Possibly Macor (machinable ceramic) from Corning, or the comparable
Vitronit, which are both glass ceramics.

Comparable to borosilicate glass.
Extremely machinable (HSS or carbide tools, use proper speeds and
coolant), continously stable until 800°C.
No safety concerns or toxic effects. The dust created when machining can
be an irritant.

Available from Corning as standard size maxi slab of 36366 cm.
Price = ?

Quoting Wikipedia for technical data for Macor:

Density: 2.52 g/cm³
Young's modulus [GPa @ 25°C]: 66.9  (FR4 = 21 - 24, Aluminium = 69,
Steel = 200)
Specific stiffness [E6 m²/s²]: 26.55  (Al = 26, Steel = 25 +/- 0.5)
Poisson's Ratio []: 0.29 (FR4 = 0.118 - 0.136, Al = 0.32, Steel = 0.27-0.31)
Thermal conductivity [W/(mK)]: 1.46  (FR4 ~ 0.3 - 1, Al ~ 300, Steel ~
15-50)
compressive strenght [MPa]: ~350
Electrical resistivity [Ohmcm]: 1 E17 (FR4 = 1 E14, PTFE = 1 E23 to 1 E25)
Coefficient of thermal expansion [E-6 m/(m*K)]: 9.3 (FR4 ~ 13 , Copper =
16.6)

Firat

Am 16.04.2017 um 13:08 schrieb John Devereux:

As a practical aside regarding surface resistance, ignoring guard
amplifiers and guard traces, the historic solution is to point to point
wire in air. With some designs using Teflon standoffs for a more rigid
approach. Some of this can still be seen today, your smoke-detector likely
has a one-layer phenonlic PCB with the sense pin of a DIP package IC bent
up off board and point-point wired in air to the Ion chamber.

On Sun, Apr 16, 2017 at 1:25 PM, fala@gmx.net fala@gmx.net wrote:

Synthetic sapphire has come down in price quite a bit, a 25mm dia 5mm thick
sapphire window can be had for just $71

https://www.thorlabs.com/thorproduct.cfm?partnumber=WG31050

Andy ◉ Bardagjy.com http://bardagjy.com/ ◉ +1-404-964-1641

On Sun, Apr 16, 2017 at 12:03 PM, Scott Stobbe scott.j.stobbe@gmail.com
wrote:

FR4 has problems with consistency.  Samples can have problems with
hook, dielectric absorption, leakage, and sensitivity to humidity.

On Sun, 16 Apr 2017 12:08:07 +0100, you wrote:

Are conformal coatings the right way to handle this?

I understand there are kinds of FR4 and G10 that don't have hook. What does
one do about hook - how are those substrates improved? How does hook
manifest in circuits?

On Mon, 17 Apr 2017 03:48 David, davidwhess@gmail.com wrote:

Hook is merely a manifestation of the variation of dielectric constant with frequency.

It affects the frequency response and transient of resistive attenuators by requiring more complex compensation than merely adjusting a trimmer cap to equalise the low frequency and high frequency attenuation. Since the dielectric "constant" (relative permittivity) of all dielectrics is frequency dependent all dielectrics will exhibit hook to some extent. Some PCB substrates like some versions of FR4 and G10 exhibit a significant variation in the dielectric constant from the dc value to a somewhat lower value for frequencies even in he audio range let alone frequencies of several MHz. Achieving a flat frequency response where the dielectric associated with circuit board capacitances exhibits significant hook is a complex task. Circuit board hook even affects the impedance of printed transmission lines (eg stripline, microstrip, CPW etc).

Bruce

Conformal coating might help with sensitivity to humidity but I would
not count on it.

Hook corrupts the AC response and settling time in high impedance
circuits.  I did not know it at the time, but I suspect I have seen it
manifest as mysterious linearity errors in integrating converters.

The way to prevent hook is to use a substrate which does not suffer
from it.  For FR4, this could include testing to qualify suppliers.
Early on when Tektronix ran across this, they used other substrate
materials like polysulfone, an amber colored high temperature plastic,
and later a white material which I have not identified.

On Mon, 17 Apr 2017 01:54:15 +0000, you wrote:

Thank you. Do you have a plot of the effect on impedance due to hook out
into high frequencies, measured on a real world material? It would tell me
a lot.

On Mon, 17 Apr 2017 04:19 Bruce Griffiths, bruce.griffiths@xtra.co.nz
wrote:

Hi David,
You could perhaps send some of that white material to Mike's Electric
Stuff, he has a mass spectrogram, which could tell you what it is.

On Mon, 17 Apr 2017 04:31 cheater00 cheater00, cheater00@gmail.com wrote:

That's quite a difference from what I knew, thanks!

On Mon, 17 Apr 2017 05:17 Andy Bardagjy, andy@bardagjy.com wrote: