical reactions. It is also this property which is being in exploited in research into light emitting diodes (LEDs), lasers, photodetectors and solar cells.

In this study, perovskites were used to monitor an elimination reaction in which haloalkanes react to form alkenes, eliminating halide ions in the process.
At the start of the reaction, the perovskite fluoresces red.
As the reaction proceeds, halide ions are released which are absorbed by the perovskite nanoparticles, and the fluorescence colour changes from red to yellow to green.
When the fluorescence colour is green, the reaction is over.
The image on the right shows a control colour on the left, and on the right, the changing fluorescence colour of the reaction as it proceeds from 0 minutes at the top to 90 minutes at the bottom.

This technology is patent-pending at the University. In the words of Matthew Maye, Associate Professor of Chemistry, "Who knows, maybe in the future, every chemist will use a Syracuse-based perovskite for monitoring their reactions."

Reference:
Tennyson L. Doane, Kayla L. Ryan, Laxmikant Pathade, Kevin J. Cruz, Huidong Zang, Mircea Cotlet, Mathew M. Maye. Using Perovskite Nanoparticles as Halide Reservoirs in Catalysis and as Spectrochemical Probes of Ions in Solution. ACS Nano, 2016; DOI: 10.1021/acsnano.6b00806

Further Reading:
Nanotechnology: http://www.ausetute.com.au/nanotech.html
Reaction Rate: http://www.ausetute.com.au/reactrate.html
Ligands and Complex Ions: http://www.ausetute.com.au/ligands.html
Naming Haloalkanes: http://www.ausetute.com.au/namhaloa.html
Naming Alkenes: http://www.ausetute.com.au/namsenes.html
Substitution Reactions of Haloalkanes: http://www.ausetute.com.au/rxreacts.html
Dehydration of Alkanols: http://www.ausetute.com.au/dehydraol.html

Suggested Study Questions:

1. Explain the terms "qualitative" and  "quantitative".
2. Explain the term "reaction rate".
3. Explain the term "nanoparticle".
4. What property of nano-perovskite is being applied by the researchers in this article, and how does this property differ for bulk perovskite?
5. Explain how these perovskites can be used to monitor the reaction qualitatively.
6. Explain how you could use these perovskites to monitor the reaction quantitatively.
7. Discuss the differences between ethane, ethene (ethylene) and bromoethane.
8. Consider ethane and ethene (ethylene), which is likely to be more chemically reactive? Explain your answer.
9. Consider ethane and bromoethane. Which is likely to be more chemically reactive? Explain your answer.
10. Explain what is meant by the term "elimination reaction" as used in the article above.
11. What is the difference between and addition reaction, a substitution reaction and an elimination reaction? Give examples of each type of reaction.
12. Write a chemical reaction to represent the elimination of bromide ions from a bromoethane to produce ethene (ethylene). 
13. Consider the structure of CaTiO₃ given in the article. What is the name of the ligand?
14. Give the formula for the perovskite in which all the oxygen atoms have been replaced with bromine.
15. Could the same perovskite be used to monitor a chemical reaction in which water is eliminated from an alkanol to produce an alkene? Explain your answer.

Borospherene

A molecule containing 60 carbon atoms in a cage-like spherical shape was first produced in 1985 and was called buckminsterfullerene, or bucky-ball. The structure is like a soccer ball, made up of 20 hexagons and 12 pentagons.
A bucky-ball is shown on the right. Each blue sphere represents a carbon atom, and each cream-coloured line represents a covalent bond between 2 carbon atoms.
One of the reasons that scientists are very interested in buckminsterfullerene is because of its ability to hold atoms of different elements inside the cage-like structure. This could enable bucky-balls to be used to deliver drugs in the body, or to store atoms such as hydrogen.

In 1991, scientists discovered that carbon atoms can also form nanotubes, and in 2004, sheets of carbon atoms just 1 atom thick known as graphene were discovered.

But can atoms other than carbon make these kinds of 3-dimensional networks at the nanometre level?

Researchers from Brown University, Shanxi University and Tsinghua University in China have shown that a cluster of 40 boron atoms forms a hollow molecular cage similar to a carbon buckyball. It's the first experimental evidence that a boron cage structure does indeed exist.
This boron cage, called borospherene, isn't quite as spherical as its carbon cousin. Rather than a series of five- and six-membered rings formed by carbon, borospherene consists of 48 triangles, 4 seven-sided rings and 2 six-membered rings. Several atoms stick out a bit from the others, making the surface of borospherene somewhat less smooth than a buckyball.

Because of the electron deficiency of boron, borospherene is likely to bond well with hydrogen. So these tiny boron cages could serve as safe houses for hydrogen molecules.

Reference:
Brown University. "Researchers discover boron 'buckyball'." ScienceDaily. ScienceDaily, 13 July 2014. .

Further Reading:
Graphene
Molecular Formula
Allotropes

Suggested Study Questions:

1. Write the molecular formula for buckminsterfullerene given the information in the article above.
2. How many covalent bonds does each carbon atom in buckminsterfullerene make?
3. Do you expect buckminsterfullerene to be soluble or insoluble in water? Explain your answer.
4. Draw a representation of graphene.
5. How many covalent bonds does each carbon atom make in graphene?
6. Do you expect graphene to conduct electricity? Explain your answer.
7. Write the molecular formula for borospherene based on the information provided in the article.
8. In the pictorial representation of borospherene given above, what do each of the following represent:
   • red spheres
   • yellow lines
9. In what ways are the structures of bucky-balls and borospherene similar?
10. In what ways are the structures of bucky-balls and borospherene different?

World's Lightest Material?

UC Irvine, HRL Laboratories and the California Institute of Technology have announced that they have succeeded in making the world's lightest material, with a density of 0.9 mg/cm³.

This material is made up of a metallic lattice of interconnected hollow tubes with walls a thousand times thinner than a human hair. Because of this open lattice structure, the material is actually made up mostly of air, 99.99% air .

But what is the metal making up this new material?

We know the density of the new material, so we can calculate the mass of a 1cm cubed volume of this material:

1cm³ of the new material would have a mass of 0.9 mg = 0.0009g.

If 99.99% of the mass of this material is made up of air, then

the mass of air = 99.99/100 x 0.0009 = 8.991 x 10^-4g (0.8991 mg)

and the mass of metal in the new material = 0.0009 - 8.991 x 10^-4 = 9 x 10^-7g (9 x 10^-4 mg)

If we assume that 0.00001% of the volume of the new material is metal, then

the volume of metal = 0.00001/100 x 1cm³ = 1 x 10^-7cm³

So, the density of the pure metallic solid would be 9 x 10^-7g/10^-7cm³ = 9g/cm³

If we compare this calculated density of the metal to a list of common metals as shown below,

Pure Substance	State	Density (g/cm3) at 25oC and 1atm
gold	solid	19.3
mercury	liquid	13.6
lead	solid	11.4
silver	solid	10.5
copper tin	solid solid	9.0 7.3
zinc	solid	7.1
aluminium	solid	2.7

then we see it is possible that the new material is made up of copper.

Reference

T. A. Schaedler, A. J. Jacobsen, A. Torrents, A. E. Sorensen, J. Lian, J. R. Greer, L. Valdevit, W. B. Carter.Ultralight Metallic Microlattices. Science, 2011; 334 (6058): 962 DOI: 10.1126/science.1211649

Further Reading

Density Calculations

Suggested Study Questions

1. Using the table of densities above, calculate the mass in grams of a
   • cubic centimetre of gold
   • a cubic metre of copper
   • a cubic millimetre of silver
   • a cubic kilometre of zinc
2. Using the table of densities above, calculate the volume in cubic centimetres of
   • 1g of copper
   • 100mg of lead
   • 4500μg of aluminium
   • 2kg of silver
3. Brass is a mixture of copper and zinc. A sample of brass has a density of 8.5g/cm³
• What is the mass a cubic centimetre volume of this brass sample?
• If the sample were made up of equal masses of copper and zinc, what is the mass of copper in the sample?
4. A sample of brass was produced using 500cm³ of each of copper and zinc.
• What mass of copper is present in the brass?
• What mass of zinc is present in the alloy?
• Assuming additivity of volumes, what is the density of this brass sample?
5. Cymbals are commonly made of bronze which is a mixture of about 10% (by mass) tin and 90% (by mass) copper. For a 100g sample of bronze, calculate
• the mass of copper present in the sample
• the volume of copper this mass represents
• the mass of tin present in the sample
• the volume of tin this mass represents
• the density of the bronze sample assuming additivity of volumes
6. Typically, bronze contains copper and about 12% (by mass) tin. Calculate the density of a sample of this bronze.
7. Bronze coins often contain copper and about 5% tin. Calculate the density of the bronze used to make coins.
8. The brass used to make springs and screws is often 65% (by mass) copper and 35% (by mass) zinc. Calculate the density of the alloy in a brass screw.

Skutterudites and Thermoelectric Generators

More than 60 percent of the energy produced by cars, machines, and industry around the world is lost as waste heat. If we could use this wasted energy we could improve the efficiency with which we use fuels, and benefit the environment.

Thermoelectric generators are devices which convert heat energy directly into electrical energy. Semi-conducting bismuth telluride, Bi₂Te₃, can be used to convert heat into electrical energy, but it is only about 5% efficient, too low to be useful in practical thermoelectric generators.
A number of scientists have been working with skutterudites to see if they can be used to increase the efficiency of thermoelectric generators.
Skutterudites have the general formula MX₃ in which M can be cobalt, rhodium or iridium, and X can be phosphorus, arsenic or antimony. The most promising of these compounds have been the CoSb₃. These compounds have 32 atoms in the unit cell and can be represented with the Co atoms occupying the corners of cubes.
The thermal conductivity of CoSb3 is too high for them to be used effectively.

So scientists have tried adding fillers to the structure to reduce the thermal conductivity. Rare earth elements and alkaline earth metals have been used as fillers.

Until recently these compounds have taken many days to make and have been expensive to produce. Oregan State University scientists have found a way to use microwaves to turn powdered metals into skutterudites in a few minutes and at a much lower cost. The first compound they produced using this technique was an indium cobalt antimonite compound in which indium is the filler.

Reference
Krishnendu Biswas, Sean Muir, M. A. Subramanian. Rapid Microwave Synthesis of Indium Filled Skutterudites: An energy efficient route to high performance thermoelectric materials. Materials Research Bulletin, 2011; DOI: 10.1016/j.materresbull.2011.08.058

Further Reading
Periodic Table
Writing Ionic Formula
Naming Ionic Compounds

Suggested Study Questions

1. Complete the following sentences:
   • A thermoelectric generator converts heat energy into ? energy.
   • A battery converts ? energy into electrical energy.
   • In a torch, the ? energy in the battery is converted into ? energy when the torch is turned on.
     
   • The ? energy in petrol (gasoline) is converted into ? energy when the fuel is combusted.
   • The ? energy released during combustion of a fuel can be converted into ? energy to move a car forward.
2. Skutterudites have the general formula MX₃. Write the formula of the skutterudite formed in each of the following situations:
   • M = cobalt and X = antimony
   • M = rhodium and X = phosphorus
   • M = iridium and X = arsenic
3. Give the name for each of the compounds formed in question 2.
4. For each of the following pairs of atoms, determine which is the most electronegative:
   • cobalt and antimony
   • rhodium and phosphorus
   • iridium and arsenic
5. Locate the elements cobalt, rhodium and iridium in the Periodic Table. In what ways do you expect these elements to be similar? Explain your answer.
6. Locate the elements phosphorus, arsenic and antimony in the Periodic Table. In what ways do you expect these elements to be similar? Explain your answer.
7. Give the names and chemical symbols of four examples of rare earth elements.
8. Give the names and chemical symbols of four examples of alkaline earth metals.
9. Write a possible formula for the skutterudite indium cobalt antimonite.
10. One structure has been represented as In_xCe_yCo₄Sb₁₂. Explain why this is an example of a skutterudite.
    
    
    
    

Sepiolite

Sepiolite has been known since Roman times when it was used to filter and purify wine. No other mineral is known to absorb more water or other liquids as efficiently as sepiolite, which is why sepiolite is commonly used in cat litter. Sepiolite is also used to absorb liquid spillages, such as in oil spills, and odours and stabilise aqueous products like paints, resins and inks.
Sepiolite is an aluminosilicate clay mineral with a typical formula of Mg₄Si₆O₁₅(OH)₂·6H₂O.

Sepiolites absorb moisture by using tiny tunnels in the crystals. The elongated, needle-shaped sepiolite crystals pack very loosely into a lightweight porous material. The surface area ranges between 75 and 400 m²/g, meaning that 20g of mineral have an internal surface equivalent to that of a football court. This is why sepiolite can absorb 2.5 times its weight in water. The tunnels in the crystal structure along with the empty space between the needles form a capillary network through which liquids can easily flow deep inside the bulk where the molecules attach to the surface of the crystals.

A team of scientists from Spain and France has obtained, for the first time, single-crystal X-ray diffraction images of sepiolite, opening the path to industrial synthesis and further improvement of its properties. In synthetic form, sepiolite could bind food products and stabilise drugs, extending their shelf life and making sepiolite an edible product.

Reference
Manuel Sanchez del Rio, Emilia Garcia-Romero, Mercedes Suarez, Ivan da Silva, Luis Fuentes Montero, and Gema Martinez-Criado. Variability in sepiolite: Diffraction studies. American Mineralogist, 2011 DOI: 10.2138/am.2011.3761

Further Reading
Percentage Composition
Balancing Chemical Equations


Study Questions

1. Calculate the percentage composition of sepiolite, Mg₄Si₆O₁₅(OH)₂·6H₂O
2. Explain what the ·6H₂O part of the formula refers to.
3. Write a balanced chemical equation for the dehydration of hydrated sepiolite to form anhydrous sepiolite.
4. Calculate the maximum mass of water you could obtain from 1kg of hydrated sepiolite.
5. If the sepiolite in question 2 has a surface area of 200m²/g, what is the total surface area of the sample in question 4?
   
6. Use the equation in question 3 to explain why sepiolite is used in cat litter.
7. Sepiolite is an aluminosilicate mineral. Explain what is meant by the term aluminosilicate.
8. Sepiolite is sometimes referred to as a zeolite-like mineral. In what ways is sepiolite similar to a zeolite mineral?
   

Multiferroic Alloy

University of Minnesota scientists have discovered a new alloy that converts heat directly into electricity. In theory, an alloy like this could be used to capture waste heat from a car's exhaust and use it to produce electricity to charge the car's battery. Similarly, a thin film of this alloy could be used to convert waste heat from computers into electricity.
The alloy is made out of nickel, cobalt, manganese and tin and has the formula Ni₄₅Co₅Mn₄₀Sn₁₀.
This new alloy undergoes a highly reversible phase transformation in which one solid turns into another solid with very different magnetic properties. This means that the new alloy begins as a non-magnetic material, then suddenly becomes strongly magnetic when the temperature is raised a small amount. When this happens, the material absorbs heat and spontaneously produces electricity in a surrounding coil.
Substances which combine unusual magnetic and electric properties such as this alloy are called multiferroic materials. Other multiferroic materials include TbMnO₃, HoMn₂O₅, LuFe₂O₄, BiFeO₃ and BiMnO₃.

In the demonstration below, "University of Minnesota researchers show how a new multiferroic material they created begins as a non-magnetic material then suddenly becomes strongly magnetic as the piece of copper below is heated a small amount. When this happens, it jumps over to a permanent magnet. This demonstration represents the direct conversion of heat to kinetic energy."




