July 29, 2022

News from Plansee 2022

Writing about web page https://www.plansee-seminar.com/

After being delayed by COVID from 2021, the long-awaited Plansee 2022 conference took place in Ruette, Austria from 29th-May -3rd June.

This is a four-yearly conference that is the place to be for anyone who works with hard metals and refractory metals - essentially most metals from group 5,6,7 of the Transition metals - namely W and elements next door.

This Seminar did not disappoint and there was a surprisingly significant number of talks on nuclear materials in terms of how W and similar materials have their place in fusion power.

There have also been numerous opportunities to connect with academic and industrial partners as well as groups such as the European Powder Metals Group (EPMA). Maintaining networks and setting up new once is as crucial to research as doing research itself - no advances get made if they are not communicated,

Alongside this were talks on exciting topics such as additive manufacture and 3-D printing techniques - both of which are significant when considering parts for fusion reactors since complex geometries are required.

The work our group presented gained some interest in that this conference is an ideal platform to demonstrate the cWC-RSB concept and the progress made so far despite setbacks. Certainly, the contacts made at Plansee will show a significant role later this year as the full demonstration of Gen 2 RSBs is realized.

Simply put, 3-D printing of powder metallurgy fabrication can be split into two main categories:

1. Direct powder bed sintering, where a laser draws out a part from a powder bed in a protective atmosphere as for 3-D printing. The aim is ultimately a near net shape part that can then be annealed and finished as for conventional PM.

2. 3-D printing of green PM materials. This comprises of a selection of techniques involving green materials including 3-D printing of organic binder into powder constituents - useful for for complex geometries - prior to a standard debinding cycles or from filaments which can be shaped and bonded into a more robust structure prior to sintering.

Both techniques show promise both for cWC and RSB materials in the later stages of this project when there will be need for demonstrator parts and full-sized boilerplace items by the mid 2020s

While it might be a long time until Plansee 2026 the work starts now to ensure that fusion-realated applications will be front and centre.

June 06, 2022

Snatching Victory from the Jaws of Defeat part 2

Picture the scene. The critical equipment you have been waiting to have ready is online and practical work is going well. You are on track to have results ready for the conference you are going to at the end of the month.

You now have a new gauge on the furnace so you aren't having to borrow one. It's in Torr not mbar but that isn't a worry since 1 torr = 1.333 mbar.

A new set of samples are loaded into the samples. First pumpdown goes without a hitch. Time to do the argon purge so follow the Standard Operating Procedure (SOP). All valves are checked, valve on vacuum pump is closed and Ar gas cylinder is carefully opened to allow gas to pressurize vacuum chamber to 1 bar Ar. Cylinder valve, flowmeter valve and lower chamber valve is opened in order.

Pressure starts to increase from its baseline near 2 x 10-1 torr baseline but starts to slacken off near 43.3 torr, despite the positive pressure at the Ar cylinder and flowmeter.

Two operators in the room, one manning the Ar tank adjacent to the flow meter and the other manning the chamber valve. Both can check the valve.

Chamber valve is opened further, pressure increases to 82 torr and levels. Valves still open but pressure just fluctuates - still short of the 760 torr target pressure-


Top of furnace arcs 10 cm from the neck of the tube and embeds itself in the lab floor. Operators act to shut off cylinder, turn off vacuum pump and shut all the valves.

Now it is time to take photos and file an incident report. Here is the resulting damage:

Damage to worktube post over-pressure incident

Here is the top vacuum assembly post incident:

Top vacuum furnace assemblage

In terms of damage, the top assemblage with the exception of the vacuum gauge is still intact. The worktube has kept its integrity with the exception of a 30 mm spall damage at the top of the tube. This will need to be removed before a seal can be established.

So while the incident report is filed, the next questions are how did it happen?

Identification of the fault showed that the over-pressure from the misreading of the gauge was 1.1 bar, which was not sufficient to trigger the failsafe valve set at 1.2 bar.

