EE 4thyr Projects 2015/2016
Project Title:	BoltSens – A multi-functional versatile diamond-based sensor
Supervisor:	Prof Richard Jackman		Submitted on: Thursday 19th February 2015

Concept
A new, diamond-based, resilient and highly sensitive multi-functional sensor for the real-time detection of water quality within nuclear installations is proposed. Moreover, such a device will lead to the establishment of a ‘family’ of similar, and interchangeable, devices capable of monitoring all aspects of water quality within a nuclear installation.

Need
The vulnerability of water distribution systems in pressurised water nuclear plants to materials failure, human error, natural disaster or deliberate attacks, which would have major public health, economic, and psychosocial consequences, is one of the main issues of concern to governmental agencies and reactor operators.

BoltSens – proposed capability of the first in the family
A single sensor head positioned on an M20 bolt, which also contains all of the (miniaturised) electronics needed for operation and readout, is proposed. Such a bolt could be readily installed on a range of cooling water installations for continuous readout of water quality as opposed to the current occasional water sampling and ex-situ measurement approach.

For the first in the BoltSens family it is proposed that the sensor head will monitor water pH, oxygen content, electrical conductivity and temperature, all with one single integrated sensor head and electronics, on, and within, an M20 bolt. The sensor will be specified to pressures greater than 200bar, and temperatures greater than 200C. The sensor head will derive its multi-functionality capabilities by being fabricated from diamond. High quality diamond, grown by chemical vapour deposition (CVD) methods, is now a readily commercially available at modest cost for engineering applications such as this. See for example, http://www.e6cvd.com/ and http://2atechnologies.com

During the R&D phase BoltSens will be powered and communicated with by copper wire. However, it is anticipated that energy harvesting (thermal, light) or optical fibre for power and non-contact communications (li-fi, Bluetooth etc.) or optical fibre for communications will be implemented for in-boat use.

BoltSens – future capability
The second in the BoltSens family is anticipated to be a sensor for trace levels of alpha radiation within reactor cooling water. An approach based on the electro-precipitation of any radio-actinides present onto a diamond sensor head to pre-concentrate them prior to measurement. Very high sensitivity should result. Initial work on this approach has started through the use of a BAES-UCL CASE PhD project.

Further additions to the BoltSens family can be anticipated to offer beta detection along with chemical and biological species sensing capabilities.

BoltSens-1 – Design Concept
A threaded bolt can be readily inserted or attached to pipework within a nuclear installation using tried-and-testing methods. It is now possible to integrate driver and readout electronics onto PCBs that are measured on the mm-scale, enabling such miniature electronics to be incorporated within the ‘head’ of such a bolt. This electronics can then be connected through the body of the bolt to a sensors’ head that is immersed in the water to be evaluated.

Diamond is a robust material uniquely offering the properties required for a multi-functional sensor head. It

• is chemically and radiation resilient
• can be doped to make regions conductive within an electrically insulating body
• offers a large ‘electrochemical window’, that is it can support relatively large voltages for sensing purposes before succumbing to the (redox) breakdown of water
• significantly less susceptible to physical and bio-fouling than most sensor materials, ensuring measurement accuracy
• can be routinely cleaned ‘in-situ’ by driving water redox reactions, which cause no degradation to the diamond itself

To avoid the need for extensive testing of the pressure containment capabilities of the diamond sensor head when in place, it is proposed to use an existing technology to support the pressure differential, and then position the diamond sensor head on this. A metal flange with electrical feedthroughs (and ceramic insulation) specified to >200bar, will form the physical barrier between the pressurised water and the interior of the bolt, whose internal bore is to be limited to 6mm.

BoltSens-1 – Scientific Concept
Electrical conductivity
Heavily boron doped diamond (>1020 cm-3) becomes quasi-metallic in character and can be used in a ‘transmission’ line configuration [J. Appl. Phys., 77 (1995), 755] to accurately determine the electrical conductivity of the water that the sensor head is immersed in.
Oxygen content
Although the use of boron-doped diamond (BDD) as the working electrode can lead to the electrochemical determination of oxygen [Electrochem. Comm. 9 (2007), 2280], the sensitivity of the measurement is significantly improved if the BDD is coated with Pt nano-particles [Anal. Chem. 2009, 81, 1023–1032]. Such an approach will be adopted here to make an amphoteric dissolved oxygen sensor, using a BDD counter electrode and a Pt reference electrode, in the form of Pt coated BDD.
pH level
The most accurate way to determine pH will be the use of an ion-sensitive field effect transistor (ISFET). Such a device is similar to a MOSFET, but the gate is missing, with the gate voltage being applied to the solution, and the threshold voltage being modified by the presence of ions in the solution. Diamond ISFETs can be fabricated using a heavily boron doped diamond, provided the channel region of the BDD is relatively thin (~400nm) [Diam. & Relat. Mat., 16 (2007), 905]. Thick BDD regions will form source and drain, with a thinner BDD channel. The Pt reference electrode for the oxygen sensor will serve as the reference (or gate) electrode of the ISFET.
Temperature
For simplicity, a miniature bead thermister will be incorporated on the rear of the diamond sensor head for temperature measurement. These are readily available at low cost (see, for example, www.sensorsci.com) and the extremely high thermal conductivity of diamond will mean that a measurement at the rear of the thin diamond sensor head will be an accurate reflection of the water temperature.
Sensor head cleaning
Two of the outermost BDD electrode will be used to generate small quantities of oxygen and hydrogen at the sensor head surface; the water flow will ensure that these will clean up the head when periodically operated.


BoltSens-1 – Sensor head methodology
A design based on concentric circular BDD regions separated by intrinsic diamond will be used; this reduces the device processing complexity when compared to individually located sensors for each measurement on the diamond chip. The layout of the BDD electrodes, i-Diamond regions, Pt-BDD reference electrode, Pt-decorated and BDD working electrode are shown below. Note the usse of a diamond-like carbon (DLC) layer for passivation of the source and drain BDD electrodes of the ISFET structure.

CVD-grown polycrystalline diamond will be obtained commercially. BDD overlayers will be grown using a sub-contractor, and the use of laser drilling for the via holes for back contacts will also be sub-contracted. All other processing will be performed in-house within the clean room available to DEG within the London Centre for Nanotechnology (LCN) at UCL. This will include mask design, photolithography, reactive ion etching (RIE) and metallisation.

Project Partner: BAE Systems (Marine) plc
http://www.baesystems.com