Reference
Vijay Srivastava, Yintao Song, Kanwal Bhatti, R. D. James. The Direct Conversion of Heat to Electricity Using Multiferroic Alloys. Advanced Energy Materials, 2011; 1 (1): 97 DOI: 10.1002/aenm.201000048

Further Reading
Metals and Non-metals
Physical and Chemical Changes
Pure Substances and Mixtures
Percentage Composition

Study Questions

1. What is meant by the term "alloy"?
2. What is meant by the term "multiferroic material"?
   
3. Calculate the percentage composition of the following multiferroic materials:
   
   • TbMnO₃
   • HoMn₂O₅
   • LuFe₂O₄
   • BiFeO₃
   • BiMnO₃
   • Ni₄₅Co₅Mn₄₀Sn₁₀
4. Which of the above substances would be classified as alloy(s)? Explain your answer.
5. Which of the substances in question 3 would be classified as mixture(s)? Explain your answer.
   
6. What similarities can you see in the multiferroic materials listed in question 3?
   
7. How do you think these similarities contribute to their magnetic and electrical properties?
   

Metallic Hydrogen Superconductor

Superconductors are materials that permit electricity to travel freely, without resistance, so they could dramatically improve the efficiency of power transmission technologies. Metallic hydrogen should be just such a superconductor.

Liquid metallic hydrogen is thought to exist in the high-gravity interiors of Jupiter and Saturn.
Scientists have predicted that electricity would flow, uninhibited, through a material made by compressing hydrogen into a metal. But so far, on Earth, researchers have been unable to use such compression techniques to squeeze hydrogen under high enough pressures to convert it into a metal. University at Buffalo chemists have now proposed an alternative solution for metallizing hydrogen by adding sodium to hydrogen which they think might make it possible to convert the compound into a superconducting metal under significantly lower pressures.

NaH₉, which does not occur naturally on Earth but is expected to be a stable compound, is predicted to become metallic at an experimentally achievable pressure of about 250 gigapascals, about 2.5 million times Earth's standard atmospheric pressure, but less than the pressure at Earth's core which is about 3.5 million atmospheres.

Reference
University at Buffalo (2011, June 13). Under pressure, sodium, hydrogen could undergo a metamorphosis, emerging as superconductor. ScienceDaily. Retrieved June 15, 2011, from http://www.sciencedaily.com­ /releases/2011/06/110613162240.htm

Further Reading
Metals and Non-metals
Kinetic Theory of Gases

Study Questions

1. Draw up a table listing the properties of metals and non-metals.
2. In what ways is elemental hydrogen like a non-metal?
3. In what ways is elemental hydrogen like a metal?
4. Use the Kinetic Theory of Gases to explain what you expect to happen as elemental hydrogen at atmospheric pressure is subjected to increasing pressure.
5. Using the Kinetic Theory of Gases, describe two ways that scientists could, in theory, make solid hydrogen.
6. Using the Kinetic Theory of Gases, explain why hydrogen might exist as a liquid in the interior of the planet Jupiter.
   
7. If 250 gigapascals is about 2.5 million times Earth's standard atmospheric pressure, what does the prefix "giga" stand for?
   
8. Why do you think that chemists suggest adding sodium to hydrogen to create a solid material capable of conducting electricity?
   

Hexagonal Boron Nitride

Graphene, a single-atom thick allotrope of carbon and an electrical conductor, is considered to be a possible successor to silicon in microelectronics applications.
Hexagonal boron nitride (h-BN) is an insulator. It is highly elastic and nearly as strong as graphene. Rice University scientists have found a way to implant sheets of h-BN into sheets of graphene, which controls the sheet's electronic character.
They have also found a way to deposit sheets of pure h-BN, 1 to 5 atoms thick, onto a copper substrate using a chemical vapour deposition process at about 1,000^oC. The h-BN material can then be transferred to other substrates. The size of h-BN sheets is limited only be the size of the copper foil and furnace used to grow it.
It should be possible to draw microscopic patterns of graphene and h-BN, useful in creating nanoscale field-effect transistors, quantum capacitors or biosensors.