The tube was overpressured as a result of a hitherto unknown nonlinearity in the behaviour of Ar at sub-atmosphere. For Pirani-type guages that rely on convection, diatomic and monatomic gasses behave differently at low pressures. The net result is that while N2 and O2 behave mostly linearly with respect to measured pressure, monatomic gasses do not:

Gas behaviour at different pressures

As can be seen, it is very easy to overpressure a furnace with Ar using a Pirani gauge, which relies on convection. Gauges that rely on absolute pressure via a plate are more reliable for this type of operation.

The actual fault was a result of a sudden loss of friction between the O-ring and worktube when at overpressure, resulting in the loss of upper vacuum assemblage while overpressure. This resulted from distortion of the o-ring at elevated temperature.

From this evidence it is apparent that the current assemblage is not fit for purpose when at high temperature, despite the highest temperature recorded at the gasket. This has lead to the following faults that need to be remedied before the furnace can be recommisioned.

  • A thermal shield is required for the top part of the furnace. This will need to sit in the neck of the furnace and sit on the top of the furnace so it can stay in place during a furnace cycle. Can be fabricated from an existing thermal shield with an additional thermal plug at the top. This will significantly cut the thermal load at Tmax when radiation is the main contributor.
  • Larger bore tubing for the vacuum pump exhaust to prevent pressure build up on turning pump on.
  • For general use, use N2 line for purging since this will prevent the risk of overpressure. This will require an additional 6-7 m of PVC tubing to connect the spare N2 line to the gas inlet.
  • A regulator can be added to the Ar cylinder to prevent gas overpressure and to limit the outlet pressure to 1 bar.
  • A new pressure gauge that does not rely on convection and will not go nonlinear at sub-atmosphere pressure.
  • Repair of worktube to enable fitting of o-rings.

Additional safety features to be added are as follows:

  • Bolting the top assemblage onto the top of the furnace cage to prevent loss of top vacuum system and enable triggering of safety valve.
  • Use of fans at top of furnace to keep top cool and increase the longevity of the seals.
  • Amending SOP to better reflect the operation of the furnace.

Once all these items are addressed the furnace can be recommisioned. Once thing is for certain, no practical work is so important that operator safety can be compromised.

May 23, 2022

Snatching Victory from the Jaws of Defeat part 1

There has not been that many updates for a while but that is hardly surprising when it is considered that the group has been consolidating data obtained from the DCF while working on sintering trials in time for the Plansee meeting for the end of May.

May has been a pivotal month for the cWC-RSB project. This month marks the first ever Generation 2 RSB sintered at Warwick, fulfilling the critical aim to showcase this work at Plansee 2022 - a highly strategic meeting for experts in W metal and cutting tools where the cWC-RSB project will meet its widest audience. Until the vacuum furnace got blown up that is - more on that later.

If there is one universal constant of experimental work is that failure is always an option. Whether it be the underpinning hypotheses, poor or inappropriate experimental design, personality clashes, equipment failure or too relevant now, war breaking out - whenever there is a long stretch of experimental work, there will be failure.

The key progress indicators are what the nature of the failures are and how they are responded to. This particular failure was equipment related and will be discussed in more detail in the next entry.

Prior to this, experimental work on RSB sintering trials had been progressing well. Recent results indicate that RSBs will expand to almost twice their volume prior to consolidation. Other results show that RSBs do not start to consolidate much before 1350oC. The 1300oC sintering trial is seen below:

RSB55 sintered at 1300C

RSB55 (target composition 30at% W, 30at% B, 30at% Fe, 7.5at% C and 2.5at% Cr) sintered at 3 hours at 1300oC.

Raising the sintering temperature to 1400oC for 2 hours had a dramatic change in the sintered RSB in that the sintered samples was observed to densify and shrink as for a generation 1 (Gen1) RSB55:

RSB55 sintered at 1400C for 2 hours

The Gen2 1400oC RSB55 is still not as dense as the original 1450oC vacuum sintered/SinterHIPPed RSB55 (86% vs 98% assuming similar phase presence) but is dense enough to be polished and considered a true Gen2 RSB. All that needs to be done is more trials at higher temperatures up to 1450oC.