Reference:
Li Song, Lijie Ci, Hao Lu, Pavel B. Sorokin, Chuanhong Jin, Jie Ni, Alexander G. Kvashnin, Dmitry G. Kvashnin, Jun Lou, Boris I. Yakobson and Pulickel M. Ajayan. Large Scale Growth and Characterization of Atomic Hexagonal Boron Nitride Layers. Nano Letters, 2010; 100722142755098 DOI: 10.1021/nl1022139

Study Questions

1. What is meant by the term allotrope?
2. What are the naturally occurring allotropes of carbon?
3. In what ways are these allotropes of carbon the same?
4. In what ways are these allotropes of carbon different?
5. If the formula for boron nitride is BN, what is the oxidation state (number) of boron?
6. Given the name hexagonal boron nitride, draw a possible Lewis Structure (electron dot diagram) for hexagonal boron nitride.
7. In what ways are graphite and hexagonal boron nitride the same?
8. In what ways are graphite and hexagonal boron nitride different?
9. Why is graphite a conductor while hexagonal boron nitride is an insulator?
   

Casting : Changes of State

The question of what happens when a material composed of more than one phase or state is heated or cooled is very important.
Many metal parts, for example, are made by casting. In the casting process liquid metal is poured into a mold and solidifies into the shape of the mold. As the liquid metal solidifies it forms tree-like structures called dendrites, and, if one of the dendrites breaks off it can lead to a change in the properties of the solidified material. The airplane industry has spent a long time developing solidification methods to avoid this problem when casting jet turbine blades.
Polymer solar cells use a complicated mixture of two polymers. When heated, the mixture evolves by a process that involves pinching which ultimately alters the properties of the mixture and the efficiency of the solar cell.
Scientists have been observing the heating process during which a rod-like phase or state embedded in another will break up into smaller domains just like droplets at the end of a stream of water, resulting in changes to the properties of the material. They have found that the shape of the interfaces during break up becomes universal, independent of the material used. This now allows them to predict the dynamics of the break-up process in a vast array of materials such as steel and polymers.

Reference:
Aagesen et al. Universality and self-similarity in pinch-off of rods by bulk diffusion. Nature Physics, 2010; DOI: 10.1038/nphys1737

Study Questions

1. Name the phase changes (changes of state) that can occur in each of the following situations:


• heating a solid


• heating a liquid


• cooling a liquid


• cooling a gas




2. Draw a sketch of the temperature-time graph expected for each of the following situations involving pure substances:
   


• heating a solid


• heating a liquid


• cooling a liquid


• cooling a gas




3. Explain why the temperature-time graph for the melting of ice differs from the temperature-time graph for freezing water.
4. Explain why the purity of a solid substance can be determined using its melting point.
5. Do you think the purity of a liquid substance could be determined using its freezing point? Explain your answer.
   
6. Explain what is meant by the term sublimation.
7. Give two examples of pure substances that undergo sublimation.

Hard Metal

Hard metal is a mixture of a hard carbide phase, tungsten carbide, and a tougher metal phase, cobalt. It is produced by sintering, a process in which fine powders of tungsten carbide and cobalt are heated up so that the cobalt melts and the material is pulled together by capillary force. This results in a solid material consisting of hard tungsten carbide grains surrounded by the tougher cobalt-rich cement phase.

The size of the tungsten carbide grains determines the hardness of the hard metal.
Scientists know that by doping the material, that is, by adding another substance in tiny amounts, they can limit the size of the grains. For example, adding a tiny amount of vanadium can limit the growth of the grains, instead of growing grains 1/1000 mm in diameter, the addition of vanadium results in grain sizes about 1/10,000 mm. Scientists at the Chalmers University of Technology in Sweden have just used high-resolution electron microscopy to observe an extremely thin layer, only 2 atom layers thick, of a cubical structure on the tungsten carbide grains which they believe is affecting the growth of the grains.

Reference:
Expertanswer (2010, June 14). Materials researchers micromanage atoms in hard metal. ScienceDaily. Retrieved June 16, 2010, from http://www.sciencedaily.com­ /releases/2010/06/100614093343.htm

Study Questions:

1. Write the symbol for each of the following elements: tungsten, carbon, cobalt, vanadium.
2. To which group of the Periodic Table do tungsten, cobalt and vanadium belong?
3. Give possible oxidation states (numbers) for tungsten, cobalt, vanadium and carbon.
   
4. Suggest a formula for tungsten carbide.
5. Would you expect larger or smaller grains of tungsten carbide to grow at higher temperatures?