That is until the vacuum furnace blew up, damaging the worktube and requiring the filing of a Near Miss Incident Report.

The basic cause of the failure was overpressuring the tube during the purge cycle - samples are vacuum sintered but an Ar purge is administered prior to pumpdown to remove trace O2 and water vapour from the tube and attain a decent vacuum - < 7 x 10-2 mbar (7 Pa) is a target for this vacuum furnace. However, the causes of the failure were not as simple as at first sight.

The next entry will look at methods used to identify the failure, the causes of said failure and how this inident can be used to better improve operator safety and furnace reliability in future.

April 06, 2022

How To Irradiate Your Samples

Following some of the successes and pitfalls at the DCF facility we now have all the irradiations completed thanks to the dedicated staff at the Dalton Cumbrian Facility. Since the energies in this round of experiments were sufficiently below the threshold of activation, samples were dispatched in the post.

The next stage will be to clean and mount sample for cross-sectional imaging, hardness testing, EBSD and TEM but that will be discussed in a later entry since much of that work will be for publication.

So how does one get to irradiate samples in the first place? And what has this specific experiment got to do with the first goal of the Radiation Dense Materials concept?

First of all as with any new science experiment, you have to talk to people first, in person, online or by email. This is critical to ensure that you can flesh out the proposed experiment and see how feasible it is or isn't. By discussing the planned irradiation schedule with the staff at the Dalton Facility we were able to acertain that ambient and non-ambient irradiation was possible with the proton beam and the gamma irradiator.

One of the inherent difficulties of current irradiation facilities is that generally samples can only be irradiated by one type of radiation at one time, unless placed in a test reactor. This is similar to reproducing a painting but with only one brush and one pigment. Hence, it is important to state the limitations of irradiation testing on samples with this in mind.

So, how are samples actually irradiated? This depends on the method and the radiation used. For the gamma irradiator it is simply a case of tracking the face closest to the 60Co source and placing it in a low-activation borate glass. The 60Co irradiator is shown below:

60Co gamma source

For low-temperature (77K) irradiations, samples were placed in a dewar full of LN2. Since oxygen liquifies at 85K this forms a pool in the dewar which under gamma irradiation transforms to O3 which is solid at LN2 temperatures. Solid O3 is an explosion hazard and thus nescessitates emptying and renewing the LN2 every 5 kGy. The low-temperature high-dose gamma sample had 90 kGy in total.

For proton beam irradiation, sample preparation is more involved. Samples need to be mounted with a heat-resistant conductive adhesive (silver paste) and need alignment such that the beam energy and radiation time can be computed, the exact figures depending on the dosage and the target temperature. Samples mounted for proton-beam irradiation are shown below:Samples mounted for room-temperature proton irradiation

Since protons are charged particles, total proton dosage can be computed from the charge accumulated from the beam current. Alongside sample irradiation, other data from samples such as emissivity can be obtained from proton irradiation at elevated temperatures.

Data analysis is at the earliest stages with the following planned sequence (1) Cross-sectional imaging (2) microhardness measurements (3) EBSD and (4) TEM (specifics to be determined following from EBSD and HV measurements).

Ultimately, this work will form the outline for subsequent radiation work as this is a first for both cWC and RSB materials.

March 30, 2022

First irradiation studies of RSBs at the Dalton Facility

Writing about web page https://www.dalton.manchester.ac.uk/research/facilities/cumbria-facilities/

Last week was a first in the field of Radiation Dense Materials in that samples of Reactive Sintered Borides (RSBs) and cemented Tungsten Carbide (cWC) were irradiated for the first time by proton beam and gamma irradiation at the Dalton Cumbrian Facility in the Westlakes Science Park during some truly beautiful springtime weather.

Dalton Cumbrian Facility

Much work on simulating the radiation response and attenuating properties of a cWC, cWC-RSB and tungsten boride shielding concepts has occured during the 2010s onward [C.G. Windsor et al 2015 Nucl. Fusion 55 023014,C.G. Windsor et al 2018 Nucl. Fusion 58 076014], however, data on the radiation response of cWCs [Shielding materials in the compact spherical tokamak https://doi.org/10.1098/rsta.2017.0443] is very sparse due to their recent adoption as candidate shielding materials. No data on radiation response exists for RSBs due to their novelty so this will be the first time any practical data will be obtained from irradiated RSB materials.

The planned experimental work at DCF fell into two main topics: Proton irradiation and gamma irradiation. Each type of radiation would take place at two different intensities and at two different temperatures.

Proton radiation most closely resembles the conditions at or near the plasma-facing component of a tokamak reactor and hence was to take place at ambient and at 600oC.

Gamma radiation will be significant at all parts of a tokamak reactor but particularly near the superconducting magnets. These irradiations were performed at -196oC (77K) and at ambient. Each experiment consisted of three samples: cWC, RSB and W metal alloy with eight different irradiation conditions with a 9th sample set to sit in the gamma irradiator for long-term irradiation studies.

So....how did the work go and what happened over last week? Read on.

The gamma irradiation mostly went to plan. Low dose gamma irradiation was set to 30 kGy for both the 77K and ambient irradiation. High dose gamma radiation samples was set to 240 kGY for ambient and 90 kGY for 77k. The lower dose at 77K is a result of the different sample positioning due to the dewar and the requirement to change the N2 the dewar every 5 kGy to prevent excessive ozone formation, which is an explosion hazard. All gamma irradiated samples except the long-term samples have been returned.

Proton irradiation was less straightforward. The high dose ambient proton irradiation was successful, with a steady state temperature of 125oC and a total dose equivalent to 1.66 x 1018 protons. Subsequent proton irradiation attempts were unsuccesful, mostly due to the plasma failing the strike and therefore enable the proton beam to irradiate the samples. However, these sample will be irradiated this week and sent by post.

This is one of the many hazards of experimental work in that failure is always an option but nevertheless this work on completion will provide an important first step in realizing the cWC-RSB concept as practical radiation shielding materials whatever the damage or lack of is present on analysis.

This work would not be possible without the dedication of the staff and researchers at the DCF who make this possible.

The Inaugural Entry – the life and times of the Radiation Dense Materials Group

Writing about web page https://warwick.ac.uk/fac/sci/physics/research/condensedmatt/radiation_dense_materials

Welcome to the Radiation Dense Materials Group Blog. This blog will showcase the latest research from this group alongside how the research came to be with notes and photos of facilities, locations and conference trips. This will give people an insight into how research takes place on a day-by-day basis and on how many steps it takes to get to where you want to be.

The Radiation Dense Materials group aims to sythesize, study and develop novel concept radiation dense materials with the initial aim of making compact radiation shielding for fusion reactors and to make practical fusion power generation possible by 2030. It is envisaged that this will only be one such application for radiation dense materials when considering the need to expand the nuclear sector as a carbon-free power source.

Nuclear fusion has the potential to be a safe, effective carbon-free power source but has been limited in part by the lack of materials that can both attenuate radiation effectively and withstand the extremes of temperature expected within a power-generating fusion reactor. The plasma-facing component is expected to withstand temperatures up to 1000oC during normal operation. Superconducting magnets require cryogenic cooling (<77K), both of which would be seperated from each other by the shielding/coolant/Tritium blanket by little more than 1-2 m in a compact spherical tokamak. The initial cWC-RSB concept aims to be able to satisfy both radiation shielding and mechanical aspects with respect to this application.

This blog aims to be as interactive as possible and to give people an insight into how scientific research takes place behind the scenes.

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